Mazda cars with the i-eloop charging system have a voltage drop resister under the drivers seat. The factory alternator charges at 24/25 volts into a capacitor in the fender well. That capacitor then feeds the voltage drop resistor that can then safely feed the deep cycle 12 volt battery. They started using this system in some of the Mazda 6 and Mazda 3 cars in 2014 or 2015 so the parts may be available from the wrecking yard soon. The only hard part would be properly wiring it in but with the factory schematic it can’t be that hard.