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Journal of Communication and Computer 10 (2013) 178-185

Power System Design for an Electric Car

Louiza Sellami, Mathew McIntyre, Linda Yin, Christian Soncini and Amanda Lowery

Electrical and Computer Engineering Department, US Naval Academy, Annapolis 21402, MD USA

Received: August 21, 2012 / Accepted: August 22, 2012 / Published: February 28, 2013.

Abstract: For many years now electrical engineering students at the US Naval Academy have been involved in renewal energy types

of projects, including electric boats and cars, and have participated in various competitions across the US. Of particular interest to the

authors is the electric car, since it involves various aspects of electrical engineering. As part of their senior capstone project, and

under the authors guidance and supervision, four students designed, built, and tested the power generation and distribution system,

the motor control system for an electric car. This was accomplished by converting an originally gas-powered car into a

battery-powered car, whereby solar panels are used to recharge the batteries. Towards this end, a mix of off the shelf parts and

components, and homebuilt circuits were used. The design considerations include selecting an appropriate control system, choosing

suitable batteries, utilizing solar panels to recharge the batteries, designing the lighting system for the vehicle, and implementing

several key safety features, including an emergency shut-off switch.

Key words: Electric cars, batteries, solar panels, DC-DC conversion, DC motor control.

1. Introduction

As early as the 1830s, engineers and inventors have

experimented with utilizing electric motors and

battery systems as a means of powering vehicles [1-2].

These vehicles are known to have very low acoustic

noise as well as zero emissions. Though they have

existed for a very long time and competed favorably

with the highly inefficient combustion engine vehicles,

the latter gained the upper hand because of the

limitations of the batteries. These included inadequate

capacity, long charging time, high replacement cost,

reduced passenger and cargo space, as well as a short

driving range [3].

With the ever increasing cost of gasoline, the use of

modern day road vehicles is constantly becoming an

economic burden. With most gasoline powered

vehicles, every mile driven is about 36 cents;

comparatively the cost of an electric car is under 10

cents per mile [3]. Nearly 85% of people in the United

Corresponding author: Louiza Sellami, associate professor,

research fields: circuits and systems, signal processing, biomedical engineering, power. E-mail: [email protected].

States own and operate motor vehicles. This number is

projected to increase, along with the number of

owners in other large countries. Because of this

increase the world faces many growing economic and

environmental problems. In the US alone, 18 million

barrels of oil are consumed daily by driving cars,

which emit 2.7 billion tons of carbon dioxide each

year [1]. With dwindling supplies of fossil fuels and

increasing environmental backlash from greenhouse

gasses, the world needs to find an alternative to the

conventional motor vehicle.

In recent years there has been a resurgence of

battery and solar powered electric vehicles which was

brought about by the issues mentioned above, and the

ensuing government regulations in terms of limiting

carbon emissions. With this resurgence came great

advancements in the research and development of

deep cycle batteries, battery chargers, MOSFET motor

controllers, DC-DC converters and sensors [4-5]. In

turn, this led to the inclusion of concepts and designs

from the green energy field in university curricula,

including the US Naval Academy.

This paper presents the design method and


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considerations for building a battery-powered budget

car, whereby the batteries are recharged by solar

panels, as well as the test results. Examples of design

constraints include: The charging time for the batteries

was limited to an acceptable ratio of charge versus

discharge. The amount of money spent on the project

had to be within the prescribed budget of under

$5,000. The car must be able to attain a reasonable

mileage and speed. Finally, the design must consider

the safety of the user during the operation and

charging of the car. Other design considerations

include selecting an appropriate control system,

choosing a suitable battery, utilizing solar cells to

recharge the batteries, designing the lighting system of

the vehicle, and implementing several safety features,

including an emergency shut-off switch.

