11.1V Stock tube battery.
Designed to be small enough to fit in 95% of stock tubes. Despite its small size it still packs 900mAh.
The biggest reasons to use a LI-PO is its smaller size and better power efficiency. Most LI-PO batteries are about the same size as a mini-type battery and is equal in power to a large type NiMH battery. No need for battery slings, battery boxes, or big batteries strapped on to guns. LI-PO is a power source for small carbines, PDW's, SMG's, etc. that doesn't require you to spoil the look of the gun, or run the risk of running out of power in the middle of a battle.
Unlike traditional batteries, LI-PO gives a stable Rate of Fire, throughout its use. In addition, it is not necessary to discharge LI-PO batteries before charging them, allowing the battery to be topped off in-between games