I've read many threads about people using deep cycle batteries and disconnecting their alternator. But the problem or me is that i live in an apartment and my parking space doesn't have an outlet.
So i got the idea of using batteries - unfortunately, lead-acid would be too heavy and lithium ion-somethings would be too expensive. Luckily (like any tinkerer) i had rescued cells from dead laptop batteries! (typically only one pair or triplet is dead and the other cells are usable - albeit with reduced capacity)
So i built this thing: (all materials free!)
Nominal capacity is 8.8Ah, but unfortunately the combined age of these cells is something like that of the Rolling stones, but otoh i have tried to be nice to them by keeping them at 60% charge and storing them in my refrigerator.
It's not completely finished yet, it needs a lid and a BMS (that's that the little white wires are for) and maybe an amp-meter. The BMS i had planned to implement by having diodes that begin conducting at about 4.1v connected to each parallel set, that would allow the sets with lower voltage to catch up during charging. I have no system planned for discharge balancing as i have yet to come up with a simple system for that. (Maybe you have an idea?)