2. Considerations and Requirements

As part of the design, first the requirements (listed

below) for the car had to be defined. Some of these

requirements were dictated by the US Naval Academy,

others by the budget constraints:

(1) An ideal distance of 40 m per charge;

(2) Under a budget of $5,000

(3) Ability to power the lights of the car using the

power system;

(4) The max sustainable weight of the chassis is

400lbs;

(5) The control system must have an acceleration of

5 mph/s and a speed limit of 25 m/h;

(6) The solar cell charge time for 80% of the battery

capacity of 8 h;

(7) The wall outlet charge time for 80% of the

battery capacity of 3 h;

(8) An emergency shut off switch for safety.

3. Design Overview

An overview of the chassis structure is shown in

Fig. 1. The motor used in the vehicle is the D&D

ES-3336-48 VDC series wound single shaft, which is

placed in the rear end of the chassis, as shown in

Fig.2.The motor is powered by ten 12V lead acid

batteries, which are placed above the motor.

The batteries are connected to the other components

of the chassis such as the speedometer, the

temperature gauge, the controllers, and the lights. In

order to charge the batteries, two solar panels are

attached to the batteries at a separate station. The

system that is used to control the vehicle is the All

Trax programmable DC motor controller, which is

shown in Fig. 3. The controller, powered by the

batteries, has inputs for the accelerator pedal and the

batteries and an output to the motor. Its function is to

vary the motor power depending on how hard the

Fig. 1 Overall view of the budget volt car.

Fig. 2 Rear of the chassis showing the motor.


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Fig. 3 All Trax programmable DC motor controller.

acceleration pedal is pressed, which is attached to a

potentiometer.

A block diagram for the whole system is shown in

Fig. 4. The corresponding circuit schematic,

illustrating the primary and secondary power loops, is

shown in Fig. 5. The major components of the vehicle

are the motor, the charging unit, the batteries, the solar

cells and the controller, which are encased in the

chassis structure.

The lead acid battery was chosen as the most

suitable battery for the vehicle for its light weight and

affordable cost. It is also the safest option. Other

design alternatives were nickel metal hydride and

lithium ion batteries; however, both types presented

problems for the safety of the car and were not within

the budget. Lithium-Iron batteries were also not

chosen because there is a greater chance they will

become overcharged and start a fire.

3.1 Chassis

The original chassis of the car was used. All of the

components are mounted on the chassis whose

specifications are shown in Table 1.

3.2 Motor

As with any car, propulsion requires a prime mover,

which in this case is the DC motor. The latter converts

the electrical energy provided by the batteries into

mechanical torque. The Motor D&D ES-33 36-48

VDC series wound 7/8 single shaft, shown in Fig. 6,

is used. It is 6.7 in diameter and 11.53 long, and

weighs approximately 57l bs. It can output from 5 Hp

at 36 V to 7.2 Hp at 48 V and can handle a current of

135-140 A. However, for the design it was found that

Fig. 4 Block diagram for the budget volt, illustrating the

major systems, and the inputs and outputs for those

systems.

Fig. 5 Circuit schematic of the power system, distribution,

and instrumentation.


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Table 1 Chassis specifications.

Part Dimensions

Front suspension Dual 12.8 ''shocks

Rear suspension Dual 15.4'' shocks

Front brakes Hydraulic disc

Rear brakes Hydraulic disc

Front tires 21 7-10 Rear tires 22 10-10

Capacities / dimensions

Weight capacity (lbs) 400

Net weight (lbs) 639

Gross weight (lbs) 739

Size Full size

Overall length 89.4

Overall width 57.9''

Overall height 59.5''

Carton dimension (LWH/in) 90.9'' 45.7 31.1

Seat height 17.7''

Wheelbase 65.7''

Ground clearance 5.9''

Fuel capacity (gal) 1.26

Safety / control

Engine kill switch Yes

Speed limiter Yes

Foot brake Yes

Horn Yes

Headlights Yes

Tail lights Yes

Turn signal indicators Yes

Fig. 6 D&D ES-33 36-48VDC motor.

there was no need to go higher than 24 V as

preliminary experimental results determined that at 16

V the car moved at the target speed.

