Were you aware that batteries can flip their polarities? I wasn’t. Fran Blanche discovered this while testing some Rayovacs that have been in the Star Trek Comm Panel that she hacked years ago.

Fran discover that one of 3 AA batteries in the toy had switched polarity. In the comments EEVBlog wrote:

Yep, this can happen if you have non-matched cells in series, or one dodgy one. The lower charge one discharges first and then the others in series push current through and reverse charge the dead one.

Many products have a warning not to mix fresh and used cells for this reason. This has popped up quite a few times in my videos over the years.

And, are Lithium cells vulnerable to this, too? FellenXD writes:

Very much so, and it can lead to growth of lithium metal crystals that can puncture the insulating membranes in the battery. This will cause a short circuit inside the battery and can lead to some explosive spontaneous self-dissasembly 🙂

Proper battery packs have balancing circuitry for the cells and a low voltage cutoff, which should stop this from happening.