Spektrum RC DX8 2S LiPo Transmitter Battery (7.4V/4000mAh)This is the Spektrum DX8 LiPo Transmitter Battery. The 4000 2S LiPo offers not only double the flight time as the stock DX8 battery but also advanced safety and features. It also includes a built in safety current that shuts down the battery, to avoid overcharging and damaging the battery and radio, as well as a built in balancing system to make sure you get the longest life and power from your pack. Features: Built in cell balancer Built in safety