BL Revo or BL E-Maxx?

[Serum]

The receiver/servo's/speedo need power to work on; they normally work on 5-6V, since the batteries are usually higher in voltage, it needs to be transformed into 5 or 6V. Normally a BEC (Battery Eliminating Circuit) takes care of that, but since the normal BEC's use a circuit which works rather inefficient, (with a simple transistor) and are rated for a rather low voltage you need a special switching BEC that takes care of that. You could also use an external receiver pack, next to the batteries you use for powering the motor. This adds weight and it needs to be charged too.