This device is used to support a house battery for loads out in nature and to maintain a reserve start battery.
The ML-7622 ACR senses voltage on House Battery Bank and the Starting Battery. When charging (either battery) the ACR links the two when set on automatic and charges both (regardless of charging source). When either battery bank is notably lower than the other, it disconnects when not charging to preserve the good battery. The device can be linked by pushing the switch to self jump start the Starting Battery from the House. Yes, it costs a bit more than a simple battery switch. But, it adds reliability and is cheaper than being towed home!
Because it is a magnetic-latch relay, there is near zero "parasitic" draw, which is good for your battery banks. Actually draws <40mA which is to monitor terminal voltages. The switch to the relay uses smaller gauge wire than would be used on a mechanical switch. The device is rated to 500A.
We used it with great confidence for several weekend trips, but have gone a different direction for charging our lithium bank. You would need to purchase a switch, which are available on Amazon:
Blue Sea Systems 2146 SPDT Remote Control ON-OFF-ON Contura Switch
The Blue Sea ML-7622 ACR lists from $190 - to over $300. I am asking $100 and you pay shipping FROM ZIP 32547. PayPal Please.
Offering here first before other marketplaces.