Advancxed EV Battery Management

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ADVANCED BATTERY MANAGEMENTAND TECHNOLOGY PROJECT

Executive Summary

Vermont Electric Vehicle Demonstration Project


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Submitted to

Northeast Alternative Vehicle Consortium

112 South Street, Fourth Floor

Boston, MA 02111

And

Defense Advanced Research Projects Agency

by

Vermont Electric Vehicle Demonstration Project

Agency of Natural Resources

103 South Main Street, No. 3 South

Waterbury, VT 05671-0402

September 20, 1999

Agreement No. NAVC1096-PG009524

Prepared By

M. J. Bradley & Associates

47 Junction Square Drive

Concord, MA 01742


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Table of Contents

1.0 Introduction

1.1 The Advanced Battery Management and Technology Project

1.2 Problem Statement of the ABMTP

1.3 EVermont Vehicles

2.0 Battery and APU Technology

2.1 Advanced Battery Design

2.2 Auxiliary Power Units

3.0 Battery Testing

3.1 Bench Testing of Ovonics NiMH Batteries

3.2 Laboratory Bench Testing at the University of Massachusetts, Lowell, MA3.3 Battery Box Design

3.4 NiMH Finite Element Model

4.0 Battery Thermal Management System

4.1 BTMS Design

4.2 BTMS Design Testing Cold Chamber

4.3 BTMS Design Testing Warm Weather

5.0 Vehicle On Road Test Evaluations

5.1 Data Acquisition System5.2 EVermont On-Road Field Test Course

5.3 EVermont NiMH Baseline Vehicle, EV13, 1997

5.4 HydroQuebec Vehicle Testing, EVHQ, Summer 1998

5.5 EVermont EV15 Vehicle Testing, Winter 1999

5.6 EV1, Solectria E-10 Hybrid Truck

5.7 Summer Testing July 1999

6.0 Conclusions and Recommendations

6.1 Cabin Thermal Management

6.2 NiMH Vehicle Performance

6.3 Battery Thermal Management

6.4 APU Integration

6.5 Battery Thermal Management System Model


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EVermont wishes to thank the many sub-contractors, companies and individuals that providedassistance to this project.

Kevin Bracey Agency of Natural Resources

Corie Dunn Agency of Natural Resources

Tom Horn Atlantic Center for the Environment

Tom Franks Department of Public Service

Lois Jackson Department of Public Service

Mary Morrison Department of Public Service

Steve Miracle EVermont

Lauren Scharfman EVermont

Bill Conn Green Mountain Power

Jean-Franois Morneau Hydro Quebec

Denis Parent Hydro QuebecSerge Roy Hydro Quebec

Denis Laurin Hydro Quebec

Tom Balon M.J. Bradley & Associates

Paul Moynihan M.J. Bradley & Associates

Amy Stillings M.J. Bradley & Associates

Joe Gagliano M.J. Bradley & Associates

Greg Wight Norwich University

Andrew Heafitz Solectria

Richardo Espinosa Solectria

Phil Girton Vermont Monitoring Cooperative

Thank you all for your hard work, research and support and most of all your enthusiasm forelectric vehicle research and technology. A special thank you to Sheila Lynch, Tom Webb andLisa Callaghan at NAVC and Robert Rosenfeld and DARPA for funding and supporting thisimportant work.

Richard Watts

Project Director, EVermont

Harold Garabedian

Research and Testing Director, EVermont


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EVermont Technical Reports

The following reports are available for $25.00 each. Checks should be sent in advance and bemade payable to EVermont, c/o Agency of Natural Resources, Building 3 South, 10 South MainStreet, Waterbury, VT 05671-0402.

Thermal Measurements and Analysis of the 1995 Solectria Force (Paul Richmond - CRREL)

Effect of Winter Conditions on Power Consumption (Deborah Diemand/Jesse Stanley - CRREL)

Electric Vehicle Traction and Rolling Resistance in Winter(Sally Shoop - CRREL)

Traction and Handling Performance of an Electric Vehicle in Winter Environment(Sally Shoop -CRREL and Harold Garabedian - Vermont DEC)

Thermal Windshield and Foam Insulation Report(Harold Garabedian - Vermont DEC)

Electric Vehicle Thermal Management - EVermont Test Results (Harold Garabedian testimonybefore the Massachusetts State Legislature)

Solectria Sunrise Thermal Analysis (Harold Garabedian - Vermont DEC)

Northeast Advanced Thermal Management Technology Project - Executive Summary

State of the Art Electric Vehicle Cold Weather Range (Harold Garabedian - Vermont DEC andAndrew Heafitz - Solectria Corp.)

Baseline Performance of a Nickel Hydride Powered EV Operating in Vermont(HaroldGarabedian - Vermont DEC, Ricardo Espinosa - Solectria, Nick Karditsas - Ovonic Battery

Corp., and Stephen Brennan - GM-Ovonic)

Thermal Efficiency Tests and Analysis of an Electric Bus in Portland, Maine; John Duffy,Professor, Mechanical Engineering, University of Massachusetts Lowell, January 1997.

Modification of Greater Portland Transit District Battery-Powered Electric Bus; EVermont,February 1997.

Electric Vehicle Noise: A Report on a Study Conducted at Norwich University; Gregory D.Wight, P.E., Professor, Civil and Environmental Engineering, March 1997.

Monitoring EVs in Floridas Environment; William Young, Florida Solar Energy Center.


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1.0 Introduction

1.1 The Advanced Battery Management and Technology Project

Claims that electric vehicles cannot be deployed in cold climates are without a doubt misleading

statements when the development and testing completed by EVermont over the last several yearsis considered. EVermont is committed to the successful cold weather development anddeployment of electric and alternative fueled vehicles and has deployed a significant number ofelectric light duty vehicles and one hybrid vehicle in the state of Vermont. This work hasprimarily centered on cabin and battery thermal management technologies, their implementationas well as their successful deployment.

EVermont has taken their extensivebody of experience in the thermalmanagement of lead-acid batteries andapplied that knowledge to the thermalmanagement needs of the advancednickel metal-hydride (NiMH) battery

technology developed by the OvonicBattery Company (OBC). EVermontwas an early adopter of the NiMHtechnology in the form of an Ovonicbattery equipped Solectria Forcevehicle. This vehicle, green in color,was registered under Vermont platenumber EV13 and was tested along

side other EVermont lead-acid vehicles under EVermonts previous Northern Region ThermalManagement Technology Project (NRTMTP), contract NAVC1095-PG009521.

EVermont and the Northeast Alternative Vehicle Consortium (NAVC) are applying this

experience in the Advanced Battery Management and Technology Project (ABMTP), contractNAVC1096-PG009524. The primary goal of the project was to identify and develop a successfulall-climate design for a NiMH battery thermal management system for application in commercialand military vehicles. The project also investigated the application of an auxiliary power unit(APU) as a vehicle thermal management system. The NAVC funded this project through theDefense Advanced Research Projects Agency (DARPA) Electric and Hybrid Electric VehicleTechnology Program.

1.1.1 Major Findings from Previous Projects

The major findings of previous EVermont projects that are relevant for this project are:

(1) For cold weather operation, a 5 kW coolant heater provided the best cabin heatingperformance for the light-duty Solectria Force passenger car,

(2) there was measured heat loss through the battery enclosures,

(3) for lead-acid batteries, an insulated battery box successfully helped the batteries retain heatuntil the vehicle is recharged,

(4) for lead-acid batteries, cooling fans were sufficient to maintain maximum temperatures belowcritical temperature levels,

Figure 1: NiMH Solectria Force, EVermont EV13


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(5) a Solectria Force with non-thermally managed NiMH batteries retained nearly 65% of itswarm weather range, similar to that of a thermally managed lead-acid vehicle,

(6) 20-30% of the cold weather range reduction was due primarily to vehicle friction losses (i.e.,increased road losses).

In order to address several additional electric vehicle cold weather thermal management

challenges, EVermont, in the NRTMTP, continued to develop and evaluate light-duty electricvehicle thermal management technologies in four areas: HVAC system improvements, improvedbattery enclosure thermal management, advanced lead-acid battery cold and warm weatherperformance and noise testing.

1.1.2 Battery Thermal Management Requirements

The factors that affect the range of an electric vehicle in cold weather can be divided into twocategories: (1) those affecting the on-road energy consumption of the vehicle and (2) those thataffect the energy capacity of the battery. Of the factors that affect on-road energy consumption,the HVAC system electrical load, tire rolling resistance and lubricant viscosity are elements thatcan be influenced from the standpoint of a conversion electric vehicle. Auxiliary electrical loads,

such as windshield wipers and headlights and tail lights, are for the most part fixed parameters.Vehicle aerodynamics are also primarily a fixed parameter, however they can be altered to acertain degree by the installation of air dams and louvers on the vehicle. Increased air density atcolder temperatures and rolling losses due to snow and slush on the road are uncontrollablefactors and affect both electric and conventional vehicle performance adversely. With respect tothe energy capacity of the battery, battery thermal effects at cold temperatures can be enhanced tosome degree in conversion electric vehicles.

