How to Measure Self-Discharge Current in EV Battery Cells
Measuring self-discharge current with a potentiostatic analyzer
Determining whether newly formed lithium-ion (Li-ion) battery cells in electric vehicles (EVs) exhibit acceptable self-discharge behavior requires a suitable self-discharge current measurement method. Lithium-Ion cells gradually discharge even without a connection to anything. Some self-discharge is normal. There are two main methods for testing self-discharge; the delta open circuit voltage (OCV) measurement method and the potentiostatic method. The OCV method takes weeks to get a significant drop in OCV to prove a reliable measurement. With the potentiostatic measurement method, a low-noise, very stable DC source is set to match the cell's open-circuit voltage (OCV). The DC source connects to the cell through a micro-ammeter, and the current flowing between the DC source and the cell is measured.
When the cell self-discharges, the DC source takes over, furnishing current to maintain the cell at a constant voltage and state of charge. As the DC source comes to equilibrium with the cell, the self-discharge current transitions from internally sourced to furnished externally from the DC source. Self-discharge current is measured using the micro-ammeter. The potentiostatic measurement method requires a self-discharge analyzer capable of quickly matching the voltage to within +/- 5 µV of the cell’s OCV before being applied. This process minimizes the introduction of any new charge or discharge changes, limiting measurement settling time to a minimum. The voltage applied to the cell must also be stable to minimize ongoing current noise on the self-discharge current measurement.
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