Right. It was too big of a drop. Besides, that 10v didn't hold & dropped off to 3.ish something as soon as you hooked up the cables, right?

The lllloooooonnnggg story...
If you overcharge a battery, say 14v, a good part of that is surface charge. When you discharge it, it'll drop down towards 12.5 pretty quickly, but then stay there for awhile. Normally, car's electrical systems run at 13v or 14v (when the alternator is charging). I thought maybe your measurement of 12v was just meter error if you had an inexpensive meter. At any rate, voltage loss overnight is a lot more reliable than measuring a SMALL voltage drop over a few minutes or an hour.

 

Ok so got a fresh battery. Metered at 12.57v. Rewatched . Finally realized he's using the 10A scale. Less than 50mA is fine.

Makes a ton more sense as to the cause pertaining to the battery. Was just skeptical since I got the same symptoms from two different batteries.

Will meter battery again in a few hours to verify it's still holding fine.

Thanks to all who contributed to this thread. Hopefully this is the end of my problem!

 

Ok, just to let everyone know that has contributed to my thread, my issue has been resolved!

Hooray!

Time to sell her now!

Thanks again for all the help.