1.2 Problem Statement of the ABMTP

Widespread deployment of electric and hybrid-electric vehicles depends on successfullyaddressing both electric vehicle (EV) range and passenger comfort demands in hot and coldtemperature extremes. As seen in NRTMTP, a NiMH battery powered car will retain 65% of its

warm weather range compared to a non-thermally managed lead-acid electric vehicle whichretains only 20% when operated in the same cold environment. This led to assertions that theNiMH battery technology is cold weather resistant and does not need or would not benefit fromthermal management of the battery modules. However, prior EVermont testing had indicated thata conventional ICE vehicle suffers only a 20% decrement in fuel economy and retains nearly 80%of its warm weather range. Experience indicates that these losses, 20% typically, are primarilyrelated to greater lubricant viscosity, higher air density, reduced tire operating temperature androad losses such as snow. However, since the mechanical drive system of both the ICE vehicleand the NiMH vehicle are similar (differential, axles, tires and wheel bearings) one would expectthat an electric vehicle should be capable of achieving 80% of its warm weather range in coldweather. Hence, one of the objectives of ABMTP is to design a battery thermal managementsystem for a NiMH battery powered EV that will perform at the same level as an ICE vehicle

during cold weather operation.

Continuing upon previous experiences with lead-acid battery powered vehicles, an additionalobjective was to determine whether the installation of an APU in a Solectria E-10 pickup couldalso achieve extended EV range and meet cabin heating demands during cold temperatureoperation. This objective would explore the problem of how to heat the cabin with the greatestefficiency and the trade-off in fuel efficiency with battery energy.

.


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1.3 EVermont Vehicles

EVermont continues to deploy numerouselectric vehicles in Vermont. EVermontvehicles have accrued over 150,000miles of in service and testing miles.Figure 2 illustrates the accrued mileagefor each EVermont vehicle since July of1996. Of particular interest are the twoNiMH vehicles, EV13 (with over 21,000miles) and EV15 (8,500 miles) testedunder this program and EVermontshybrid truck, EV1 (with 15,000 miles).The total accumulative mileage for thethree NiMH vehicles is 36,000 milesover the two-year study period.

1.3.1 Project Test Vehicles

This project tested three Solectria Force NiMH vehicles referred to as EV13, EVHQ andEV15, as well as one modified lead-acid Solectria truck referred to as EV1. Thesedesignations are a result of the Vermont license plate numbers for each vehicle.

1.3.1.1 EV13, Solectria Force

EV13 is a 1995 Solectria Force with the following equipment: auxiliary fuel-fired heat, standardelectric heat, non-insulated warm weather battery thermal management, standard preheat, one 220volt charger, light weight air conditioning and Ovonic NiMH batteries.

Since it was delivered in late 1996,EV13 has accrued 21,632 miles (as ofJuly 20, 1999). EV13 is powered by 15

Ovonic NiMH battery modules with anominal voltage of 198 volts and 85Ah.Six of the battery modules are located inthe front battery box while theremaining nine modules are located inthe rear battery box. The total weight ofthe batteries is about 587 pounds (267kg). The pack has a total energycapacity of 17 kWh providing a usefulrange of about 85 miles.

The 85 Ah Ovonic NiMH, battery packin EV13 utilizes a warm weather BTMS

that does not optimize for cold weatheroperation. This means that the battery boxes are fairly open to atmosphere and batterytemperatures remain at ambient when the vehicle is not in operation. This car was used as abaseline NiMH thermal management vehicle, against which the remaining two cars with refinedcold weather thermal management systems were judged.

EV13 maintained roughly 65% of its warm weather range when operated at temperatures below -15C. Data collected from the baseline NiMH powered vehicles indicated that non-thermally

Figure 2: EVermont Vehicle Fleet Mileage

Mileage Chart

0

5000

10000

15000

20000

25000

EV1(EVTTruc

k)

EV2(GMPTruc

k)

EV3

EV5(VYTruc

k)

EV7(EVTCar)

EV9(GMPCar)

EV10(EVTCon

tro

l)

EV11(CVPSCar)

EV12(EVTCon

tro

l)

EV13(NiMHForce

)

EV14(De

lco

Force

)

EV15(98GPForce

)

EV16(96NYPAForce

)

EV17(96PEPCO

Truc

k)

EV18(96PEPCO

Truc

k)

Vehicle

Miles

July-96 November-96 July-97 November-97 July-98 November-98 April-99 Aug-99

EV-7 started with 21,000 miles. All other vehicles represent actual miles.

Aug. data was not available for EV2, EV3, EV5, EV10, EV11, EV12, EV17 and EV18

Figure 3: GM Ovonic 85Ah NiMH Battery Modules


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managed NiMH batteries canprovide near design capacityenergy, even in cold conditions

(-22C), however total batteryvoltage is suppressed (9.2%)and total voltage fluctuation is

increased (40.2%). Within thepack, individual modulesexperience a 30.3% increase inthe difference between thevoltage of the module with thehighest charge vs. that with thelowest. This depression of totalpack voltage resulting from thefluctuations in the state-of-charge of individual cells canbe deleterious to batteryperformance and life. Note that

the battery compartment of the baseline vehicle was designed to reject heat generated by NiMHbatteries operating in warm/hot climates, and no modifications were employed for cold-weatherbattery thermal management.

Technologies and strategies to improve NiMH battery performance should account for the fullrange of temperatures and climates in which these batteries may operate. It is imperative thatresearch be continued to integrate battery thermal management systems into NiMH vehicles,which address a wide range of operating climates. These types of systems will insure thereliability of NiMH batteries as an energy source for EV operation.

1.3.1.2 EVHQ, Solectria Force

EVHQ is a 1997 Solectria Force.

Since it was delivered in February1998, EVHQ has accrued 5,734miles. This vehicle has been testingin Quebec in warm weather undercontrolled route conditions. Forcold weather operation this vehiclewas also tested within a coldchamber at the Laboratoire desTechnologies Electrochimiques etdes Electrotechnologies dHydro-Quebec (LTEE) test lab inShawinigan (Quebec) Canada. A

complete report with respect to thistesting is available as a separate report independent of this document. A summary of theconclusions from the report is provided in later sections of this report.

This vehicle is outfitted with a refined BTMS design, optimized for both warm and cold weather.This new BTMS system includes additional insulation and reduced airflow to aid in the retentionof heat generated by the batteries. This vehicle is equipped with revised battery boxes whichcontain insulation and are also equipped with plugs to partially close off the ventilation holes.This allows for ventilation of the battery pack so that on-road operation does not result in

NiMH Powered Solectria Force

Traction Battery Voltage

Under Two Ambient Conditions

For The Same 65 Mile Test Course

160.0

170.0

180.0

190.0

200.0

210.0

220.0

230.0

240.0

0 500 1000 1500 2000 2500 3000 3500

Time, Seconds X 2

Traction

Battery

Voltage,

Volt

s

Total Volts: 58 F Total Volts: -7 F

Figure 4: NiMH Battery Pack Voltage vs. Temperature

Fi ure 5: EVH GP NiMH Solectria Force


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significant induced ventilation of thebattery pack. For summer operation theplugs can be removed if necessary. Thisvehicle is equipped with cabin preheatand a fuel-fired heater (4,000 watt) butno air conditioning.

EVHQ is powered by 15 Gold Peak GPNiMH battery modules (Ovonic license)with a nominal voltage of 180 volts and90Ah. As with EV13, six of the batterymodules are located in the front batterybox while the remaining nine modulesare located in the rear battery box.

1.3.1.3 EV15, Solectria Force

EV15 is a 1997 Solectria Force. Since it was delivered on November 25, 1998, EV15 hasaccrued 8,466 miles. This vehicle was tested in both cold and warm weather in Vermont. The

battery box design is similar to that of the EVHQ vehicle, however the insulation and the plugsize has been optimized for cold weather operation. This vehicle is equipped with cabin preheatand a fuel-fired heater. Like EVHQ, EV15 is powered by 15 Gold Peak GP NiMH battery

modules (Ovonic license) with anominal voltage of 180 volts and90Ah. As with EV13, six of thebattery modules are located in thefront battery box while theremaining nine modules are locatedin the rear battery box.

In addition to the refined BTMSdesign, the third NiMH vehicle isequipped with a high currentcontroller to increase the torquespeed envelope of the SolectriaForce by 30% for driving on hillyand high-speed roads. As in theother vehicles the controller hadthree forward settings, economy,

normal and power, which allowed the vehicle to be operated both at high power and at a reducedlevel (normal) that will be consistent with the power setting of the first two vehicles.

EV15 was entered by EVermont and Solectria into the 11th annual Northeast Sustainable EnergyAssociations American Tour de Sol in May 1999. The vehicle captured first place overall in

points for the weeklong, real-world performance testing of nearly fifty electric vehicles. EV15scored well for its efficiency, reliability, acceleration, handling and consumer acceptability. Alsothe vehicle traveled 142 miles on a single charge during the tour.

