Hi
A power cap..aka. glitch buster.. will only Keep voltage from going below your desired volts say its 6vdc.. that will last for a short time as in less than a few micro seconds .all depends on caps size as in micro ferrads..its a boom and then gone till cap recharges.. also micro seconds.

A bec is hooked to your power source .battery.. and does not have a time limit.. it is constant supply. at whatever the bec is set..say 6vdc 5 amps.

There is a Giant difference.. you might need both..

have you set the castle bec on a higher voltage?