Battery Management Systems (BMS) using next generation Li-Ion battery chemistries such as LFP (LiFePO4) have a lower maximum cell voltage 3.6V vs. ≥4.2V and ~100mV SOC (State of Charge) drop from 80% – 20% vs. other Li-ion chemistries SOC drops ranging from 250 -280mV.

The more accurate the cell voltage measurement; the more reliable and repeatable the “State of Charge” (SOC) , “State of Health” (SOH), or in the case of an Electric Vehicle “Distance to Empty” (DTE) become. This has pushed differential BMS cell voltage measurement accuracy requirements down from ±100uV to ±50uV.

The QMS has excellent resolution and accuracy but needs to be tighter over time and temperature to meet future BMS test requirements. Spot Calibration (Spot Cal) requires monitoring Ambient Temperature (using QMS Air Temperature or a DIB Temperature Sensor) and / or the drift of a high precision voltage reference. During Spot Cal the Keysight 3458A DMM and the QMS both measure the same precision voltage reference.

The delta between the 3458A and QMS measured values is used to offset (Spot Cal) the QMS for any drift cause by temperature or time.

This presentation will detail the temperature response data measured while the QMS is cycled in a temperature chamber from 25°C +5°C and -5°C; with and without the use of a Spot Calibration.
The implementation of the Spot Cal procedure enables the QMS to meet BMS’s required +/-50uV short term measurement accuracy.