Figure 6: GP NiMH Battery Module

Figure 7: EV15 GP NiMH Solectria Force


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Table 1: NiMH Program Vehicles Summary

NiMH #1, EV13 NiMH #2, EVHQ NiMH #3, EV15

Operator EVermont Hydro Quebec EVermont

Operating Environment Vermont Canada Vermont

Battery Pack 85Ah NiMH, 198 Volt 90Ah NiMH, 180 Volt 90Ah NiMH, 180 Volt

Battery Manufacturer Ovonic Gold Peak Gold Peak

BTMS Open air fan cooled Restricted air flow Restricted air flow

Drive system 42kW, Single speed 42kW, Single speed 55kW, Single Speed

1.3.1.4 EV1, Solectria E10 Pickup Truck

This vehicle is a 1994 Solectria E10 pickup truck, which was purchased from Solectria by anothercompany and subsequently acquired and used by EVermont. This vehicle previously participatedin NRTMTP. This vehicle, operated with lead-acid batteries, was chosen to have an APU

installed to test cabin thermalmanagement and rangeperformance.

This vehicle was originallyequipped with three strings of lead-acid batteries. Each string wascomprised of 12 modules in seriesfor a total operating voltage of 156volts. Two of the battery strings arelocated in battery boxes below therear pickup bed and the third batterystring was located in the engine

compartment. To facilitate theinstallation of the APU the batterybox in the engine compartment wasremoved. While this did not affect

the battery voltage of the vehicle, it did decrease the energy capacity of the vehicle by one third.This vehicle is currently equipped with 24 group 22 gel-cell batteries. Each battery has a nominalvoltage of 13 volts and a nominal capacity of 33.5 Ah when discharged at a C/1 (depleted over aone-hour period) average discharge rate. As the pack is generally depleted in less than one hour acapacity of about 30 Ah is used. This yields a total pack capacity of about 9.4 kWh with twostrings versus a total previous pack capacity of 14 kWh with the three strings.

Figure 8: EVermont EV1 Solectria E-10


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2.0 Battery and APU TechnologyThe United States Advanced Battery Consortium (USABC) identified mid-term,commercialization and long-term goals for advanced batteries, shown in Table 2.

2.1 Advanced Battery Design

The EVermont project team members considered a number of potential advanced batterytechnologies to test and evaluate in cold climates. The project teams investigation into batterytechnologies revealed that given the current state of battery technology and commercialization,the only new near-term battery technologies that were realistic candidates for consideration in thisproject were advanced lead-acid and NiMH batteries. NiCd batteries were considered to alreadybe a proven technology. Table 2 provides an overview of these batteries.

2.1.1 Advanced Nickel Metal-Hydride BatteriesNiMH batteries have been available in consumer electronics for several years, but only recentlyavailable in sizes suitable for electric vehicles. Ovonics Battery Company (OBC) reported thatthe NiMH battery provides three times the energy density, an energy to weight ratio, of a lead-acid battery. Additionally the cycle life is expected to be at least twice that of lead-acid batteries.While the lead-acid battery performance decreases considerably in cold weather, NiMH batteriesare more sensitive to high temperatures.

The primary disadvantages of NiMH batteries are that the batteries typically require an activebattery energy management system and to date they have been produced in limited quantities.Both of these factors contribute to a very high cost for the battery pack ($45,000), a cost highenough to be unattractive for all applications except where performance is at a premium.

However, the California Air Resources Board (CARB) predicts that NiMH batteries will be inproduction quantities (greater than 10,000 batteries per year) by 2003 which may reduce futureproduction costs. Table 4 compares the production modules specifications and performance asprovided from the manufacturers for the two NiMH batteries used in this study.

Table 2: Performance Characteristics for Advanced Batteries

Life Cycle

Specific Energy(Whr/kg)

Specific Power(W/kg)

Cost($/kWhr)

USABC GoalsMid-term 600 80 15 < 150

Commercialization 1,000 150 300 < 150

Long-term 1,000 200 400 < 100

CARB Estimates for 2003NiMH 1,000 90 300 250

Li-Ion 1,000 120 300 300

Li-Poly 1,000 150 315 < 250Source: CARB, 1998 Zero-Emission Vehicle Biennial Program Review, July 1998


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Table 4: Ovonic and GP Battery Production Module Specifications and Performance

NiMH Manufacturer Ovonics GPNominal Voltage (Volts) 13.2 12

Number of Cells 11 10

Nominal Capacity (Ah) 85 90

Nominal Energy (kWh) 1.2 1.08

Specific Energy (Wh/kg) 70 70

Energy Density (Wh/liter) 165 170

Dimensions (mm) 102 x 176 x 409 102 x 186 x 388

Weight (kg) 18.2 17.8

Life Cycle > 600 > 600

Source: GM Ovonics and GP Batteries International

2.2 Auxiliary Power Units

During the first year of the electric vehicle testing in Vermont, it was discovered that heating thepassenger compartment of an electric vehicle was a major obstacle to successful winter operation.Electric resistance heaters, though light and inexpensive, proved to be too large a load on thealready overburdened batteries. Efforts to reduce the quantity of heat required, through the use of

Table 3: Comparison of Advanced Lead-Acid and Nickel Based Batteries

SonnenschienLead-acid

Ovonic BatteryCompany NickelMetal-hydride

Saft AdvancedNickel-Cadmium

Range (miles) 50 100 100

Voltage (Volts) 156 184 / 198 168Capacity (Ah) 50 85 100

(kWh) 7.8 15.6 / 16.8 16.8

Estimated Life (cycles) 400 1,000 2,000

Estimated Life (miles) 20,000 100,000 200,000

Warranty None 3 year 4 year / 25,000 miles

Cost $1,500 $45,000 $14,000

Energy storage(cost/mile)

$0.075 $0.450 $0.070

Data acquisition andcontrol

No Yes (DAQ) No

Maintenance None None (Data reporting) Distilled water - singlepoint every 6,000 miles

Cold weather Battery warming needed Good without warming Very good withoutwarming

Experience inproduction (years)

3-4 1 4-5

Recycling 100% 100% 100%

Source: Solectria Corporation, 1997


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seat heaters, air recirculation and vehicle insulation, were experimented with. While some ofthese methods did improve the effective performance of the electric heaters, window fogging andreduced range persisted. In the end, burning fuel in an efficient heater was chosen as the mostpractical solution to the problem.

Even though we had a working solution to the EV heating problem, there was still a significant

reduction in vehicle range on very cold days. Our project team was compelled with the idea ofcapturing the expansion energy of the heated air in the cabin heater and using it to create asupplemental source of electric energy that could be used to offset this reduction in range. Thuswas born the idea of creating a hybrid vehicle which would burn fuel, primarily for the creationof heat, while at the same time, extend the vehicle range to that expected at warmer temperatures.This would be, by definition, a co-generation project, trading the unlimited continuousextended range that could be obtained by installing a larger auxiliary power unit opting for aunit that would provide just enough heat to the cabin.

2.2.1 Combustion Engines

The burning of fossil fuel (gasoline, diesel, etc.) in an ICE provides significant waste heat energy,which can be used to heat the vehicle, as well as, generate electricity, extending the EV range.

The problems associated with an ICEs use as an APU are the emissions from combustion and thenoise generated by a typical reciprocating ICE.

Most of the prime movers available at the optimal ~5kW size were either carburetor gasolineengines or injected diesel engines. Neither engine would maintain the low emission/noisestandard that an EV offered in pure electric form. Even with further emission reductions,accomplished by equipping the ICE to burn propane or natural gas, the unit would still producethe lawn mower in the trunk effect due to the excessive vibration and exhaust noise inherent tothis type of engine. Nonetheless, a small unit operating at low rpm and located in the bed of apickup truck was assumed to be easily integrated and fairly unobtrusive.

2.2.1.1 Prime Movers

There were several prime movers which, when coupled to an appropriate alternator, wereidentified as being able to provide the desired results. The first engine the team reviewed was a1100 cc BMW motorcycle engine. This powerplant utilized the best fuel management systemcurrently available on IC motorcycle engines, as well as a three way catalytic converter in theexhaust. It is a smooth running four-cylinder water-cooled engine, currently available and couldbe coupled to a generator/alternator. Shortcomings include its relatively large size and weight(which exceeds the 200 lb target), use of gasoline as a fuel and excess power capacity. Thegenerating capacity of this engine is in excess of 30 kW at full power. Fisher ElectricTechnologies indicated that they could couple an efficient, permanent magnet generator to it andpossibly provide a controller. However, the team concluded that this type of engine would be faroversized for the job.

The second possibility was the use of a rotary-type engine. An engine of this type possesses avery good power to weight ratio and generally produces lower emissions than a similar sizereciprocating engine. This units smaller size when coupled to an alternator provides for a verycompact APU.

There are several four stroke gasoline engines available from the small engine manufacturingdivisions of Honda and Kawasaki. The Honda units are generally sold as complete motorgenerator sets providing AC voltage for residential and commercial use. The Solectria pickupoperates at a nominal 144 Volts and is not amenable to this type of generator unless it is used tosupply the onboard battery charger limiting output and incurring additional transfer losses.


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Several conversations with Fisher Electric Technology provided information on a general purposeKawasaki four stroke gasoline engine that Fisher was familiar with. This unit is rated at 20 hp, isfairly compact, readily available and relatively inexpensive (~$1,500 for the engine).

2.2.1.2 Prime Receivers

Fisher Electric Technology in St. Petersburg, Florida, was the prime candidate for a supplier of anefficient DC alternator. This company produces permanent magnet motors and generators andhas experience in the electric vehicle industry. While they offered to create a custom builtalternator to match any prime mover the project team sent them, the team was specificallyinterested in the previously mentioned Kawasaki engine with which they were familiar. Thiscompany also had a working relationship with Moller International and was interested inproducing an alternator for the BETA 2 engine. At the time of the discussion, Fisher hadsupplied more than two dozen alternators for use in hybrid electric vehicles. The advantage to aDC alternator is that it can be tied in directly between the battery controller and the batteriesproviding power to either charge the batteries or supply power directly to the motor.

2.2.2 Integration of APU

Once our goal was established, webegan the process of choosing apower unit, with an ICEdetermined as the best choice.ICE options ranged from an air-cooled Briggs and Stratton to aliquid cooled BMW motorcycleengine. We knew that a liquidcooled engine would be quieterand enable us to efficientlytransfer heat into the vehicle. Wealso knew that it would take about

5,000 watts of waste heat from theengine to heat the vehicle. If half

of the waste heat energy from an ICEgoes out the exhaust, we would needan engine that produced about10,000 watts of total waste heat.This size engine would also produceabout 2,500 watts of mechanicalenergy. Our goal for a successfulcogeneration / hybrid was to heat thevehicle while offsetting the 20% coldweather range reduction. This range

reduction translates into about 1,200watts. We either had to extractadditional waste heat from the APUexhaust or go with a greatergenerating capacity than we reallyneeded.

Figure 9: EVermont EV1 Solectria E-10

Figure 10: Kawasaki FD620D, Fisher A7/28AF


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Investigation of commercially available APUs resulted in the purchase of a Fisher alternatormated to a Kawasaki four-stroke gasoline engine. The Kawasaki FD620D 617cc engine is ratedat 20 hp maximum with 16 hp available at 2700 rpm. This operating rpm corresponds to thelowest specific fuel consumption for this engine. The unit is coupled to a Fisher model A7/28AFbrushless alternator rated at 10 kW (~70 amps) output at 144 volts and 2700 rpm. The totalweight of the system is approximately 130lb (30lb for the alternator, the remainder for the engine,

fluids and connections). It was clear that this power unit was much larger than what we hadinitially set out to install, but the potential for further extending the vehicle range along with thequality and efficiency of the Fisher products enticedus to choose this path. Based on the systemmechanical output approximately 25,000 watts ofuseful heat energy would be available from this unitat full power.

2.2.3 Physical Integration

The physical dimensions and weight of the unitdrove the physical integration of the APU into the

vehicle. We had originally intended to mount theunit along with the fuel tank in the bed of the truck.Because of the need to lift the bed of an E 10 truckin order to access the controllers and batteries, thebed would have required extensive modification.This location also would have required long hoses tothe vehicle heater core, which would have led toexcessive heat loss.

The idea of removing the front batteries andassociated box (the vehicle originally had front andrear battery boxes), and installing the power unitunder the hood emerged as the most sensible

solution. This reduced the battery storage capacityby 1/3 (the vehicle originally had three parallelstrings so voltage remained the same), but if the range reduction became an issue, these batteriescould be re-installed in the rear of the vehicle. The APU would now have to overcome a 47%

loss to return winter range to summercapacity. Taking into account theremoval of the front twelve batteriesand their enclosure, the installationof this power unit has created a totalvehicle weight reduction of over 100pounds.

The APU installation in the enginecompartment of the truck was fairlystraightforward. A steel frame wasfabricated which was mounted to thevehicle frame via rubber mounts. Tothis frame was mounted the powerunit and the radiator assembly. Thebelt driven cooling fan was replacedwith an electric unit and a dual

Figure 11: EV1 Lift Bed

Figure 12: EV1 Rear Battery Box


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thermostat assembly was created. The heater core is supplied with hot water by the opening ofthe first thermostat at 180F. The second thermostat controls flow to the radiator and opens at195F. The engine contains a mechanical coolant circulation pump that is capable of providingan adequate flow to the heater core. No heat regulation valves are presently installed in thissystem but one may be added in the hose to the heater core for summer operation.

Included with the Kawasaki engine is a built in alternator that has been coupled to the vehicles12-volt system to provide redundancy for the DC to DC converter. In order to utilize this layout,a diode was installed on the output side of the DC to DC converter to protect it from any voltage

fluctuations that might be producedby the alternator. The factoryinstalled starter motor is used tocrank the Kawasaki engine and asmall battery has been added to the12-volt system in order to satisfythe surge of current required by thestarter.

An automatic (electric) fuel

enrichment device has beeninstalled on the APU carburetor tofacilitate cold starts. A 15-gallonfuel tank has been mounted underthe cab, between the frame rails(transmission tunnel), and its fillerpipe runs up to the right front inner

fender well. The hood must be opened in order to add fuel but this was determined to be anacceptable compromise. A charcoal canister has been installed in order to absorb fuel tankvapors. This canister has a simple evacuation system, which consists of a small hose connectedto the intake manifold of the engine. We found that a solenoid control system was not necessaryfor acceptable idle quality because of the small size of this hose and the moderate rpm operation

of the APU.The engine possesses an internalflyweight type throttle control thatis presently attached to the vehicleaccelerator pedal. This setupincreases alternator output asincreased current flow to the drivemotors is called for. The exhaustsystem begins with a header pipegoing down to a section offlexible pipe with a connectionflange on the bottom. From there,the exhaust passes through acatalytic converter and twomufflers before leaving thevehicle at the rear bumper. Theunits ignition switch is mountedon the center console and is run in series with the vehicle ignition switch. The starter motor isthen engaged by turning the ignition switch to the crank position.

Figure 13: APU Installed in Engine Compartment

Figure 14: Fisher AC/DC Converter


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APU instrumentation in the instrument cluster includes fuel level, oil pressure, water temperatureand DC volts (12-volt system). Solectria instruments located in the center console includebattery pack volts, current and a state of charge (Ah) meter. Additional instruments in the centerconsole include power unit output current, Fisher alternator and vehicle battery temperatures,manifold vacuum, hour meter and a tachometer. The Solectria installed electric power steeringand brake vacuum pump has been retained along with an Espar kerosene burning air heater. The

General Motors anti lock brake system was disabled during Solectria conversion of the vehicle.Removing this unit saved 23 pounds and freed up valuable space under the hood.

The electric interface between the Solectria drive system and the APU is between the batteryconnection to the motor controllers via a three-phase rectifier that converts the alternating APUoutput to DC current. This means that any power produced by the alternator will flow directlyinto the controllers or the batteries depending on which has the lowest potential. Initial testingindicated that a problem could occur with this set up that results from the APU being operatedwhile generating current via regenerative braking with the batteries fully charged. This produces

excessively high voltage and thecontrollers drop off line. Cycling thedrive selector switch through the off

position will reset the controllers butthis situation needs to be avoided.We have come up with three ways toprevent this occurrence. First, do notrun the APU with the batteries fullycharged. This should always beobserved so as to avoid overchargingbut precludes the option of warmingthe cabin on a cold morning.Second, operate the electric heaterwhile the engine is running. Thiswill absorb excess energy andprevent over-voltage. Third, switchoff the regenerative brakes.

Two other problems associated with this installation were vibration and noise. These werepredictable and by no means a surprise. Both have been addressed and significant improvementshave been made. The power steering assembly was originally attached to the engine-mountingframe. The location and mass of this assembly produced a harmonic vibration that wasunacceptable. Relocating this from the frame to the vehicle chassis eliminated this portion of theproblem. Modifications to the engine mounts were also necessary to further reduce vibrationsbeing transmitted into the frame of the vehicle by the APU itself. The exhaust system wasoriginally assembled with one muffler. The sound level emanating from this system wasexcessive. An extension was added to the pipe and a second muffler was hung on the outside ofthe frame rail near the rear of the bed. This effectively reduced the exhaust noise to an acceptable

level.

Figure 15: Completion of APU Installation


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3.0 Battery Testing

3.1 Bench Testing of Ovonics NiMH Batteries

NiMH battery capacity is significantly less susceptible to low temperatures than lead-acid

batteries. Lead-acid batteries typically suffer about a 50% reduction in discharge capacitywithdrawn at a C/3 rate when battery temperatures drop below freezing. NiMH batteries on the

other hand can deliver 80% or more of their rated capacity at 0F. Below 0F increased internalresistance of the NiMH batteries does result in more significant decreases in capacity. NiMHbatteries are however more susceptible to high operating temperatures than their lead-acidcounterparts and as such any thermal management system which provides insulation to thebatteries for extreme cold weather operation must compensate with additional coolingcapabilities. It is estimated that repeated operation of NiMH batteries in excess of 40C (104F)will result in reduced cycle life, about 60% of specification.

