Use any 4 cell lipo, but I would not go under 3 Amp Hrs. That will cost triple digits though. Also to avoid damage, make sure there is an on/off switch to disconnect the battery, otherwise it has the possibility of slowly draining, and figure out a way to add a voltage cut off, otherwise you wont feel that it has lost too much voltage therefore damaging the cells and turning it into a dangerous paperweight. I bought a voltage alarm for one of my helicopters that you wire in parallel to the battery, and it is programmable at what voltage the alarm goes off. It is the CSM Lipo Alarm.
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