Erle-Copter makes use of rechargeable Lithium Polymer (LiPo) batteries. A good rule of thumb is to use 1,000 mAH (milliamp hours) per motor.
For a Quadcopter, a 4,000 mAH LiPo should work nicely. While your copter can use only one battery at a time, we recommend having at least two batteries in stock; more batteries means more flight time. You’ll also need a charging station for your batteries.