Battery charge when sun goes down

[DannyB1954]

You realize one 12 volt battery is six cells in series, just like two 6 volt batteries?

With one battery it is not too hard to figure out which battery is bad.
With more than one 12 volt battery you are back to separating them for a while without charging or draining and comparing them.
I would add it is not a bad idea to charge them separately then let them sit.