L9961 Cell voltages deviate during discharge

[jondobrinc]

Hello, we are having an issue where our cell voltage readings are drifting when discharging. When there is no current, the voltages all read correctly. However, during discharge, we see that cell 5 and cell 1 have lower voltage readings by about 150mV at 5A. Once discharge is ended, the voltages level off again. I have checked that this is reflected in the registers.

These are our filter settings:

[Peter BENSCH · Accepted solution]

Based on the L9961 data sheet, the behaviour is likely related to the measurement timing and input filter settling under load, rather than a pure ADC accuracy issue.

A few key points:

• The cell measurements are taken cyclically, not continuously
• Your T​MEAS_CYCLE = 50ms must be longer than thefull measurement task duration
• With TCELL_FILTER = 16.67ms, the task may be too close to the cycle time, which can cause unstable or skewed readings during discharge
• The 150mV shift at 5A is much larger than the device’s specified cell measurement error, so I suspect a dynamic settling / layout / sense path effect

Recommendations:

• Increase TMEAS_CYCLE to ensure it is safely greater than the total conversion time
• Temporarily reduce the cell filter time and check whether the drift decreases
• Compare the L9961 readings with a direct measurement at the cell terminals under load
• Check whether cell 1 and 5 have any layout or routing asymmetry, especially around the sense filters and references

Regards
/Peter