A Universal Battery Elimination Circuit (or UBEC, for short) is needed if the Electronic Speed Control (ESC) being used for the motor does not have a built in Battery Elimination Circuit (BEC), or if the BEC of the ESC is inadequate to power the number and/or size of servos being used.
Using a UBEC is a safer way to go on large models for the following reasons:
1. Most built-in BEC circuits are one- to two-amp "linear mode" circuits, which are only useful for two or possibly three standard-size servos when using a 3-cell lipo battery. If you use more servos, or a higher voltage battery pack, you will almost certainly overload the BEC, causing a crashed model.
2. In an ESC with a built-in BEC, excessive heat generated in the ESC by the current draw of the motor and/or BEC can cause total loss of power to the receiver/servos, resulting in a crashed model. The chance of total loss of power is greatly reduced when a separate receiver battery pack or UBEC is used. If the ESC overheats and shuts down (no power to the motor), you will still have power to the receiver/servos, and will be able to maintain control of the model.
Installing a UBEC
The red (+) and black (-) power input wires of the UBEC are connected directly to the main battery pack of an electric model, the same as the ESC. In fact, the power input wires of the UBEC are often spliced into the power input wires of the ESC, so that the UBEC and ESC power on at exactly the same moment. The power output wires of the UBEC have a receiver connector on them, and this is usually plugged into any open channel of the receiver.
NOTE: If the ESC being used has a built in BEC, that BEC must be disabled when using a UBEC. To disable the BEC of an ESC, simply disconnect the red wire on the receiver plug of the ESC.