How old is the battery?
Try disconnecting it and see if the voltage rises
Disconnect all the 12v from the panel and measure the voltage again, if it's 12v then connect them one by one until you find the one that's dragging it down and start there.
Yeah, any decent ones keep the temp, but strangely or not I can't re boil a kettle, I always empty and refill it. Not sure if true or not but I remember something about boiling oxygen out of it or something?