I'm posting this for a friend that has a question about charging an aux battery from a solar panel.
I have no experience with this stuff but figured someone on this forum would.
They were camping on a remote beach in Baja recently.
"We used the solar panels to keep the cameras, laptops, and phones charged. During the day we were able to charge everything simultaneously. I have a feeling that if we had a newer battery things would have been even better. I think the battery was topping out at about 13.5V when newer batteries should charge up to 14.5V (or so I've heard). Anybody have experience with this kind of stuff? We were using a 2 x 45W panels in parallel kit charging a gel battery."
Probably they don't have enough solar power to fully charge the batteries.
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Actually proper charge voltage is about 13.8 to 14.1 for a gel cell battery and some manufactures say the float charge voltage can be lower. A fully charged battery will level out at about 12.8 with no load or charge applied after it sits for 24 hrs
I agree with Herb, we would really have to have information on the solar controller itself. Even after that you could have a wide range of other things that could affect the voltage at the battery, and/or where that is being measured.