Hey, Jove, had a cheap to do idea for an easy voltage check.  The cigarette plug got a lighted double USB plug that also shows the voltage from my 12 volt reducer for lights, smokes, or whatever.  So, I just plug it in fully, and there is the voltage, for a 12 volt single batt.  Just double that and thee is the 24 volt status, or use the single figure to see when to turn back home.  A common sense idea, that backs up my depth finder going dark at 11.5 volts -- or a low battery pack overall.

Catch you Later, ---  Cal