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.
Its internal battery system do not self-charge, like 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, dead batteries can always be disposed of and replaced with fully-charged ones.