This simple option would allow the use of the load output for a solid state relay or normally open relay contact to control mains voltage to a back-up charger.
As it is now, it needs a normally closed contact and the relay coil is powered in normal operation. A solid state relay is not an option. I tried using a sink resistor on load and the solid state relay input from Vbat to load. That seems to confuse the load output circuit.