I put one of those warning beepers in a pane in Hawaii due to the rough terrain that a plane can get lost in after one day of the annoying beeping I canned that thing but fast. I used volt watches on my planes. They are cheap and easy to use. Look at the lights when you move the surfaces on preflight. If the lights that are yellow light up do not fly until you charge. I always did the readings while moving servos to get an idea of loaded voltage as opposed to just sitting there and getting a no load reading.