ORIGINAL: CGRetired
And, another good reason to check before the flying begins on a flying day to make sure that the battery is up where it belongs. Then, upon landing, since it is not practical to dismantle everything to check batteries, the voltwatch 2, for example, obviously there others, do just fine because they do check under the existing load.
CGr.
Every plane I've ever built has a charge jack, no need to pull wings or pop the canopy to test battery condition. These are work great,
http://www.troybuiltmodels.com/items/MPI5470.html
It's a combo switch, charge jack, and battery status. Never had one fail and I've put many of them through heck.
Hints: Since this is only a three light, don't start a flight with yellow solidly lit. If the plane is a high wing, if possible mount it under the wing, the yellow light will stand out on a fly-by in the shade of the wing, think about landing soon if you see it. Always mount the switch so that it is "power on" to the rear and "power off" towards the front. The wind or a brush with something wont turn it off and the switch will turn off on impact from inertia. (hopefully few of those). Never mount any switch down wind of the exhaust or fuel spray or leach points like the carb or cowl outlets. Don't over tighten the mounting screws and do use removable thread locker sparingly, (on the threads only!) thread locking fluids cause most plastics to crack in time.