This is the original Big Brother in a two cell AA box. It uses a linear regulator that is not as efficient as the Vari-cool model, but still produces good battery life. It operates between 0 and about 6.5 volts. Dial it to zero when putting in pocket. It uses two 14500 3.7v LI protected or high drain batteries, not included. Size is approximately 1.27X2.6X.6. The high drain batteries might require magnetic spacers to work in the mod. Kits are available by ordering the Big Brother kit option. There is a 7 day wait on the Big Brother mod