I don't think these devices are very useful, they simply indicate the NO-LOAD voltage. Pretty much worthless information IMO (albeit you can stir the sticks and observe the voltage reading). An expanded scale LOADED volt meter (.5 to 1.5 Amp load) on the other hand is a must again IMO. The ignition system cannot load the battery dynamically like a flight system accordingly the information is of little value with regard to the reserve of the battery under test.

Anywho even ESV dynamic battery loading only offers a kinda-sorta-maybe feel for what’s left in the battery. A poor mans fuel gauge so to speak. One can garner information of the specific battery pack and develop a relationship with regard to expected discharge time intervals, characteristics and the like. This allows you to form an educated “feel” for the batteries well being and expected flight times.