The battery is the most important component of an electric vehicle (EV). Electrical performance testing is necessary to ensure that the battery meets its performance specifications for operational use. The challenge is that EV batteries are high-power devices, and testing EV batteries requires rigorous and precise testing protocols using the proper power instrumentation. This blog will outline the most critical tests for EV battery testing and present instrumentation options to enable efficient, simplified solutions.
EV battery manufacturing process
The chemistry of the most often used EV battery type is Lithium-ion due to its high energy density and capability to extract energy quickly for vehicle acceleration. The battery’s performance is a significant factor in determining the range of the EV.
Testing the battery to obtain maximum performance is essential in the manufacturing process. Figure 1 shows the steps of the manufacturing process for a Lithium-ion battery. Manufacturers combine a large number of Lithium-ion cells into proprietary series and parallel combinations. The final step requires conditioning the battery by formation and aging. The formation and aging process involves charging and discharging the battery to stabilize the internal chemistry. The process requires repeated charging and discharging at varying rates. Power instrumentation, such as programmable DC power supplies and programmable DC electronic instrumentation, can perform the necessary testing.
Figure 1. An overview of the Li-Ion battery manufacturing process
EV battery test methods
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.
Figure 2. Electrochemical dynamic response
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.
Figure 3. Battery discharge with an electronic 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.
Figure 4. Battery charge with a power supply. A diode prevents battery current from flowing into the power supply and damaging it.
Selecting the properly sized power instrumentation
Batteries in EVs currently on the market have operating voltages between 200 V and 800 V. Newer models are trending toward a higher voltage range of 400 V – 800 V to provide the potential for greater power, smaller current-carrying cables, greater driving distance, and faster charging. Therefore, your instrumentation should have a voltage rating higher than the battery’s maximum voltage that will be tested. You may even want to have a higher voltage capacity higher than 800 V, as some future vehicles may use voltages up to 1200 V.
EV battery power ranges from 16 kWh to 100 kWh in personal EVs. Battery manufacturers typically recommend charging a Lithium-ion battery at a level of 0.5 C to 1 C. Thus, DC power instrumentation should have a power rating of at least 1 C based on the capacity of the battery you are testing. You will need a power rating of a minimum of 16 kW and potentially as much as 100 kW if you are testing very high-capacity batteries.
Consider DC power supplies and electronic loads with voltages as high as 1200 V and power levels as high as 100 kW. If you need greater than 30 kW, you may need to parallel power supplies and parallel loads to obtain the necessary power.
An alternate solution
An alternative test solution to a power supply and an electronic load is a single instrument, a bidirectional DC power supply. A bidirectional DC power supply is a two-quadrant instrument that can both source and sink current. As a power supply, the dc power supply sources current and can charge a battery. As a load, the bidirectional power supply can sink current and act as a load for the battery. Figure 5 shows the single instrument solution for battery charge and discharge testing. This eliminates the need for protecting the power supply from reverse current with additional circuitry due to the seamless transition from source to sink.
Figure 5. Use of a bi-directional power supply to both charge and discharge a battery
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:
- A true autoranging output characteristic to accommodate increased load current draw as battery voltage declines
- A built-in function generator that can generate load pulses for the electrochemical dynamic response test
- 60 kW power capacity in a single 6U, full rack enclosure. No other power instrument manufacturer can pack as much power in as small a package. Up to 60 kW, a single instrument can provide a complete solution for charging and discharging.
- Paralleling of multiple instruments with a Master-Auxiliary bus to simplify interconnection and control and a Share-Bus™ interface to ensure all instruments equally share the source or the load.
- Regenerative energy recovery for returning absorbed energy to the grid with 96% efficiency
- Automated test with a wide range of interfaces for connection to a PC or a PLC
Source for solutions to EV battery testing
EA Elektro-Automatik offers dependable programmable products for EV battery testing, utilized by numerous customers, as seen in the video with NI, an innovator in EV battery testing. For more information, see www.eapowered.com; for technical assistance, contact us at firstname.lastname@example.org.