3.2 Laboratory Bench Testing at the University of Massachusetts, Lowell, MA

A 15-module pack of Ovonic NiMH electric vehicle batteries was purchased and evaluated at theUMASS Lowell battery evaluation laboratory. A total of 30 cycle tests were performed of eachindividual module. Moderate rate 20 amp cycles were performed at 20, 0, 20 and 40 degreescentigrade. Module voltage, current, impedance and temperature were measured at one-secondintervals and average values were stored as time series files at three-minute intervals. Batteryperformance characteristics such as capacity, round trip efficiency, energy density, power densityand heat dissipation were derived from the data. High rate 80 amp cycles were performed at 20and 0 degrees centigrade. A final experiment was a 15-module pack test.

The battery pack delivered toUMASS Lowell consisted of 15, 11-cell modules with nominal ratings of13.2 volts and 85 Ah as provided by

the manufacturer. The 11-cellNiMH module has roughly the sameoperating voltage range as a 12-voltlead-acid battery module comprised

of 6 cells. At 20C the averagecapacity value was approximately87.75 Ah at a 20-amp discharge rate(this is approximately C/4). Thiscapacity correlates well to the 89Ahaverage determined by Ovonicutilizing a C/3 rate. On a kWh basisthe batteries had an average round

trip efficiency of 83.5%.

Average Battery Capacity

0

20

40

60

80

100

120

-20C 0C 20C 40C

Degrees Centrigrade

Ah

CC discharge Ah output Ah input

Figure 16: Average Capacity for Ovonic NiMH Battery


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Figures 16, 17, 18 and 19 showthat the impedance and voltage

were both optimized at 20C,however these parameters were

most impacted at -20C.Discharge voltage was depressed

at -20C and further depressed athigher discharge currents.Generally speaking OvonicNiMH batteries exhibited a lossof specific power (W/kg) at coldtemperatures, however, specificenergy (Wh/kg) capacity fellonly a small amount. On theother hand at high temperatures

specific power was retained, however,total energy capacity began to fall off.

This was in part due to higher self-

discharge rates (1.5% per day at 20C) athigher temperatures. The results obtainedat UMASS Lowell agreed well with theinformation provided by OBC. Thetesting identified an optimum operating

temperature range of 0C to 30C for theOvonic NiMH batteries.

Above 40C actual cycle life begins to fall

off and above 60

C a failure is possible.Above 30C energy capacity began to falloff to less than acceptable levels and

below -10C specific power fell off farenough to indicate vehicle performance

could be hampered.

In summary, at temperatures above

30C the NiMH batteries exhibit adrop in total Ah capacity anddischarge voltage that results in aloss of kWh capacity and reduced

range. At temperatures below 0C

the batteries generally maintain theirAh capacity but voltage is againdepressed resulting in a loss of kWhcapacity and reduced range.Optimum power and capacity aremaintained between approximately

0C and 30C. Self-discharge isrelatively high at 1.5% per day at

Discharge Voltage

12.6

12.8

13

13.2

13.4

13.6

13.8

14

-20C 0C 20C 40C

Degrees Centigrade

Volts

Voltage

Figure 19: Ovonic Battery Average Discharge Voltage

Figure 17: Round Trip Efficiency for Ovonic NiMH Batteries

Round Trip Efficiency

0

20

40

60

80

100

-20C 0C 20C 40C

Degrees Centigrade

EfficiencyPercent

Efficiency

Figure 18: AC Impedance of Ovonic NiMH Batteries

AC Impedance

6

6.5

7

7.5

8

8.5

-20C 0C 20C 40C

Degrees Centigrade

Milliohms

Impedance


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20C, which increases dramatically with battery operating temperature.

A complete report of the results from the ULowell testing is available as a separate report. Thereport is titled Characterization of Ovonic Nickel Metal Hydride Electric Vehicle Batteries,authored by Dr. Ziyad M. Salameh and Dr. William A. Lynch and completed August 1998. Thereport is 284 pages in length including all appendices.

3.3 Battery Box Design

Because NiMH battery technology is generally exothermic, a conservation of energy approachhas been taken in the application of passive technologies to retain heat with the batterycompartments. These design modifications have been applied to maintain the batteries above theambient temperature without the addition of an active heat source. The conservation technologiesemployed have been battery compartment insulation and reduction of battery box ventilationthrough the application of proof of concept flow restrictors and ventilation system flappervalves. The addition of the insulation reduces conductive losses from the system, whereas themodifications to the ventilation systems are designed to reduce convective losses.

Solectria considers the exact specification of the battery box for the Solectria Force confidential,

however an overview of the design strategy is contained here. Each battery box wasmanufactured in aluminum to conserve weight. Ventilation holes were cut into the bottom of thebattery box to allow for vertical ventilation and drainage should water enter the box. A plasticweather shield was located about one inch below each box to limit the amount of water intrusion.The battery box is lined with rigid insulation. The NiMH batteries were installed in plasticsupport frames that act as both insulators and spacers. Ventilation fans are located in the top ofthe box to draw cooling air through the pack. Round plugs with smaller holes are used to limitcooling airflow during winter operation. These plugs are easily removed for summer operation.

The results of prior testing from EVermonts NRTMTP project on different battery insulations,concluded that the choice of foam board alone would provide good heat retention during vehicleoperation.

In EV13 the battery boxes were left somewhat open, employing openings in the bottom of thebattery boxes, which allowed intrinsic cooling of the batteries from airflow induced when thevehicle was in motion. Maximum heat exchange was achieved by narrowing the spacing betweenthe battery modules such that the velocity of the cooling air was increased.

Battery box modifications were pursued in the second project vehicle, EVHQ, to maintain thebatteries at optimal temperature regardless of ambient temperature, design modifications werealso made to the motor controller to overcome some of the side effects associated with theinstability in battery operation. NiMH batteries experience a voltage sag under high currentload. This instability was exacerbated at cold temperature to the point where system controls toprotect the controller from over current would come into play and disable controller operation.

A microprocessor controlled data acquisition system (DAQ) monitored the temperatures andvoltages of the battery modules to implement control strategy algorithms to maintain the batteriesin the front and rear compartments at uniform and optimal temperatures. The voltages are used assafety limits to prevent the batteries from being overcharged or over discharged. Temperaturesensors, two in each box, were monitored which the DAQ in turn uses to controls fans. The fansdraw air through the front and rear battery boxes to keep the two boxes in equilibrium with eachother by cooling the warmer of the two packs. The boxes themselves were designed internally toequalize the fans cooling effect on each battery. The DAQ also monitored the overall averagetemperature in the boxes and cooled the entire pack as appropriate.


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3.4 NiMH Finite Element Model

In addition to the areas of modifications (insulation and air restriction) initiated by SolectriaCorporation on the battery boxes, there are a number of other governing parameters that influencethe performance of the system. The construction of different prototypes corresponding to variousconfigurations in order to test them for a design optimization purpose is a costly undertaking. Analternative to this kind of analysis is numerical modeling, which offers better flexibility and lowercosts compared to prototype testing.

The NiMH Finite Element Model focuses on the thermal modeling of the system rather than thestudy of parameter effects. The model is able to predict the velocity and temperature distributionsinside both battery boxes (front and rear).

3.4.1 Results - Hot Case

The hot case corresponds theoretically to an ambient temperature of +20C. In reality, the

ambient temperature depends on time as temperature increased to +23C by the end of thesimulation. The comparison of the predicted temperature evolution with the experimental datacollected (from the sensor located on module #2 in the front battery box) is shown in Figure 20.Close agreement is found between numerical and experimental results regardless the state of thefans. In fact, the model accurately predicts the temperature profile of the box. The averagerelative error is approximately 0.2% based on the Kelvin absolute temperature scale.Examination of the results for the rear box shows that the predictions are also in accordance withthe experimental data. The average error is 0.19% for the left side (module #11) and 0.12% forthe right side (module #14). The highest value of the absolute error in prediction is approximately1.5C.

Figure 20: Temperature Evolution in the Front Box (terminal #2)

20

22

24

26

28

30

32

34

0 600 1200 1800 2400 3000 3600

Time [sec.]

Tempera

ture

[oC]

Monitoring

Model

F a n s O n

F a n s O f f

L a m i n a r f r e e

c o n v e c t i o n

T u r b u l e n t c o m b i n e d

f r e e a n d f o r c e d

c o n v e c t i o n


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The results demonstrate that the simulation model describes with satisfactory accuracy thethermal behavior of the modules for the hot case even though the insulation is ignored in thesimulation. This can be explained by the fact that when the fans are running, the insulation doesnot play a significant role, because the fans extract the heat generated by the modules from theboxes and the air is continuously renewed. In contrast, the effect of the insulation is expected tobe strong if the fans are inactivated (Off position). However, the results show that there is no

effect in this case. This result may be attributed to the fact that the fans were not running for thefirst 1,977 seconds (~33 minutes).

