Confusion about NIMH polarity reversal

I have searched in the forums for the past Q&A on this topic, here's one thing that I am still confused with -

I have a headlamp that uses 4xaaa in series. I am using some kinda old energizer 850 mAh cells from more than 5 years ago - there are numerous occasions when I take off the lamp and found that the cell voltage of one of the cell UNDER LOAD is slightly negative at around -0.04V. Once switched off the cell open circuit voltage springs right back to around 1.2V.

Question - is this considered reversed polarity where the cell clearly still have some capacity left but nonetheless got forced into "reversed polarity" due to high internal resistance (I am assuming this is the reason why it jumps from -0.04V to 1.2V)?

Thank you for your help.

Hello Kyzhkcpf,

Welcome to CPF.

That usually occurs as a result of cell capacity mismatch. The cell that is going negative is empty and being reversed charged by the other cells in series. If that goes far enough you will ruin the cell as its polarity will reverse permanently.

You should be seeing a drop in light output when that cell reaches empty. At that time you should shut the light off.

Tom