Incoming Batch Inspection and Degradation Testing Procedures for Sodium Ion Shipments
Immediate AC-IR screening and differential capacity testing reveal hidden transit degradation and cell capacity variance in sodium ion shipments.
DC Resistance Pulse
Meaning
The diagnostic measurement of the internal resistance of an electrochemical cell by applying a short high-amplitude current step and recording the resulting voltage change. This procedure determines the total internal resistance, including both the rapid ohmic contribution and the slower charge-transfer and polarization effects. The value is calculated by dividing the voltage change at a specific timestamp by the applied current step.
The boundary of this measurement is defined by the duration of the pulse, which must be long enough to capture chemical changes but short enough to avoid heating the cell.
Pulse Execution
To perform this test, the cell is placed under a stable open-circuit condition or a steady-state bias current. A precise current step of defined magnitude and duration, often between ten seconds and thirty seconds, is applied by the testing equipment. This step can be a discharge pulse or a charge pulse, depending on the test protocol.
The voltage is recorded at high sampling rates, especially during the first milliseconds of the pulse to isolate the instant ohmic drop. This rapid sampling enables the separation of the electronic resistance of current collectors and active material contacts from the ionic resistance of the electrolyte.
Transient Response
The voltage curve during the step shows a sudden jump followed by a slower asymptotic transition. This secondary phase is caused by the development of concentration gradients and charge-transfer resistance at the electrode interfaces. Analyzing this transition phase provides insight into the diffusion limitations of the active materials.
If the voltage continues to drift without stabilizing, it indicates that the pulse length has exceeded the transient regime and is entering the steady-state region. This transient analysis helps engineers model the dynamic performance of the cell under peak demand conditions.
Cell Evaluation
The resulting data is used to construct impedance models that predict cell performance under heavy load conditions like acceleration or fast charging. This test is a standard part of characterization protocols for cells destined for electric vehicles where peak power delivery is essential. Comparing the results across different states of charge and temperatures allows developers to map the safe operating limits of the battery.
It provides the empirical foundation for the thermal and power limits programmed into the battery management system.