I am not quite clear on what you are trying to do.
If you are just trying to "maintain" (from another post) a pair of 110AH marine batteries for LED lighting, then that does not take a whole lot. Certainly not the 20A mentioned elsewhere.
Some people use a modified version of the Ghurd Controller for a backup grid powered charger.
When the battery gets low (12.25V), then the relay feeds grid power to a battery charger.
When the bettery gets full enough (maybe about 12.8V, but depends on the system size and loads) the relay shuts off grid power to the charger.
Works fine with cheap simple transformer chargers.
G-
SSR is Solid State Relay.
The "about 500 ohms" needed reduced to maybe 220(?) with his SSR.
Notice both fets are N-channel.
