micky was thinking very hard :
I have a CR2032 mercury battery, and when measured with a digital VOM,
it starts around 3.5v and climbs slowly to a little over 6v .

What does that mean?

It's a cheap meter but it's worked fine for years, especially for DCV.

Bad battery? Sometimes a car storage battery won't turn an engine over,
but using the headlights for a while heats the battery and it works
better at the new temperature. Maybe this is a miniature form of that
phenomenon.