Some battery packs have a "smart shutoff" feature that turns the battery pack off when it detects a low current draw.  This is used when charging battery powered devices like cell phones and keeps the battery pack from being drawn down unnecessarily.  SMOBOT only consumes about 80mA of current, and will often not be enough for these smart battery packs.  Look for a battery pack that does NOT have this smart shutoff feature.  A very cheap 2200mAH battery pack that you can purchase for $5 at most convenience stores is great and will run your SMOBOT for around 24hours or more.