Testing Electric Vehicle Batteries

TESTING ELECTRIC VEHICLE BATTERIES

The battery is a key element in the acceptance of the EVs. In order to test and develop the battery pack, it is important to determine the effect of battery cycle life versus the peak battery power and battery rest periods, and to determine the impact of the charge method on the battery cycle life. From the battery pack management standpoint, it is important to study the thermal management characteristics and the utilization of active material in the batteries.

The United States Advanced Battery Consortium (USABC) has defined procedures for battery pack testing. These test procedures include battery pack life-cycle testing, destructive safety and/or special testing, destructive abuse testing, and posttest analysis.

A typical battery test flow includes the following test steps (not necessarily in the same order):

• Core battery performance testing • Performance safety/abuse or life cycle testing • Life cycle reporting • Posttest analysis and reporting

Core Battery Performance Tests

The core battery performance tests include the following:

• Constant current discharge • Peak power • Constant power • Variable power discharge • Federal Urban Driving Schedule (FUDS) regime • Dynamic stress test (DST) regime • Special performance • Partial discharge • Standloss • Sustained hill-climb power • Thermal performance • Battery vibration • Charge completion or optimization • Fast charging

A brief description of each of the above-mentioned tests provides an insight in to the nature of the battery test and how the test

improves the development of the EV from the traction battery pack standpoint.

The Constant Current Discharge Test

This test determines the effective battery capacity of the battery pack using repeatable, standard tests. A series of predefined current levels are applied to characterize the effect of discharge rate on the battery pack. The test is terminated when the rated battery capacity or the minimum discharge voltage (as specified by the battery manufacturer) is reached, or whichever occurs first.

Peak Power Test

This test determines the peak (maximum) battery power capability of a battery at various DOD. The value is calculated at 80% DOD and is important since it provides a comparison with the USABC power goal values. This test does not measure the actual peak power available from the battery.

Constant Power Test

This test determines the ability of the battery to provide a sustained discharge over a range of power levels that represent EV applications. The tests perform a sequence of constant power discharge/charge cycles that define the voltage versus power behavior of a battery. The behavior is as a function of DOD.

At each power level, the battery undergoes a discharge to rated capacity or a specified termination point, whichever comes first, a minimum of two times.

Variable Power Discharge Test

This test produces the effects of EV driving behavior (including regenerative braking) on the performance and the life of a battery pack. The variable power discharge profiles specified under this test are based on the auto industry standard Federal Urban Driving Schedule (FUDS), a 1,372-second time-velocity profile originally based on actual driving data. The schedule has been approximated to include the EV application of battery pack charge/discharge cycles.

Federal Urban Driving Schedule Regime (Variable Power Discharge) Test

This represents the best possible simulated condition of the actual power requirements from the EV standpoint. This test provides a demanding profile with respect to the occurrence of high power peaks and the ratio of maximum regenerative charging, to the discharge power applied to the battery pack.

Dynamic Stress Test Regime (Variable Power) Test

This test provides a simplified profile of the Federal Urban Driving Schedule Regime (FUDS)-based power-time test. This specific profile can simulate the dynamic behavior of the EV undergoing discharges and regenerative braking conditions.

Special Performance Test

The special performance test, as the name suggests, is defined only under certain conditions and/or environments that are encountered rather infrequently, and is also applicable under specific battery technologies.

Partial Discharge Test

This test provides a measure of the response of the battery pack to successive partial discharges, to identify any resulting capacity loss, and to verify the proper charging procedure for a partial DOD operation of the EV.

Standloss Test

This test measures the battery capacity loss when the battery is not used for an extended period of time. This test simulates the condition when the vehicle is not driven for a period of time, and the battery is not placed under charge. This standloss is primarily due to self-discharge, or other mechanisms that could result in permanent or semipermanent loss of battery pack capacity.

Sustained Hill-Climb Power Test

This test determines the DOD at which the battery will support a 6-minute discharge at approximately 45W/kg before it is completely dis-

charged. The test results are plotted with respect to time for which the 45W/kg power can be sustained. This test provides a graphical determination of the maximum DOD at which the power can be provided for at least six minutes before the battery is fully discharged.

Thermal Performance Test

This test characterizes the effects of ambient temperature variation on the battery pack performance. The characteristics of the battery that are affected are in most cases, technology related. Thus the number and the types of charge and discharge cycles to be performed cannot be generalized for all battery types. The results of this test provide useful data to determine the need for battery thermal management or the allowable temperature range for a battery that may incorporate thermal management at a later stage.

Battery Vibration Test

This test characterizes the effect of long-term, road induced vibration and shock on the performance. This test characterizes the service life of candidate batteries. The intent is to either qualify the vibration durability of the battery or identify the design deficiencies that must be corrected.

Charge Completion on Optimization Test

This test establishes a charge algorithm or optimizes an existing algorithm for use in charging the battery pack in case the charge algorithm supplied by the battery manufacturer or developer does not adequately charge the battery, or a stable battery capacity is not obtained. Also, the charging method is a contributor to this problem.

Fast Charge Test

This test determines the fast charging capability of a battery by subjecting the battery to high charging rates, and determining the efficiency and other effects of accelerated charging. The USABC goal for fast charging is to return 40% of the state of charge (SOC) of the battery, starting from approximately 60% DOD in 15 minutes.