I think you need something better than an "idiot" light. An LM3914 would give a better indication. You can make one or more LEDs yellow to indicate caution, some red to indicate dangerously low, and some green to indicate okay.The purpose of the circuit is to tell the pilot if they need to charge the battery before starting the system or if they can start the system and there will be no problem for 20min only using battery power.
The IC can drive up to 10 LEDs.