Will these work without a battery in the system?
Help me understand...you want a battery-low voltage protection circuit that works in a circuit without a battery???
Seriously,with such a question,I think you should hire a professional to work on your project. While it is only 12 volts,it can kill or cause much damage and mayhem.
I worry as no inverter I know of has an 8 volt warning and most ,of any power capability,have automatic low voltage shutdown at 10.5 to 11 .2 volts under load.