As electric vehicle (EV) markets continue to grow, so does the demand for the key components built into them. Batteries represent one such type of component, and just like all other components, batteries need to be tested at various stages — for example, at the cell level and the completed battery-pack level.
Battery tests present unique considerations and challenges that aren’t found when testing other units under test (UUT). Knowing how batteries are constructed and behave is key in deciding what type of instruments are best suited to test them.
Often thought of as the main solution for testing batteries, general-purpose bidirectional programmable power supplies satisfy some requirements. As shown in Figure 1, the bidirectional supply can source and sink power, so it can exercise a battery’s charging cycle, where the supply provides power to the battery, and it can exercise the battery’s discharge cycle, where the supply can sink energy from the battery and regenerate it to the ac grid.
Figure 1. A bidirectional power supply can source power to and sink power from an unit under test (UUT), such as a battery.
However, a standard off-the-shelf bidirectional supply leaves many battery test challenges untouched. As an alternative, purpose-built battery testers target these specific requirement shortfalls and ensure the proper testing of batteries and their supporting components.
Isolation challenges
First, among the specific challenges, batteries are energy-storage devices that store a finite amount of energy and can be inadvertently discharged, causing problems. If not tested carefully, this discharge can permanently damage the battery or connected equipment, increasing test times and costs.
Many standard bidirectional dc power supplies lack output isolation and polarity detection, meaning irreversible damage can occur if a battery or high-energy UUT is connected incorrectly. Additionally, suppose a battery is left connected to such a supply, even with the supply turned off. In this case, the internal resistance of the supply can discharge the battery over time, making it impossible to continue a test and potentially permanently damage the battery, depending on the chemistry.
Large inrush currents
Typical bidirectional dc supplies have large output capacitances that present low impedances to a fully charged battery when that battery is first connected. Because batteries are capable of high discharge rates, they can deliver large inrush currents until the supply’s output capacitance charges. Without additional considerations for inrush-current control through precharge, voltage-matching, or slew-rate-control circuits, the battery and test equipment can be damaged.
Battery-specific parameters
Battery testing uses specific measurement parameters not typically associated with other types of UUTs. These parameters include ampere-hour (Ah) and kilowatt-hour (kWh), which many general-purpose power supplies will not support without requiring additional higher-level software steps.
Having these measurements directly available on the test instrument saves programming time and adds efficiency and accuracy to the system by reducing programming complexity.
Dynamic response
A battery is extremely dynamic. It can both provide current and change its voltage extremely quickly, depending on the current magnitude. As such, to sufficiently test a battery, the test equipment must be as quick or quicker in its voltage and current transient capabilities.
Typically, general-purpose power supplies typically have high output capacitance — on the order of tens of thousands of µF. This is coupled with slower transistor switching frequencies and sluggish control operation, making their voltage and current changes slower. This translates into the inability to test real-world scenarios that the battery will see properly.
A different testing approach
One approach to battery testing is to use a configurable bidirectional supply equipped with a battery-test option that turns it into a purpose-built battery tester. Such a configuration would include software that enables the creation of multiple charging and discharging profiles to mimic real-world battery operation.
Like any purpose-built battery tester, it would support seamless transitions between charging and discharging operations to provide uninterrupted testing. The system would also include an output isolation relay that can disconnect a battery under test in response to a fault condition or when the power supply is off.
Such a configurable system would offer an advantage over other custom solutions by simplifying the process with fewer external controls. For example, a user testing batteries may also need to test a battery charger or battery management system (BMS). In this case, the user could choose a battery-simulation option that can be activated under software control or via the instrument’s front panel (Figure 2).
Figure 2. Depending on the installed hardware and software configuration, the bidirectional power supply in this example can be adapted to support various specific applications, including battery simulation (BATSIM) and battery testing (BATTEST).
With the battery-simulation mode, users can simulate any battery chemistry at any state of charge or capacity, essentially creating an infinitely tuned battery. Figure 3 shows a programmable supply acting in a bi-directional mode, where it can source or sink power from a UUT exactly as a battery would in the real world.
Figure 3. When configured as a battery simulator, the bidirectional power supply (Mi-BEAM) can either source or sink power to and from the UUT, effectively emulating real-world battery behavior.
Conclusion
Given the batteries’ energy capacity, extreme care must be taken when testing them. Otherwise, failures such as component damage, exothermic events, and even facility damage can occur. Many bidirectional power supplies do not have full galvanic output isolation, precharge circuits, or polarity detection, opening up the possibility for these types of failures.
For effective, safe, and reliable battery testing, choose a purpose-built battery tester or a modular bidirectional supply with dedicated battery test and simulation options. These will include the hardware and software to address inrush currents, dynamic performance, and isolation. Also, they can provide measurement results in parameters specific to the battery test and simulation environment.
Filed Under: Batteries, FAQs, Testing and Safety