On Mon, 22 Jul 2019 05:06:05 -0700 (PDT), "
wrote:

As to how you would get 'negative' voltage. Consider what happens when you connect your VOM with the leads reversed.

The chemistry in that battery had "flipped over" such that it was producing current opposite to the marked poles. Not uncommon with cheap batteries that often blow up even before being unwrapped.

Peter Wieck
Melrose Park, PA
I know the chemistry somehow reversed. What I would like is an
explanation of what happened in detail. How the chemistry reverses.
Eric