We have always recommended AW cells for powering the Billet Box, but there are many other batteries that work great too. Until recently the Black Efest cells were just fine, however a recent change in their production has the cells way over sized and will no longer fit in the Billet Box. The numbers on the battery are a representation of their size, the billet box takes 16340, this equals 16mm diameter x 34 mm long. The new E-fests are 37 mm long, and will not fit in the Billet Box. Forcing them in will take a massive amount of force and will likely damage the box.