Battery eliminator circuit
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A battery eliminator circuit (BEC) is an electronic circuit designed to deliver electrical power to other circuitry without the need for a battery. Historically the expression was sometimes used to describe devices used to power battery-driven equipment from mains electricity.
In a radio controlled model, a 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 'steering' servo(s) with enough power to quickly bring the model safely back to the operator. In the case of an 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, probably resulting in the destruction of the model.
BECs are also used in some motorcycle and ATV applications to reduce the weight penalty involved in carrying a battery. The battery is typically replaced by one or more large but light-weight capacitors which smooth out the fluctuating electrical pulses coming from the alternator, without themselves generating power. As there is no electrical power source, electrical starters cannot be used.
In the case of the radio controlled model BEC's its simpiest form is a linear fixed [[voltage regulator]] with its standard circuit suggested in the manufacturers datasheet - usually the power supply of the receiver needs 5 V. Low drop types are prefered - exspecialy for batteries with only a few cells. For Small models 1.5 to 2 Ampere are enougth, for midsize models a 3 Ampere type needs to be considered. BEC's for large models have to provide current of 5 Ampere or more. Here a more complicated switched mode regulator should be used, as the BEC has to deal with losses. These Losses are proportional to the difference of the target voltage of 5 volt and the voltage of the main battery, as well as they are proportional to the provided current. For example a 10 cell NiMh(nimh) akkumulator with a normal volatage of 12 Volt. With a peak current of 5 Ampere the BEC will have losses of (12V-5V)*5A = 35W. With a linear regulator these 35W will be converted to heat and so require a large heat sink. In all cases it is a good idea to mount some large capacitors to buffer the regulated output. In large plane or ship models another posibility is to buffer the power supply with a further capacitor near the aktuators (Servos).
