Power source brakes
Yes the brakes requires battery power to work , if no other source is available , ie ext power or Apu. In case of towing the fwd battery ( main) will be used as pwr source to release and apply the brakes, if batteries for instance are removed from the aircraft with brakes set to on, the brakes cannot be released until ext pwr or Apu are available. The brakes are electro mechanically operated . The aft battery( Apu batt) can also be used to pwr the navigation lights for towing if no other pwr source is available , ie broken Apu or not want to start it.
That's how the system is set up to able towing with lights partly on and Apu off. Captains radio is powered by batt ( fwd) so radio ca be used if needed to tow with Apu off