Cylindrical lithium ion battery with JST connector. The included protection circuitry keeps the battery voltage from going too high (over-charging) or low (over-use) which means that the battery will cut out when completely dead at 2.5V. However, even with this protection, it is very important that you only use a LiIon/LiPoly constant-voltage/constant-current charger to recharge them at a rate of 0.5C (1 Amp) or less.