battery charging

I think there are modern fuse taps that plug in to the fuse box. I installed one, but now cannot remember which vehicle or what for. It will get the job done without messing with wiring.

Adding on a 12V to 5V converter for USB socket will be a universal way to charge any device like tablets, phones....

A gel cell floats at 13.5V, so it will overcharge, dry out, with long term use in a normal flooded lead acid vehicle. A Schottkey diode 0.3V drop, in series will likely provide enough drop, and will prevent the little battery from trying to supply vehicle loads. Choose an amp rating sufficient for battery capacity.