You don't need ALL that battery, you may be burning up your plug. A C-cell will work just fine since your only using it at below 1/4 throttle.
One system I use on a S.T. 90 is a Radio Shack nicad D-cell and a toggle switch.
I turn on the switch, start the engine and fly with the battery on all the time. I have put four 15 minuet flights on the plane in one day and the battery still had juice in it.
My Saito 180 uses a C-cell nicad OBG system I made but it's set up to shut off at above 1/3 throttle, it will last all day as well but I must remember to push the throttle past 1/3 after killing the engine and turning off the receiver otherwise the driver stay's on and drains the battery.