Rechargeable 405070 LIPO Battery ( 3.7V 4000mAh )

From the shop's description

These are very slim, extremely lightweight batteries based on the new Polymer Lithium Ion chemistry. This is the highest energy density currently in production. Each cell outputs a nominal 3.7V These batteries require special charging. Do not attempt to charge these with anything but a specialized LiPo charger. Battery includes built-in protection against over voltage, over current, and minimum voltage.