Examination of the results concerning the front box reveal that the hottest region in the box islocated in the core of module #2 (and module #5 by symmetry). Even though the fans are

running, the corresponding temperature is as high as 303 K (30C). It is interesting to mentionhere that the temperature at the location of the sensor is lower than that at the center of themodule; the difference is approximately 2C.

In terms of the velocity distribution, the air movement underneath the modules seems to be quasiuniform because of the existence of the plenum. Similarly, the flow between the modules is quitehomogeneous except for the air space at the front face of the box (between module #3 and thewall) where the velocity is less important than elsewhere. On the other hand, the mass of air on

the top of the module #3 seems to be still. This is attributed to the vortex (flow recirculation)created by the ascendant and descendant flows. This phenomenon is not observed for the othermodules since they are closer to the fan and the vortex cannot occur.

In the rear box, the hottest modules are those located at the center (#10, #11, #12 and #14) and themost critical one seems to be the last one, module #14. This is a result of poor air circulation onthe backside of this specific module. In this particular region, the fan is relatively far from themodule, which favors free convection forces to drive the cooling air by density or temperaturedifferences. It should be mentioned here that the heat transfer deteriorates when the convectionmechanism changes from forced to free. Consequently, the temperature rises rapidly as the heatremoval decreases.

3.4.2 Results - Cold CaseThe cold case corresponds theoretically to an ambient temperature near -20C. In reality, theambient temperature depends on time as increased to 16.6C by the end of the simulation.

The comparison between the numerical predictions and the test data are presented for the frontbox in Figure 21. As shown, the modeled temperature diverged from the actual temperature overtime by an increasing margin for the front battery box. This same result also occurred for the rearbattery box. There are several variables that could result in the discrepancy between themonitored and modeled results, such as insulation and airflow. The effect of insulation on thetemperature evolution was investigated for both boxes.

Insulation of 10 mm thickness was added on each side of the box. The results of the insulation onthe temperature evolution in the front box are shown in Figure 22. It should be noted here that

similar simulations were performed on the rear box, but the results are not presented since theydrive to the same conclusion: the insulation does not have a great impact on the temperatureprediction. Indeed, the insulation decreases the temperature difference between the model andthe experimental data by approximately 1C.

The second suspected parameter is the mass flow induced by free convection forces, as the fansdid not function in the cold case. In reality, the flow at the inlets cannot be imposed since itdepends on the order of magnitude of the free convection forces. In other words, the larger thetemperature differences between the outlet and the inlet, the greater is the mass flow. Hence, as


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Figure 21: Temperature Evolution in the Front Box (cold case)

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

0 600 1200 1800 2400 3000 3600

Time [sec.]

Tempera

ture

[oC]

Monitoring

Model

Figure 22: Effects of the Insulation and the Plugs on the Temperature Evolution in the Front Box

Model 1: No insulation, no plugs

Model 2: No insulation, with plugs

Model 3: With insulation, with plugs

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

0 600 1200 1800 2400 3000 3600

Time [sec.]

oC]

Monitoring

Model 1

Model 2

Model 3


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the box temperature increases with time, the mass flow of air becomes more important. From themodeling standpoint, it is more realistic to compute this quantity than to impose it. Nevertheless,a negligible value representing the air flow through the inlets was imposed when the fans werenot functioning to keep the model relatively simple. It is difficult to determine the exact amountof air crossing the box to impose it as a boundary condition at the inlets.

The option corresponding to the imposed mass flow was chosen in the present study to minimizethe complexity of the model and consequently the cost of the study. Now the question is: doesthe value of air mass flow imposed represent the reality? The issue does not arise when the fansare functioning because the driven flow is known (mass flow of the fans). When the fans are notfunctioning, the No plugs situation for the same test parameters was used as a reference casefor the study of the mass flow effect.

The results reported in Figure 22 reveal that the air mass flow has no effect on the temperatureevolution. In order to confirm the modeled prediction, a comparison between the data collectedas temperature evolution in the front box for the No plugs and the With plugs situations is

displayed on Figure 23. The original curves show a constant difference of 3.75C in average. Thedifference is mainly due to the temperature offset at the initial time. The original curvecorresponding to the With plugs situation has been adjusted by subtracting the average

difference (3.75C), and the resulting curve reveals that the difference collapses. The slightremaining difference may be attributed to the range of precision of the sensors since the

maximum difference is approximately 1C. The same trend was observed for the rear box.Therefore, we can state with confidence that the air mass flow has no effect on the temperatureevolution when the fans are not operating during the tests corresponding to the cold case.Consequently, the air mass flow imposed in the model is credible since it has no impact on theaccuracy of solution according to the modeled results and the experimental data.

Figure 23: Comparison between the Case with and without Plugs for the Front Box

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

0 600 1200 1800 2400 3000 3600

Time [sec.]

T

empera

tureevo

lution

[oC]

No plugs-monitored

With plugs-monitered

With plugs-adjusted


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The isotherm contour results show that the temperature distribution in this case is quite differentin shape compared to the hot case. Contrary to the later case where the fans start working afterapproximately 33 minutes of running, the fans stay off during the entire simulation. Therefore,the flow of the air is induced by ascending forces associated with free convection as explainedbefore. In this situation, the velocities involved are too low to generate a significant flow, whichwould reject the heat generated by the batteries outside the box. As a result, the heat is trapped

inside and a relative uniform temperature distribution is observed through the batteries. A reversetrend is observed in the mass of air located on the top of the modules. In fact, the isothermcontour at this location is characterized by many regions presenting different temperatures (the

difference could be as high as 4C). This phenomenon is attributed to the nature of the air flow.As a consequence of free convection, the flow is disturbed and is characterized by manyrecirculating zones. The velocity reflects the complexity and the disturbance of the flow on thetop of the batteries. This observation may be a key to understanding the disagreement betweenthe model and the monitored data for the cold case. In fact, in this case the geometry details andlocation of the heat generation within the battery could change the entire velocity field andconsequently, the thermal field as the two fields are coupled. This problem was not observed forthe hot case as the flow is driven mechanically. Hence, the flow is less disturbed and thetemperature distribution through the region in question is relatively uniform. Furthermore, when

the fans are functioning the heat transfer is due to forced convection. In this case, the temperaturedoes not interact with the velocity. These reasons could explain why the models accuracy isbetter for the hot case.


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4.0 Battery Thermal Management System

4.1 BTMS Design

A battery thermal management system (BTMS) for NiMH batteries was initially developed by

Solectria and Ovonics to thermally manage the dual battery compartments of a Solectria Force.The system performed two major tasks: fan cooling of the batteries to dissipate heat generatedduring use, charging and self discharge, and balancing the temperatures in the front and rearboxes with each other, again by fan cooling the warmer of the two packs. No active heating wasincluded in the design as NiMH batteries were perceived as cold weather resistant. All heatingwas provided by the charging and discharging of the batteries.

In light of a design objective to maximize the energy capacity and power output of the NiMHbattery pack in cold weather, Solectria designed several new improvements to the NiMH BTMSwhich was baselined using EV13. EVermonts goal was to determine the reduction of batterymanagement power loads and maximization of available battery capacity by selectively insulatingand reducing airflow through the battery boxes in cold weather. These reduced power BTMScars, EVHQ and EV15 were tested side by side with EV13 for comparison. Areas that the project

observed to determine successful operation of the new design included: less frequent use of thecooling fans (i.e., less power consumption), better balancing of the battery temperatures, bettertemperature balance between the front and rear battery boxes, higher overall battery temperaturesin cold weather (below the minimum fan operating points) and no battery overheating.

The Solectria BTMS utilizes a combination of rigid insulation and an air gap to limit conductiveheat loss to the ambient air and maintain uniform battery temperatures within the battery box.When necessary, battery cooling is provided by drawing air over the batteries using a smallblower. Ambient air is drawn in, flows between the batteries and is then exhausted. If there is atemperature differential between the front and rear battery boxes the cooling fans are used tobring the warmer of the two boxes down to the temperature of the other box. This design basiscontrols battery temperature by all but eliminating conductive losses (insulation) and utilizing airto control battery temperature by convection.

The existing system relied on both convection and conduction to remove heat from the batteries.The new system design relied more heavily on convection. By insulating the battery boxes with0.25 to 1 of insulation, conduction heat losses were reduced. The primary method for heatremoval from the batteries was the operation of the fans, drawing ambient air between thebatteries. The main difference was that the batteries would retain heat from their exothermicchemical reactions much more effectively while the fans were off, increasing their overalloperating temperature in cold weather. The fans would still have the ability to fully cool thebatteries, but the batteries retained more heat in cold weather.

