Because member SouthArk370Z is an amazing and helpful member, after I had posted my response and closed the thread, he sent me a PM stating:
I got to thinking about you getting the high (12.99V vs 12.4-12.6V) reading on your battery when not being charged. Lead-acid batteries will hold a "residual" charge for a few minutes - something about local spots of higher potential on the battery plates IIRC. The "extra" charge will equalize over time - I've seen claims of from 5 minutes to 12 hours. You may get a more accurate reading before you crank up in the morning. There will be some drain on the battery from the always-on electronics, but it shouldn't a large drain so voltage should still be above 12V after sitting all night (but I'm not sure just how much drain is involved, so don't hold me to that figure).
Probably not that important but thought you might want to know.
I replied:
Man oh man, you're awesome for coming back to shoot me a PM!
I really appreciate it. I got the 12.99 after about half an hour, but tomorrow morning I'll check before I fire it up!
Today after the car has been sitting all night it's at 12.82V, and I tried 2 voltmeters, and they tested the same, and they both tested a brand new Duracell AA battery at 1.79V, and my home AC outlets are at 112V... The car/garage is around 60 degrees F, sooooo, not sure how much that makes a difference, but I assume colder would have a lower readout vs before when the garage was 70 degrees F, and under the hood was quite warm.
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