Another advantage to the onboard volt indicators is that if you have a servo going bad or sticking and draining too much current without you knowing it, you will get a low voltage indication telling you to check things out. Another idea is to use a charger that indicates how many milliamps charge a battery takes. For example, if you usually have to recharge 100 mA after a flight, and suddenly your having to recharge 300 mA, something is wrong, fix it to prevent crash. Or if you are using 500 mA per flight, and have only 600 mA battery, you better get a bigger battery!