The battery thermal characterization showed that a tight hysteretic control loop using one of thebattery terminals for the feedback point was the best control algorithm. Solectria programmedtwo advanced BC1600 chargers with this algorithm and sent them to EVermont for installation intwo of the cars. They have been in use, and anecdotal observations showed that energy useduring charging had been reduced as an unexpected beneficial side effect.

4.2 BTMS Design Testing Cold Chamber

Following the on-road field study during the winter of 1996-1997, the EVHQ car was equippedwith design modifications to minimize the effect of the cold on the NiMH batteries. Themodifications were passive in nature and included better insulation of the battery compartmentsand the use of airflow restrictors in the compartments ventilation system. In order to validate the


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efficiency of the design modifications that were undertaken, Jean-Franois Morneau of Hydro-QuebecsLaboratoire des Technologies lectrochimiques et des lectrotechnologies (LTEE) inCanada was contracted to perform a series of controlled cold chamber NiMH vehicle tests. TheLTEE Report for this testing, Evaluation of Cold Temperature Performance of a NiMH batteryPowered E.V. is available as a separate document.

The EVHQ was installed in the coldchamber and a load bank consistingof eight resistors was used to applya load profile to the batteries,simulating real driving conditions.The tests were made with twoconfigurations (full flow throughventilation system and restrictedflow conditions), at twotemperatures (-20C and 20C).Each combination of temperatureand configuration were performed

three times, making a total of twelvetests.

The results obtained from these testsare very interesting and show thatthe modifications are efficient in

maintaining battery performance in cold weather operation. After one test cycle, it was foundthat temperature tended to equilibrate to a temperature over 20C (for the conditions used in thetests), and consequently the battery voltage response came very close to the ones observed inwarm weather conditions. Also, the standard deviation of the highest and the lowest individualvoltages was much lower than observed during the winter field study conducted by EVermont:0.86V and 0.95V compared to an average of0.55V. With the BTMS modification, the results

obtained tend to demonstrate that the designmodification would be efficient for the regularuse of the vehicle.

Some care has to be taken in the analysis and thegeneralization of the results obtained in thesetests. The results are directly dependent on thetest conditions that were used. For instance, iflonger test cycles or greater wind speed had beenused, the results obtained would certainly bedifferent. However, the same tendencies wouldhave been observed (augmentation of meanbattery temperatures and voltages). Theimportant thing to remember from these tests, isthat the heat generation of NiMH batteries is sogreat, that bringing the heat exchange betweenthe battery compartments and the ambientenvironment to the minimum, will contribute tomaintain battery performance. NiMH batterieswould then have a big advantage over theircompetitors since no other thermal management

Figure 25: Resistive Load Bank

Figure 24: LTEE Cold Chamber


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techniques, like the use of electric heaters, would be necessary. This passive type of thermalbattery management is ideal since it uses wasted energy to maintain battery performance.

The efficiency of the two modifications is however directly dependent on how the vehicle is used.In order to maintain temperature within the battery compartments at an optimum level, the EVhas to be used on a regular basis.

Another factor that can influence the efficiency of the design modifications is the effect of thewind. When operating in cold weather, care must be taken to restrict the airflow through thebattery compartment as best as possible and during extreme cold weather, the flow should becompletely blocked. To ensure that flow conditions are optimum for all weather conditions,automatic variable flow restrictors should be used. This would also prevent damage to thebatteries if flow restrictor plugs are left in place during extreme hot temperatures.

Based on the results obtained with the tests presented here, it is recommended that themodifications performed on the Solectria (more insulation of the battery compartments and use ofair flow restrictors) be implemented for better NiMH battery management. Additionally, evenmore improvement may be possible in cold weather and for safety precautions for the operationin hot weather, with airflow restrictors that operate in response to the operating ambienttemperature.

4.3 BTMS Design Testing Warm Weather

Due to the nature of the modifications made to EVHQ, it was also important to study the thermalresponse of the system when operating in warm weather conditions. Normally, for a non-thermally managed Solectria Force with NiMH, forced ventilation of the battery compartments isnecessary in warm temperature, in order to prevent batteries from overheating. There was apossibility that with the addition of insulation, the forced ventilation might be insufficient. Thebatteries could then overheat, which would result in permanent battery damage.

The objective of the on-road tests was to evaluate the warm weather performance of a SolectriaForce modified for cold weather operation. These tests were necessary to complement on-roadwinter testing and controlled laboratory testing to demonstrate the performance and the viabilityof the system over a wide range of real world operating conditions. Summer testing was done inShawinigan, Qubec, at Hydro-QubecsLaboratoire des Technologies lectrochimiques et deslectrotechnologies (LTEE), on the same car used in Vermont for winter testing. The coursedesigned for the tests presents characteristics similar to the one used by EVermont. This coursewas run seven times, on seven different days (ambient temperatures ranging from 23.15C to28.99C). The LTEE Report for this testing, On-Road Summer Testing of NiMH BatteryPowered E.V. is available as a separate document.

4.3.1 Test Description

This project had the objective to conduct on-road testing in warm weather conditions of awinterized EV equipped with NiMH batteries. The warm weather performance will be

compared to the performance obtained during winter testing. To ensure that the results from thetwo test series can be compared, great care must be taken to ensure that winter test conditions arereproduced as best as possible. The parameters that must be reproduced are the ones not directlyrelated to the weather, which are the driving conditions and the test course.

During the tests, all usual parameters were recorded by the CR10X data acquisition systeminstalled in the vehicle by EVermont. This system records all electric parameters (batteryVoltages, current and temperatures, ambient temperature, Ah, kWh). The tests were done with

the vehicle speed selector set to power mode and the vehicle speed held within 10 km/h of the


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speed limit at all time. A total of seven runs was done, on seven different days, from August 19,1998 to August 31, 1998.

4.3.1.1 Test Course

For comparison purposes, the course used for the tests had to present similar characteristics as the

one used in Vermont. The developed course is 32.7 km long, which is approximately the samelength of EVermonts course (32.64 km). For each test, this course was run three times, for atotal trip of around 98.1 km., Figure 26 presents the test course in light green.

The test course starts at LTEE facility on avenue de la Montagne, in Shawinigan, in a 50 km/hzone. The first stop sign is encountered at about 1.2 km from the starting point. The course turnsleft at this intersection on to Garnier, toward the city of Shawinigan. Another stop sign isencountered and the road then goes downhill and uphill under Highway 55. The first stop light isencountered at 3 km. At this point, the course enters an urban zone for about 2 km (2 stop signsand 2 traffic lights). A right turn is made to exit the city limits at 5 km, where a 6% downhillgrade is encountered before arriving to Baie-de-Shawinigan at 6.3 km. The course then followsShawinigans bay, where the speed limit increases to 90 km/h at 7.1 km. An 8% grade hill is thenencountered from 7.9 km to 9.3 km, where highway 55 is taken (speed limit of 100 km/h).

The course follows highway 55 until it reaches the 17th km. During the highway portion of thecourse, between exits 211 and 220, the road goes mainly uphill, with a significant downhill-uphillcombination when passing over Garnier Street. The highway is left at exit 220 and a left turn ismade, heading north, passing over the highway and entering a rural road for 6.9 km, towards St-Grard-des-Laurentides village. The first part of this course portion is in relatively flat terrain,with a speed limit of 70 km/h. After a left turn on 103rd street, at 21.1 km, the speed limit

decreases to 50 km/h for a fewhundred meters and then increases to80 km/h. At this point, the coursecontinues with a combination ofslight up and downhill elements,until turning left at a stop sign andentering the St-Grard-des-Laurentides village at 24.5 km fromthe beginning (50 km/h).

After going through the village, aleft turn is made on to route 351,heading south, in Shawinigansdirection for about 6.2 km. Road351 presents relatively flat rollingconditions, with a speed limit of 80km/h. When re-enteringShawinigans city limits, the speed

limit is reduced to 50 km/h and astop is encountered (30.7 km). Aleft turn is made back on to avenuede la Montagne, towards LTEE forthe end of the course (32.7 km).

Figure 26: EVHQ Warm Weather Test Course


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4.3.1.2 Summer Testing Results

The testing results show that for the conditions of the six tests, the temperature did not reachcritical levels. The highest temperature observed in the battery compartments was 47.5C, whichis relatively high but not dangerous. This temperature was maintained only for a short period oftime, during a harder part of the course. On a lower stress part of the course, the ventilation

system was enough to cool down the battery compartments. However, the general trend of thebattery temperature is upwards. As a general concept, the temperature in the battery compartmentis a balance between the heat generated by the batteries and the heat loss of the batterycompartments. A temperature elevation means that all the generated heat cannot be evacuatedfrom the compartment. This shows that for heavy, and maybe even medium duty cycles, atambient temperatures of over 20C, the ventilation system might not be sufficient to keep thebatteries cool, which could then lead to overheating.

