A UBEC is often used in model aircraft to take the voltage of the batteries down to about 5 volts that the radio controller can then use. The ones that are usable with a raspberry pi can take a voltage of 5.5 volts up to about 24 volts. The UBEC then converts this down to the 5v that the Pi uses, and does it in an efficient way. A standard Pi needs about 700 milli amps in order to run, so be sure to buy a UBEC that supports at least 1 amp. A 2 or 3 amp UBEC is also suitable.