T h e m a g a z i n e d e s i g n e r s r e a d march 2009

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automotive march 2009

batteries to only 40% cyclingin order to increase batterylongevity by a factor of 3, thebattery size must be doubledto achieve the same usablecapacity as the 80% cyclingcase. This would double theweight and volume of thebattery system, increasingcosts and reducing efficiency.

Electric vehicle batterypack systemsAn electric vehicle batterypack consists of dozens ofbatteries stacked in series. Atypical pack might have astack of 96 or so batteries,developing a total voltage inexcess of 400 V for Li-Ionbatteries charged to 4.2 V.While the vehicle powersystem sees the battery packas a single, high-voltagebattery—charging anddischarging the entire battery pack at once—the battery control system must considereach battery’s condition independently. If onebattery in a stack has slightly less capacity

than the other batteries, then its SOC willgradually deviate from the rest of thebatteries over multiple charge/dischargecycles. If that cells’ SOC is not periodically

Remarkable progress has beenachieved on battery technologies forEVs and HEVs. Battery energy

densities have steadily increased, andbatteries today can be reliably charged anddischarged thousands of times. If designerscan effectively exploit these advancements inenergy capacity, EVs and HEVs have thepotential to be competitive with traditionalvehicles in terms of cost, reliability, andlongevity.

A battery’s specified capacity refers to theamount of charge the battery can supplyfrom 100% State of Charge (SOC) to 0%SOC. Charging to 100% SOC or dischargingto 0% SOC will quickly degrade a battery’slife. Instead, batteries are carefully managedto avoid complete charge or dischargeconditions. Operating between 10% SOC and90% SOC (80% of capacity) can reduce thetotal number of charging cycles by a factorof 3 or more, when compared to operatingbetween 30% and 70% SOC (40% ofcapacity).

The tradeoff between effective batterycapacity and battery lifetime createschallenges for battery system designers.Consider the above case of 40% cyclingversus 80% cycling. If a system limits

BATTERY MANAGEMENTARCHITECTURES FORHYBRID/ELECTRIC VEHICLESNext generation Electric (EV) and Hybrid Electric Vehicles (HEV) arepushing the development of new battery technologies. To minimisecost and maximise efficiency, vehicle system designers need toexploit the full usable battery storage capacity.

Table 1

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AccuracyTo take advantage of the maximum possiblebattery capacity, the battery monitor needsto be accurate. A vehicle, however, is a noisysystem, with electromagnetic interferenceover a wide range of frequencies. Any loss ofaccuracy will adversely affect battery packlongevity and performance.

ReliabilityAutomobile manufacturers must meetextremely high reliability standards,irrespective of the power source.Furthermore, the high-energy capacity andpotentially volatile nature of some batterytechnologies is a major safety concern. Afailsafe system that shuts down underconservative conditions is preferable tocatastrophic battery failure, although it hasthe unfortunate potential of stranding

passengers. To minimise both false and realfailures, a well-designed battery pack systemmust have robust communications,minimised failure modes, and fault detection.

ManufacturabilityAdding sophisticated electronics and wiringto support an EV/HEV battery system is anadditional complication for automobilemanufacturing. The total number ofcomponents and connections must beminimised to meet stringent size and weightconstraints and ensure that high volumeproduction is practical.

CostMinimising the number of relatively costlycomponents, like microcontrollers, interfacecontrollers, galvanic isolators, and crystalscan significantly reduce total system cost.

PowerThe battery monitor itself is a load on thebatteries. Lower active current improvessystem efficiency and lower standby currentprevents excessive battery discharge whenthe vehicle is off.

Linear Technology has introduced a devicethat enables battery system designers tomeet these difficult requirements. The

balanced with the rest of the batteries, thenit will eventually be driven into deepdischarge, leading to damage, and eventuallycomplete battery stack failure. To preventthat from happening, each cell’s voltagemust be monitored to determine SOC. Inaddition, there must be a provision for cellsto be individually charged or discharged tobalance their SOCs.

