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Re: Reversed polarity on AA Alkaline batteries????? (2023 Update)

I picked up a plastic robotic horse at a rummage sale for the kids. It

>walks and makes sounds. But it didnt work. For a couple bucks it was >cute enough the kids would like it even if it didnt work. I removed the >4 aa alkaline batteries and using my multimeter I found 2 of them were >good and two of them were weak and had the polarity reversed. > >They appear to be the batteries that came with the toy. The toy is >dated 2018. Both toy and batteries were made in China. > >How can batteries get reversed polarity???? Thats a new one for me.... >(Of course if they were made in China, so anything is possible I guess).

I believe that can happen if you have several batteries wired up in series (as here) and one or more of them is substantially weaker than the others. If you keep operating the device beyond the point at which the weakest battery runs down to zero, the other batteries will keep pushing current through it. This can create a situation called "over-discharge", and depending on the specifics of the battery chemistry it can cause the over-discharged battery to "charge up in reverse" a bit, and develop a reverse-polarity output voltage.

Over-discharge will damage NiCd (and I suspect NiMH) cells, and I believe it can cause alkaline cells to start leaking.

The better brands of alkaline batteries usually come with an on-the-package warning that all of the batteries in a set should be replaced at the same time, and that you shouldn't mix different types of batteries in a set. This is why. You really want 'em all to run down at the same time.

Reply to
Dave Platt
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Dave Platt wrote: =====================

** +1

.... Phil

Reply to
Phil Allison

Would not have expected to ever find a battery with reversed polarity, but yep, there it was. Very weak charge, which made sense when I sussed it out .. especially as it was one of two AA batteries in the device I was using (the other one tested Strong). I replaced both with different-brand-off-the-shelf-out-of-the-package-pre-tested-verified-fresh-new-batteries. If the situation happens again, it will be very interesting to try and map the occurrence back to the device itself, as perhaps it's internal wiring could be the root cause. And that is all I have to say on that. Cheers mates!

Reply to
ben ford

This is exactly the reason. When I was a child toys were fully analog and had no low battery cut-off, so I used to run my remote controlled car until it stopped completely. I had a battery tester and commonly saw the needle go the opposite direction with some cells. It used four AA cells in series. Once I was curious and installed one of these cells reversed in a clock and it ran some days.

To avoid reversing rechargeables moden gadgets have a low-battery cut-off, for example if using four 1,2V cells it should cut at 3,8V so that the worst case three full cells and one empty will not reverse the empty one.

Reply to
Jeroni Paul

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