A battery's most critical performance test is its discharge and charge testing. The discharge time determines the battery's capacity to power the vehicle over a distance. The battery's charge time determines the state of health of the battery and how quickly it can be charged.

Another test is an electrochemical dynamic response which measures the state-of-health (SoH) of the battery. The test evaluates the dynamic response of the battery. Figure 2 shows the response to a pulse of a battery with full capacity compared to that of a battery with only 70% capacity.

A programmable DC electronic load meets the requirements for discharging an EV battery. DC electronic loads can discharge the battery at varying rates. One representative discharge rate is based on the power level consumed by an electric vehicle during steady speed, typically highway driving. That rate is estimated to be around 20 to 30 kW. An electronic load with the appropriate capacity can draw a variable current represented by drive power/battery voltage. The electronic load should disconnect itself from the battery when its voltage falls to the manufacturer's recommended complete discharge level. Figure 3 illustrates the test circuit for battery discharge.

For the electrochemical dynamic response test, you will need an electronic load that can generate a pulsatile load. Selecting a load with this capability will avoid the need to create the pulse using external circuitry with the load.

A programmable DC power supply satisfies the requirements to charge the battery. Initially, the power supply must charge the discharged battery using a constant current. When the battery voltage reaches the charge threshold level defined by the manufacturer, the power supply should switch to a constant voltage mode. Finally, when the current drawn by the battery falls to under 3% of the battery's rated current, the power supply should cease to supply charge energy and terminate the charge cycle.

Battery manufacturers recommend charging Lithium-ion batteries at rates between 0.5 and 1 C. The rate, 1 C, is equivalent to 1 A flowing for 1 hour. EV battery manufacturers rate their battery capacities in Wh. For example, a battery rated at 1 kWh with a 400 V rating would have its Ah rating equivalent to 1 kWh/400 V = 2.5 Ah. Thus a 1 C charge rate for this battery would require a 2.5 A charge current for a 1-hour duration. A 0.5 C charge rate would require 1.25 A and a 2-hour duration.

Figure 4 shows a battery charge circuit. Since conventional, single-quadrant power supplies are damaged by current flowing into the power supply, we recommend using a diode to ensure that the battery cannot discharge into the power supply.

EA Elektro-Automatik offers bidirectional power supplies that are ideal for EV battery testing. See EA-PUB 10000 6U - EA Elektro-Automatik (eapowered.com) and Bidirectional DC Power Supply | EA Elektro-Automatik (eapowered.com). Valuable features include: