You didn't say WHICH JR you have... however almost all of them with the LCD display have a built in low voltage warning.

The low voltage warning is a series of beeps UNLIKE the ones used for the plane timers ( you know about the plane flight timers, and TX "on" time timer too don't you? )

Typically you can fly until the low voltage warning kicks in, at which point you have a 2-4 minutes to land your plane before the voltage levels get too low.

The 7+ channel TX's also have a little "notch" on the LCD voltage scale that indicates where this is expected to happen, so you can gauge how much time you have left.

I usually replace the TX battery with something more heafty... e.g. 1800mAh or above, and then I'm able to fly all day long w/o worrying about the TX's battery level.