choosing the correct relay/isolator

Would like to know how to properly choose the correct Relay/Isolator for My Vehicle to Install an Auxiliary Battery for Subwoofer System. Is there a System to follow in regards the the amount of Amps the Relay is rated at?

It's the max current that your amp/system draws, plus the recharge current drawn by the battery. The latter is the tricky part, a 70AH AGM can absorb up to 100A though usually only for short periods. And adding max amp draw PLUS max battery draw can lead to an excessively oversized relay. Generally I'd suggest a relay that at least matches the total amp etc fusing plus 10-50A for (each) battery depending on their size and type (and level of discharge etc). The interconnect fuses (minimum 2 of, each located near the main and aux batteries) are chosen to protect the relay else the cable (whichever is the smaller). Fuses and relays should be able to handle short excessive currents upon initial battery charging.

Thank you very much oldspark...