Low voltage switch (12v)
To charge lead-acid batteries, you need a proper regulated or "smart" battery charger designed for that application. 

A "smart" charger will have three charging states - bulk charge (full current until 14.4 Volts), acceptance (hold 14.4 Volts til current drops to an Amp or so), (I've forgotten the other term...)

12 Volts is not sufficient to charge a "12 Volt" lead-acid battery.  A fully-charged battery will be over 13 Volts.

If you have invested in 800 Ah of batteries, surely you can afford a proper charger to maintain them.