Jug,
I’ll try to give the quick version, sorry for the long response. And sorry if I tell you stuff you already know.
As ee79 said, nominal voltage for this battery should be 12.8-12.9 under no load. As stated, Just being connected to the car puts a minimal load on the battery for things like alarms and clocks or whatever so the theoretical 100% state of charge won’t necessarily always be observed. The other thing is that as the battery ages, 100% gradually becomes unattainable. The last thing I’ll say is if you open car door to pop the hood, you won’t be reading resting voltage because the displays come on and the car powers some other untold number of things (you can hear what sounds like some type of fan under the hood near the back of the instrument cluster very quietly running for 3-5 minutes that costs about .1v from what I’ve noticed as well as what sounds like the motors for the air conditioning duct system cycling two times after the “key” is off. In this case the load doesn’t shed for several minutes and then resting voltage takes some amount of time to come back up. I can’t say exactly how long, but at least an hour or two probably. Maybe more.
When I had the battery drain issue, my battery would only charge to about 12.3-12.4v with an AGM charger. I charged it at the remote terminals. After it rested for several hours it would usually read 12.2-12.3v. This reading was also taken at the remote charging terminals under the hood with a standard electrical multimeter.
All of that to say this: one night I ripped the battery cover and AC vents out and disconnected the main battery. Read the voltage on the battery terminals. It was close to what I had been reading under the hood before disconnecting the main. For grins i went back to the remote terminals and I was still getting 5-6 volts on the meter with the main battery disconnected. If I opened and closed the door the car’s displays were still trying to come on! It dawned on me that I had isolated the aux battery and it was in fact tied to the remote terminals as well as the car’s electrical system and it was reading 5-6 volts. I thought I had found my problem so I accessed the aux battery under the passenger seat and took a reading, sure enough 5 or 6 volts. When I disconnected the aux there was no longer power at the remote terminals….or the car.
Out of curiosity I reconnected the aux, checked voltage, then connected the main and checked voltage on the aux. To my surprise, the aux was up a volt or two and the main was down some amount. When I pulled the terminals off again the aux was back down, main was back up. What it appears is that these batteries are not isolated from each other and, despite being very different capacity-wise, are connected in parallel. I disconnected and then charged both batteries fully and when I checked the resting voltage the next day (batteries still disconnected) the aux was 12.8 and the main was back down to lower/mid 12’s voltage. The dealership diagnostic ID’d a bad main battery. To be honest I’m not sure that they tested the aux.
Bottom line is this, I had a what I believe to be a failing cell in my main battery (probably due to multiple deep drains) and it was “stealing” charge from the aux battery. This was after the latch was replaced, but I knew the battery wasn’t healthy prior to that because I had been watching it.
In your case, it appears one of the batteries is getting near the end of its life, but still might be testing “fine” as the dealer says as long as the current it’s providing is within their parameters. Or you still have something in the background draining it…but I hope not!
