The Station Builder Tug is a maneuverable, dockable craft. Once attached to the station, it can be used to move or re-position the station from a central point. Due to its very large fuel capacity, it is often used to position station parts and attach them to stations, thus its name.
It has no solar panels or RTG reactor, and therefore the internal battery system does not self-charge unlike certain spacecraft, such as the Stand-Alone Laboratory, Soyuz, Orion, or Draco Trunk. It uses 6 power, and must be docked to an external power source to allow its batteries to recharge.
Of course, if one is in a hurry, dead batteries can always be disposed of and replaced with fully-charged ones.