3.3 DC Motor Controller

A DC motor controller in an electric car can be

compared to a carburetor or fuel injection system in a

gas powered car. It is a device that controls the output

power of the motor by controlling the input power

drawn from the batteries. By stepping up the amount

of voltage and current available to the motor it

controls the speed of the motor and how quickly the

motor can attain this power [6].

The All Trax programmable DC motor controller

used is a durable, high tech DC motor controller. It

has the advantages of being waterproof,

corrosion-proof, vibration proof, and is fully

programmable. Performance characteristics that can

be programmed include: max output current, throttle

response profile, acceleration rate, plug-brake current,

under/over voltage cutback, high pedal disable, and

throttle input type. The 4855 model whose specs are

shown in Table 2 was selected.

3.4 Contactor

Because a large amount of power is used by the

electric motor and the accessories there is a need for

a safe and reliable way of turning on the electrical

system without either leaving the system running or

reconnecting the batteries to the system. Unsafe

conditions can occur as a result of a high

current which can cause damage to the system,

hence the use of a contactor in the design. The latter

is an electromagnetic relay that, when a voltage of

12 V is applied across the solenoid end, causes a

large metal plate to shift up and create an

instantaneous connection between the batteries and

the rest of the electrical system. This acts as a dead

mans switch as it stops the car by interrupting

energy flow if anything goes wrong. The

white Rogers 586 shown in Fig. 7 was selected,

which is capable of carrying up to 200 Aa

current that is below the current at which the car

operates.


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Table 2 AXE controller specifications.

AXE model 4855 Specs

Battery voltage 24-48 V

Current limit:

2 Minute rating 650 A



Voltage drop @ 100 A < 0.08 V

Fig. 7 White Rodgers 586 on budget volt.

3.5 Solar Panels and Solar Controller

Solar panels and solar controller serve as the

charging station for the car. The two available models

are the MSX120 and the SX80V. Each solar panel

model specifications are shown in Table 3. The

Solarex SX-80 photovoltaic module is constructed of

36 polycrystalline silicon solar cells. The cells can be

utilized in configurations directly with DC loads or in

an inverter-equipped system for AC loads [7]. The

overall dimensions of the rain-tight structure are

19.75 in by 57.31 in. Each solar cell is rated at a

maximum power output of 80 watts. The voltage at

maximum power is 16.8 V and the current is rated at

4.75 A. The guaranteed minimum power output is

75 W. The short circuit current is rated at 5.17 A and

the open circuit voltage is rated at 21.0 V. The solar

cells are utilized in configuration with the DC load of

the 24 V battery array, which is used to power the

motor and the auxiliary electrical systems in the car.

The solar controllers used in this project are the

SunSaver SS-20L-12V and the SS-20L-24V whose

specifications are listed in Table 4. The solar

Table 3 Solar panel specifications.

Model MSX120 SX80V

Pmax 125.8 80

V (V) 17 16.8

I (I) 7.41 4.75

Voc 21..4 21

Isc 8.39 5.17

Table 4 Solar controller specifications.

Model SS-20L-12V SS-20L-24V

Solar rating 20 A 20 A

Load rating 20 A 20 A

System voltage 12 V 24 V

controller utilizes switching pulse width modulation to

charge batteries.

3.6 Batteries

Batteries are portable sources of electrical energy

which the DC motor converts to mechanical. There

are many new types of batteries that are suitable for

electric vehicle application. These include lead acid,

various types of nickel-based (iron, cadmium, and

metal hydride), lithium-based (polymer and iron), as

well as sodium-based (sulfur and metal chloride) [8-9].

However, at the present time deep cycle lead acid

batteries are still the most commonly used. They have

the advantage of being fully recyclable and their cost

is significantly lower. As a result, the use of a

standard 12 V lead acid battery was the best option.

In order to design the power system, it is important

to find the power needs of each of the components.

