Think about the battery state monitor.....
One of the surplus battery alarms from the MightyBoy electric vehicle project.....
I have six of these from the early MightyBoyEV project that are now surplus (updated to Li-Ion Technology a couple of years ago - www.MightyBoyEV.com )
They will be modified for 24 volt use - and trigger an LED as a warning when approaching a low battery condition followed by an audio alarm at the critical state.