I don't understand your comment about disconnecting the second battery, does it work with only one battery connected?
A general question don't chargers generally stop charging or switch to trickle charge when the battery is at 12 volts or so. Then why does the capacity of the battery make any difference. Two batteries connected in parallel would feed each other, that is the weaker one would get charged by the stronger one. I wonder with a volt meter would the reading be the average of the two. Anyway I don't think a charger would care how many batteries are connected. So if it works on one battery, hook up the second and don't mention it to the control panel.
For some reason my old EBL stopped charging the engine battery, probably due to something I did so after a couple of years of getting a dead engine battery I hooked a wire directly from it to the other two house batteries with an on off switch and a fuse on each end. So they all get charged together from current or the solar panel. The idea being to turn off the switch before starting the engine for fear it would blow a fuse or something, then of course I forgot and nothing bad happened.
Hi, think you have misunderstood my original post, I shut the system down to fit a second battery, the comment I made later in the post" reverting to 1 battery" was simply an attempt to give full information of what had occurred to anyone with possible helpful advice.
I've not suggested that the charging system does not work indeed in my reply to 747 I stated that the charging was fully operational.
Having now had time to fully research the problem, it would appear 99% certain that it is the LT500 panel that is at fault.
In reply to your general question, a 12 volt battery requires somewhere in the region of up to 14.2 volts to charge, a fully charged good condition battery will be at 12.6 to 12.9 (or so) volts
A decent modern type of charger will put maximum charge into a battery until it is at around 75% charged if reading with a multimeter about 13.8 volts, then will progressively drop the rate until it reaches full charge but will continue to supply a charge to maintain a level of around 13.8 volts.
If you are reading only 12 volts on your multimeter then you're battery is only around 25% charged.