I believe LiPo batteries are actually the batteries with the highest power density on the market currently. They're extremely powerful, and highly compact. The only downsides to them are the fact that they require special chargers and the fact that they are a bit unstable if not used carefully. If they're punctured, they explode in a fireball. They also can swell up when used improperly, which is a sign to dispose of the battery immediately. They're a bit dangerous, but they're just about the only battery that's both small enough and powerful enough for this build.