Not a question for the forum, just a learning to share with those who, like me, may never have come across this before.
One of my LoRa sensors stopped working yesterday because it's battery was flat. It is powered by 3xAAA NiMH cells of the "stay-charged"/low self-discharge variety. I thought this was strange, as I had only replaced them a couple of weeks before. My first thought was that the wires from the battery holder had failed, this has happened to several similar holders I purchased together. So I measured the voltage from the wires, which, as expected from what the sensor was reporting, was less than 2V. Then I removed each cell and measured them separately for comparison. Two cells measured around 1.4V, but the third measured -0.9V!
Apparently this phenomenon is called Battery Polarity Reversal and can happen when multiple cells are used in series. The cells in question are all the same brand and capacity, and I assume must have been purchased together because they are the only cells of that brand that I have. But they were quite old and must have been reused many times. One of them must have been starting to fail before the other two. It's voltage dropped more quickly, and then the other two cells, which were still stronger, had "reverse charged" the weaker cell.
I recharged the faulty cell and it went back up to 1.4V. I am waiting for my smart charger to tell me the cell's capacity. I expect will we be pretty low.
Needless to say, this cell will be going into the recycle bin.