BEC = battery eliminator circuit. In the esc, theres a device called the BEC that takes full battery voltage and steps it down for the rx, servos and the brain board (in the esc) to use - usually 6v and maybe as much as 5amps.

There is heat associated with stepping the voltage down the way its usually done, the bigger the voltage step down, the more the heat. Usually the BEC is the limitiing factor driving an esc cell limits.

ou could run 4S and 5S with the old Mamba esc (rated to 3S) if you'd disable the esc's BEC and use an external BEC.