Hi there is a lot of great advice for homeowners who want to add large amounts of renewable energy. I am in a different position, I rent and am bound to move again inside a year yet I am trying to use as much renewables as possible. This means attaching panels where you can and running cables in through preexisting holes in walls etcetera. Again I do this on the cheap and I use rescued cable and leisure batteries I pick up cheap and resuscitate if need be. Because of this everything needs to be kept very simple and low level to keep safe. Basic safety rules I stick to is 1) fuse everything, 2)only ever stick two 12 volt leisure batteries together and over estimate likely cable width on all peripherals, with them big thick ~1cm cables between paired batteries. My question is this. I have another couple of 12v leisure batteries and I would like to be able to set my system to automatically charge them when the first pair are fully charged. They are lead batteries like the others so the solar panel controller is compatible. Is it in theory OK to use a single pole, double throw relay to measure the first battery packs charge (on coil) and to have the relay switch the battery input/output to the solar control to battery pack B. I assume the relay would need to be set at something like 14.6v, would that be right? Is this a daft idea? Will the relay go crazy on and off, if so how can I avoid that? is there something on the market that does this for me for a few quid anyway? The two battery packs will be several meters apart so I need a solution that only requires thin cable, which is why I thought a relay would be good. Any thoughts? Thanks in advance.

Adam