This issue comes up here in the beginner's forum quite a bit, and there's a lot of confusion about it every time it comes up. Normally the discussion is about the receiver battery, but it's the same with the transmitter battery too because all batteries have a similar discharge curve. First of all, if you watch the voltage of the battery only and quit when the voltage hits 9.6 v (4.8v for receiver batteries) then you are losing what could be as much as 50% of the batteries capability. Batteries do not have a linear discharge rate, meaning they do not drain constantly. Instead (see attached diagram) they will quickly discharge down to ~9.6v (again, 4.8 for receivers) and then stay at that level for long time, and then after that they will quickly drop off towards being unusable. How long your battery will stay at that 9.6v level is hard to say, this is why batteries need to be cycled. Cycling the battery will let you know how long that battery will last in a usable state. If you don't have a battery cycler you can just turn on your radio with a fully charged battery and observe how long it takes to drop down to 8.8v (1.1v/cell times 8 cells), since radios have a constant power draw you can get a very accurate idea of how long your radio will last. Two very very important notes here, if you do this make sure that you have your antenna fully extended when you do this or you will burn out the RF transmitter portion of the radio, and secondly make sure you are not withing a 3 mile distance from any flying field.

If interested everybody should visit the [link=http://www.rcbatteryclinic.com]RC Battery Clinic[/link] . Anything and everything you ever wanted to know about the batteries we use are covered here. It's definitely a must read for anybody in this hobby.

Hope this helps

Ken