What you need is a 11.1V 2100-2250. The 400 likes its juice and will destory anything weeker than 2000mah at full power in a hurry. I run my 400 with a Thunder power 1900mah 11.1v and you dont want to go full power for more than 5seconds otherwise you will begin to damage the battery. Something like Thunder Power 3cell 11.1 2100mah and or go to www.maxamps.com and click on the 2250mah link and look up the 2250mah 11.1 3cell. That would be just about Ideal if the battery diminsions fit your specs. The Thunder power packs are kinda compact. The best combo I have had so far was the 370 with the 1900mah 11.1v thunder power and the Phoenix 25. It will lift something like a YAK, or Tribute off the ground and stright up at 80% throttle. In dealing with the brushless and lipo stuff you always want to be mindfull of what kind of amps your pulling. A 1900mah has a Maximum Continuous Current of: 19A. The Eflight 400 4200kv motor can burst 24amps and continually run on 18amps. The 1900 3cell is not enough to run flat out for anytime at all without damage to the battery. The Thunder power 2100mah 11.1v has a max continous current at 24amps. This is the kind of power cell required to allow the 400 to pull all the amps it needs to run continous without damaging the battery in any way. It will also be kind to the motor while running it at 110%. I know you are thinking what is the difference of 200mah in the two batteries and its a lot. Just look them up and see there max continuous amps. one is like 18 and the other 24amps. Big difference when you are running a motor that pulls 24amps. You must always be mindfull of how hard you run the 400 inrunner because the thing gets extremely hot. Run in the 75-85% range with a little spike here and there and you will be fine. I hope I have helped you and if you have anyquestions just let me know.