There is an easy-to-use charge indicator! When off, press and hold the button. The light will blink a color indicating your charge level - ranging from red to green - and fade out.
Red = really low, less than 10% left
Orange = okay, about 50% left
Yellow = should last a while, 50-90%
Green = good to go! 90% or more
Note Colors are approximate. Runtimes vary greatly with different modes.