An important consideration for the batterypack monitoring system is thecommunications interface. Forcommunication within a PC board, commonoptions include the Serial PeripheralInterface (SPI) bus and Inter-IntegratedCircuit (I2C) bus. Each has lowcommunications overhead, suitable for low-interference environments. Another option isthe Controller Area Network (CAN) bus,which has widespread use in vehicleapplications. The CAN bus is very robust,with error detection and fault tolerance, butit carries significant communicationsoverhead and high materials cost. While aninterface from the battery system to themain vehicle CAN bus may be desirable, SPIor I2C communications can be advantageouswithin the battery pack.

Battery monitoring requirementsThere are at least five major requirementsthat need to be balanced when decidingbetween battery monitoring systemarchitectures. Their relative importancedepends on the needs and expectations ofthe end customer.

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Parallel modules with CANgateway (Figure 2)Each 12-battery module contains a PC boardwith an LTC6802 and a digital isolator.The modules have independent interfaceconnections to a controller board containinga microcontroller, a CAN interface, and agalvanic isolation transformer. Themicrocontroller coordinates the modulesand provides the gateway to the vehicle’smain CAN bus.

Single monitoring module withCAN gateway (Figure 3)In this configuration, there is no monitoringand control circuitry within the 12-batterymodules. Instead, a single PC board has 8LTC6802 monitor ICs, each of which isconnected to its battery module. TheLTC6802 devices communicate through non-isolated SPI-compatible serial interfaces. Asingle microcontroller controls the entirestack of battery monitors via the SPI-compatible serial interface, and it also is thegateway to the vehicle’s main CAN bus. ACAN transceiver and a galvanic isolationtransformer complete the batterymonitoring system.

Series modules with CAN gateway(Figure 4)Each LTC6802 is on a PC board within its12-battery module. The 8 modulescommunicate through the LTC6802 non-isolated SPI-compatible serial interface,which requires a 3- or 4-conductor cable tobe connected between pairs of batterymodules. A single microcontroller controlsthe entire stack of battery monitors via thebottom monitor IC, and also acts as thegateway to the vehicle’s main CAN bus. Onceagain, a CAN transceiver and a galvanicisolation transformer complete the batterymonitoring system.

Battery monitoring architectureselectionThe first and second architectures aregenerally problematic due to the significantnumber of connections and the externalisolation required for the parallel interface.For this added complexity, the designer hasindependent communication to each monitordevice. The third (single monitoring modulewith CAN gateway) and fourth (seriesmodules with CAN gateway) architecturesare simplified approaches with minimallimitations. The LTC6802 can address allfour configurations, leaving the choice to thesystem designer.

Two variants of the LTC6802 have beencreated, one for series configurations andone for parallel configurations. TheLTC6802-1 is designed for use in a stackedSPI interface configuration. MultipleLTC6802-1 devices can be connected inseries through an interface that sends dataup and down the battery stack withoutexternal level shifters or isolators. TheLTC6802-2 allows for individual deviceaddressing in parallel architectures. Bothvariants have the same battery monitoringspecifications and capabilities.

JIM DOUGLASS works as a Design Managerfor Linear Technology

10 epd

LTC6802 is a battery stack monitorintegrated circuit that can measure thecell voltages of up to 12 stacked cells. TheLTC6802 also has internal switches thatprovide for the discharge of individual cellsto bring them into balance with the rest ofthe stack.

Battery monitoring architecturesFour architectures for battery monitoringsystems are depicted in Figures 1-4 anddescribed below. Table 1 summarises thepros and cons of each architecture,assuming a 96-battery system organisedinto 8 groups of 12 batteries. In everycase, one LTC6802 monitors each groupof 12 batteries. For example, using 4.2 VLi-Ion batteries, the bottom monitoringdevice would straddle 12 batteries withpotentials scaling from 0 V to 50.4 V.The next group of batteries would havevoltages ranging from 50.4 V to 100.8 V,and so forth, up the stack. Eacharchitecture is designed to be anautonomous battery monitoring system.Each provides a CAN bus interface tothe vehicle’s main CAN bus and isgalvanically isolated from the restof the vehicle.

Parallel independent CANmodules (Figure 1)Each 12-battery module contains a PCboard with an LTC6802, a microcontroller,a CAN interface, and a galvanic isolationtransformer. The large amount of batterymonitoring data required for the systemwould overwhelm the vehicle’s main CANbus, so the CAN modules need to be onlocal CAN sub-nets. The CAN sub-nets arecoordinated by a master controller thatalso provides the gateway to the vehicle’smain CAN bus.

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