Obviously, the motor uses the majority of the power,

but the peripherals such as lights are also important to

take into consideration. Comparing current capacity,

weight and cost, a pack of 10 lead acid batteries

(model: CSB-GP-12260 shown in Fig. 7) each with a

weight of 18.6 lbs and a current capacity of 26 Ah.

They are connected in five branches with two batteries

in series per branch in order to provide 24 V each.

This was done to reduce the amount of current draw

from each battery, since the capacity is only 26 Ah. A

is a total current of 18A is drawn, and the car runs for

an hour and a half before it needs to be recharged.


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3.7 DC-DC Power Conversion and Instrumentation

A DC-DC converter is necessary in order to power

the lights and the other car accessories, since the latter

operate at 12 V whereas the battery pack provides 24

V. The Pyle PSWNV720 shown in Fig. 8 was chosen

due to its ability to sustain a 360 W output, which low

enough so that it does not reduce traction power or

range of the car. It has a converse efficiency of 87%,

and has several safety built-in features.

To fully implement a working set of turn signals

and hazard lights that blink, the use of the LM555

timer is essential. Two identical timers were designed

and calibrated to have an output time of 0.808 second

and a shut off time of 0.503 second. This was done by

using a 2 K and a 3.3 K resistor and a 220 F

capacitor as seen in Fig. 9. The first timer powers the

left front and back turn signals or the right front and

back turn signals, depending on how the switch is

pressed. The second timer powers a relay that turns on

and off, which then powers all four lights based on

direct input from the DC-DC converter. This was done

to overcome the output current limitations of the

LM555 chip, which was not sufficient to power all

four lights at the same time.

To keep track of the cars electrical system, a

battery meter is added that to monitor the capacity of

the battery pack. The battery manager 3 by BSDesigns,

which is shown in Fig. 10 was selected. It determines

the remaining battery capacity by way of a DC shunt

through which the load current flows. The meter

measures the voltage drop across the shunt and

performs a comparison in order to determine the

current being used by the car, and displays it in

Amp-hours.

3.8 Safety Considerations

There are numerous safety devices that are

integrated into the power system of the car in order to

prevent accidents. Aside from the contactor and dead

mans switch, fuses are placed in series within each

battery branch and connecting the lights on the car.

Fig. 7 Battery pack on the rear of the budget volt.

Fig. 8 DC-DC converter mounted on the chassis.

Fig. 9 555 timer circuit used for turn signals.

Also, the additional battery and passenger weight

and the effects on the brakes, axles and front

suspension are considered. Battery placement so as

not to change the center of gravity of the car, and the

safety brackets to hold the batteries in place are

another safety key design that is carefully planned.

4. Testing and Design Specs Verification

There were many different stages during the testing

process. During the initial stage the motor was


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Fig. 10 Battery manager 3 (top), shunt (bottom).

Fig. 11 No load and full load test results.

physically removed from the car and powered by four

batteries connected in series to determine its

maximum speed. A secondary test was done with a

dynamometer. This is what is called the light load

test, which resulted in an average current draw of 40

A (Fig. 11). Based on the results of these preliminary

tests, the decision to use a 24 V battery pack was

made.

Once the car was fully loaded with driver and

batteries there was a much larger power draw than

was expected originally. There was only an

introduction of about 300 lbs but the current draw was

around 120 A which is much higher compared to the

increase from no load to light load. With 120 A there

is a draw of 24 A from each battery pack which is

about twice as much as expected. The road results

were verified using a standard Garmin forerunner 410

which was used to calculate the speed of the car, in

mph, and the distance that the car travelled, in miles.

5. Conclusions

In this paper a mechanism for converting an

originally gas-powered car to a battery-powered,

whereby solar panels are used to recharge the batteries

was presented. Specifically the design method and

considerations were defined and explained, and the

selection of the various components was discussed.

Also discussed are the test results and the safety

features.

Acknowledgments

The authors wish to acknowledge receipt of funding

from NAVSEA and IWS for the project and this

publication.

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