Made by the same company that manufactures the popular ( electronic flasher) we use when
we installed LED tail lights, Innova.
when engine is off it tells you the battery
charged level. When engine running it tells you
how much the alternator is putting out.
Two years ago while road tripping in North Carolina during the Vintage I pulled into a gas station and glanced over to see a yellow light then the red light. I turned off the engine and saw my battery was at 11 !!!!!
the problem turned out to be a wire had dropped off the volt regulator.
without this I may have run the battery down to
a point that I would have needed a battery charger and it may have been in the middle of nowhere. $24 on amozon
move also used it to help fellow o2 guys diagnosis