I have a unit that looks like a charger, but when it receives AC a relay cuts out most 12V devices from the house battery and supplys power to them thru a transformer. The water pump is not cut out and stays on the house battery. This isolates the battery so it can be charge from a seperate battery charger. This charger was missing when I bought the rig and I spent lots of time trying to figure out why the battery didn't charge when the rig was plugged into the AC. Testin the water pump with isolation and non-isolation revealed it just work better when isolated, which is what the rig manufacture intended.
