This is the RX battery voltmeter that I use. Like I said, i fly it until it says 70% and then quit. That may be conservative, but it's worked for me so far.

http://www.horizonhobby.com/Products...ProdID=EXRA502

The Voltwatch is a similar setup, but you mount it right to your plane and you can always tell how your battery is doing.

http://www3.towerhobbies.com/cgi-bin...?&I=LXHDJ2&P=0