It needs a separate power source.
The best thing to do is splice a 9V connector onto it then power it off a rechargeable 9V battery (2-3 hrs of battery life), an 8-AAA battery holder from radioshack (7-9hrs of battery life), or an 8-AA battery pack (20-25 hrs of battery life).
Mine's been working flawlessly for a year of nonstop daily commuting.

One thing to keep in mind is, if you have a rough commute, have some kinda strap around the 8-battery holders so the batteries don't pop out of the contacts while in your saddle bag.