Big lead acid batteries when they are empty can accept 150+ amps, so rate the wires at the maximum current of the alternator (see the Q/A section below).

It will allow the extra battery to be charged at whatever rate the alternator can put out, so it needs big enough cables to do that. Neither will he house battery be used to start the car (unless asked to). The isolator won’t let the auxiliary batteries be discharged by the vehicle’s gadgets. This battery-isolator is used to control extra battery systems that charge off the vehicle’s alternator. This hybrid system is really the best of both worlds, solid-state control and solid-metal contacts. This almost never happens, but this (to us) is a fail-safe condition, the auxiliary battery will still charge, and to fix it requires only some judicious banging on the relay to get it working again.

The worst thing that can happen with a contactor is that the relay contacts will weld together if the big over-current happens while the contactor is trying to open. If you have a 1500 amp spike, the relay’s terminals can probably handle that, whereas MOFETS will simply fail, and when one fails the rest cascade like a waterfall. Seriously, we can design a solid-state battery isolator, but we like the forgiving nature of a big contactor. It uses state-of-the art microprocessor solid state control of the charging and isolation functions, but uses a solid tungsten points contactor relay to control the big currents.

Battery isolator relay