Battery for Futaba R127DF
You can go with a 600 mAH. That is the size battery that comes with the flight packs. I am running 5 servos on my 4 star and had a total of 2.5 hours of actual flight time today (according to my timer on my 9C). I checked the voltage after that and to my amazement the batteries were showing 4.77 Volts and that was under a load. I was surprised!! Remember, Backwing the larger the battery the more your plane will weigh, and with a trainer you want to keep as light as possible...