Why does the battery show a minus voltage when checked with the voltmeter?
There are several reasons why this can happen:
You may have measured the wrong way. Make sure that the plus pole and the minus pole are measured with the corresponding measuring ends of the voltmeter. If they are switched, there will be a negative voltage.
If the poles of your voltmeter are correctly connected, there is a possibility that the battery suffered a phenomenon called “polarity reversal”. This is a rare phenomenon that can happen at the end of a discharge with 2 or more batteries in series.
When battery A (high electrical capacity; not yet drained) and battery B (low electrical capacity; nearly depleted) are discharged in series, battery A will still have a lot of remaining capacity whilst battery B is near depletion. When this is the case, it is possible that battery B will be further discharged by forced current from battery A. In this case there is the possibility that battery B will get a negative voltage. This can happen when there is a mixed use of old and new batteries together. If this happens the likelihood of leakage will raise, so please take out and dispose these batteries.