The combiner will charge both if 1) "on" is selected, and your cranking battery meets the set charge criteria. If it doesn't meet that criteria, your motor alternator will continue to only charge your cranking battery until it meets that threshold. This is the isolation protection to ensure you can always get home. Or 2) "combine batteries" is selected, which is the 1+2 position from the 4-position switch.So the Blue Sea Charging Relay allows both batteries to be charged by the motor IF the battery is switched to both?
I don't crank that much so it's possible throughout the day I'm always above the voltage limit and my house is getting constantly charged. But I have the benefit of the built-in safeties.