LiFe Receiver Battery, 2200mAh 2S 6.6V, by Spektrum

Lightweight and simple, these LiFe batteries offer a high discharge capacity with no memory effect. The slightly lower voltage of LiFe packs allow for their use with standard servos without the need for regulators. The energy density of LiFe is much greater than NiCd and NiMH, resulting in a much smaller size and total weight for equivalent capacities. Due to the Stability of the chemistry, these cells are noted to have far greater shelf life as well as working life span over Li-Po batteries and even NiCd and NiMH batteries.