In order to prevent any overheating damage in any type of conditions of vehicle utilization, itwould be necessary to modify the ventilation system or the configuration of the batterycompartments. A better airflow between the battery cells or higher volume of air passing throughthe compartments would help to evacuate the excess generated heat. However, with the current

configuration of the system, the driver should be aware of the potential overheating danger whenoperating the vehicle in more stressful driving conditions (weather and type of road). A possiblesolution for this problem is a warning light that alerts the driver when battery temperaturereducing measures are necessary. In these cases a more conservative way of driving is required(operation in normal or economy mode) in order to keep the heat generation as low as possible, toprevent battery damage.


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5.0 Vehicle On Road Test EvaluationsTwo intensive vehicle evaluation campaigns were undertaken on EV13 and other EVermontvehicles. One in warm weather to baseline the vehicles and the other in extreme cold weather toevaluate system performance. During these periods the vehicles were fitted with appropriate

electronic sensors to measure energy use and temperatures of the vehicles, components andambient conditions. This data washigh resolution time series data,collected and stored via theCampbell CR 10 data logger.

The vehicles involved in thesetests were also driven on a dailybasis as a way of collectinganecdotal performanceinformation and, when necessary,to gather additional data toreinforce the tests preformed in

this project.

Under the guidance of EVermont,the data was handled by VermontMonitoring Cooperative (VMC).VMC, under the auspices of the Vermont Department of Forest, Parks and Recreation, maintainslong-term environmental monitoring data sets, stores data in a data management system and has astaff dedicated to maintaining and analyzing environmental data.

5.1 Data Acquisition System

The Solectria Force is a four-door conversion electric vehicle based on a Chevrolet (Geo) Metro.It is powered by a Solectria UMOC 440 controller coupled to an AT1200 transmission with an

ACgtx20 AC induction motor. Through a Power Selector Switch, the driver controls this fixedreduction drive system. The Drive Selector has three positions: economy, normal and power. Thesystem offers regenerative braking for capture of braking energy. The drive system componentsweigh 58 Kg, or only 5.5% of the total weight of the vehicle.

Battery management is accomplishedwith a proprietary battery managementsystem, DAQ, developed by Solectria.This system monitors individual batteryvoltages at all times and preventsindividual batteries from being over-discharged or over-charged.

DAQ also provides thermal managementcontrol for the battery pack. Its maingoal is to maintain temperatureuniformity between batterycompartments as well as to keeptemperatures within optimal operatingrange. DAQ has various algorithmsallowing proper thermal-managementcontrol under a variety of conditions,

Figure 27: EV15, EV13, EV1, EVHQ and Richard Watts

Figure 28: Solectria Drive Selector Switch


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depending on the ambient environment.

5.2 EVermont On-Road Field Test Course

EVermont designed a 20.4-mile (32.8 km) road course establishing a structured setting to collectdata. The test course was used to assess and compare vehicle performance on a single real-world

driving route. Reducing the number of variables between vehicles (day, time, temperature, roadsand driving conditions) enabled legitimate comparisons to be drawn among vehicles. At the sametime, data was collected from operating on-road, in real-life situations throughout the projectsperiod. The initial test course evaluations were held in February 1997 to collect cold weatherdata and in July 1997 to provide comparison points during warmer weather. Additional warmweather testing was performed in July and August 1999.

The 20.4-mile test course, shown in Figure 29, begins in Middlesex, Vermont at the VermontGeneral Services Division (GSD) facility on U.S. Route 2. Out of the GSD facility the coursefollows Route 2 East to Montpelier. This section of the course consists of seven miles of rollingrural secondary two-lane road. Exiting from the GSD facility, the vehicle encounters an uphillsection of roadway with a posted speed of 50 MPH. This uphill continues for 0.3 miles, at whichtime the vehicle descends the hill as Route 2 travels through Middlesex Village where the speed

limit is reduced to 35 MPH and mile 1 of the course is completed. Once through the Village, thespeed limit returns to 50 MPH, with Route 2 continuing east. The road generally follows theWinooski River, heading upstream, and consists of rolling terrain.

Between miles 2 and 3, Route 2 crosses over Interstate 89. At about mile 5 the speed limit isreduced to 35 MPH and then to 25 MPH at about mile 6. The reduced speed limits signify theentry into Montpelier. At mile 7, a traffic light is encountered, and course conditions changefrom rural secondary roads to urban traffic conditions. This leg through the center of Montpelieris approximately 2 miles in length, and includes two additional traffic lights. The course rejoins

Route 2 east briefly, and by mile 9 is on Hill Street in Montpelier. From the base of Hill Street inMontpelier to where it plateaus in Berlin is a 12% grade. This 0.7-mile section of road has asurface that starts out as asphalt in need of repair and then changes to gravel. The coursecontinues on the gravel surfaced road with a down and uphill section while bearing left ontoStewart Road, but overall climbing in elevation.

Figure 29: EVermont Test Course


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At mile 10 there is a stop sign at the intersection of Paine Turnpike. Paine Turnpike is a pavedsecondary road and the terrain is generally rolling but climbs in elevation. Another traffic light isencountered at the intersection with Vermont Route 62 (mile 11). Route 62 is taken a shortdistance (0.1 miles) to the entrance ramp of Interstate 89. At this point the course changes torural interstate conditions. The course continues to climb in elevation to mile 12. At this point a6% downhill grade is encountered on the interstate for a distance of 2 miles. The roadway then

changes to a rolling terrain with slight up and downhill elements. The interstate is exited at exit9, where a stop sign is encountered. A local road is taken to Route 2 and then the course endswith a return to the GSD facility.

5.3 EVermont NiMH Baseline Vehicle, EV13, 1997

Solectria Force EV13 has been in continuous use since being delivered on January 15, 1997 andhas logged over 21,000 miles since that time. The data presented here is the baseline informationused for the project. The first three data sets (Figures 30, 31 and 32) present data collected fromoperating the vehicle under two distinctly different ambient conditions, one Cold at 22C, theother Mild at 14C. Ambient temperature was the major factor that varies between these datasets. The vehicle was operated by the same driver, in the same manner, over the same test course.

The test course was an on-road test course where the posted speed limits range from 25 to 55miles per hour. During these data collection periods the vehicle was always operated within 5MPH of the legal posted limit, as well as following all other traffic codes. Prior to each testcourse run the vehicle was left outside overnight and allowed to charge in the same manner.

Measurements of battery temperature (four separate), battery voltage (individual, plus total),current, total amp-hour and kilowatt-hours are recorded every other second through an on-boardDAQ.

The total voltage of the batterypack is presented for twoconditions in Figure 30, arelatively Mild day when the

average temperature during thetest course run was 58o F (14o

C), and a Cold day when theaverage temperature was -7o F(-22o C). On the Cold day, theovernight low temperature was-13o F (-25o C).

The general pattern of tractionbattery voltage for the twodays track parallel each other,a demonstration of theeffectiveness of the

operational controls (seeFigure 30). On the Mild day

the total traction voltage averages 208.0 volts, whereas on the Cold day it averages 189.4 volts, or9.2% less. The standard deviation of the total voltage is 9.51 volts on the Mild day and 13.35 onthe Cold day. Based on these observations there is 40.4% more fluctuation in total voltage on theCold day versus the Mild day.

Figure 30: Baseline Traction Battery Voltage from EV13


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The DAQ monitors theindividual voltage of eachbattery within the total batterypack as illustrated in Figure 31.The system is set up to recordthe highest voltage and lowest

voltage of each battery in thestring every other second. Forthe Mild day the average of theHigh battery voltage was 14.0volts, whereas the comparablevalue for the Cold day was 12.9,or the average High was 7.9%less on the Cold day. Therespective standard deviationsfor these readings were 0.66 and0.86 volts, or the fluctuation involtage among the high battery

was 30.3% more on the Cold day versus the Mild day.

For the Low battery, the average for the Mild day was 13.8 volts, whereas for the Cold day itwas 12.3, or the average Low battery voltage was 10.9% lower on the Cold day. The standarddeviation on the Mild day was 0.62, and on the Cold day 0.95, therefore there was 53.2% morefluctuation in the average Low battery voltage on the Cold day versus the Mild day.

The temperature of four batteries was monitored during the two test course runs and the data isillustrated in Figure 32. These batteries were located at the extreme corners of the two batterycompartments; one in the front of the vehicle, the other in the rear. The initial temperaturesrepresent the standing temperatures of the batteries prior to the start of the test course run, withtemperature increasing throughout the run. On the Mild day, the temperatures within eachcompartment are generally within two degrees centigrade of each other and generally within six

degrees centigrade between the front and rear compartment.On the Cold day, the rearcompartment battery temperaturesremained within two degreescentigrade of each other throughoutthe test course run. The frontcompartment temperatures displayeda different pattern. While the frontleft battery tracked well with bothrear batteries monitored, about two-thirds of the way through the run,there was a point of departure. Thefront battery compartment containeda lesser number of batteries and hadgreater exposure to the ambientcondition. The data for the frontright battery produced a noticeablydifferent pattern than all otherrecordings.

Figure 32: Baseline Battery Temperatures for EV13

Figure 31: High and Low Battery Voltage for EV13


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There was concern that the sensors may be biased by the ambient condition, and therefore wasnot an accurate represen

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