Are you using this for racing or bashing? If you're racing, the sanctioning body has rules on battery types allowed. if you're bashing, check with Castle Creations to see the maximum voltage the ESC/Motor combo is rated at. 3.7 is 1s. 7.4 is 2s and 11.1 is 3s. Always try and get the largest capacity in mAH that you can afford. The more capacity the longer the run time.