bkenobi wrote:
I have no problem buying a pair of batteries, a generator, some level of solar, etc. I have a problem buying something without knowing it will help or understanding the problem I'm trying to solve.
Good for you.
Assuming that the batteries are 12 V each and are wired in parallel.....The kind of problem you are seeing is EXACTLY why the consensus recommendation is to NOT do 2 twelves in parallel because if one goes bad it drags BOTH down and complicates the testing.
You need to catch it when it is down around 10.5 and then disconnect enough cables that you can measure each battery individually with the converter OFF.
Hopefully that will let you identify which one is really going bad.
If that doesn't work, you can run the system for a while on only ONE battery to see which one is bad.
BUT.....if both have really gone down to 10.5 or below several times and stayed like that very long, then even the "good" one probably has been weakened too.