Reverse Polarity?

rescraft

I was changing the 4--AAA batteries out of my son's calculator and only had 3 fresh ones to replace them with so I decided to check the voltages on the ones in the calc. and see which one I would keep. The third one I checked, the polarity on the battery was reversed. It wasn't leaking or damaged. Does this happen often and why? I've never come across it before.

dSilanskas

They dont sell perfect batteries at the dollar store

Electric_Light

Because, it suffered from polarity reversal.

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When the first cell dies, the other cells will keep pushing the current. Eventually, the first dead battery will reach 0v, and continued current charges it backward, reversing polarity. If sufficiently pushed, it will leak.

This is why they tell you not to mix batteries.

This is also a good reason to not drain your power tool batteries completely to the ground. The pack is made of multiple cells and they're not perfect, so there's a variation in each battery within the pack and that cell will always empty first. If you keep using it, it will polarity reverse.

Since that weak cell gets hit every time, it will eventually fail, and when one cell in the pack fails, it increases the resistance of the whole circuit leading to a pack that doesn't hold a charge.