A battery eliminator circuit or BEC is a feature of some electronic speed controls which allows an electric model's drive battery to power the onboard radio system as well. The BEC can sense the voltage drop caused when the battery has little charge left in it. It then cuts the power to the drive motor in order to provide the control servo(s) with enough power to quickly bring the model safely back to the operator. In the case of an electric aircraft, the power to the propeller would be cut but the operation of the control surfaces would be maintained in order to perform a "dead stick" landing. Without this feature, all control would be lost when the battery expired, almost certainly resulting in the destruction of the model.
Eliminator circuits are also used in some full-sized all-terrain vehicle applications to reduce the weight penalty involved in carrying the battery. The circuits frequently involve replacing the battery with a large capacitors that serve to smooth out the electrical pulses coming from the alternator. Loss of the battery means that any electrical starters cannot be used.