Dave Platt wrote:
> In article <1smibg5oe0fmg55cm...@4ax.com>,
> Unlisted <unli...@nomail.com> wrote:
>
> >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.
>

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.