The Nitecore NL2150HPi is a proprietary 21700 battery designed for use with multiple Nitecore products including the i4000R and the 21700i Series of accessories. This battery has both positive and negative polarities at both ends combined with a super high energy density. This allows the battery to have an efficient output while also minimizing power loss from internal resistance. The NL2150HPi has many internal and external safety features including a built-in protection circuit and a thermal regulating design to keep your battery safe and working in peak condition.