Why won't my android phone charge from my computers USB port? It actually seems to drain the battery
To expand on Matthew's comments, from wikipedia (http://en.wikipedia.org/wiki/USB):


  As with previous USB versions, USB 3.0 ports come in low-power and
  high-power variants, providing 150 mA and 900 mA respectively, while
  simultaneously transmitting data at SuperSpeed rates.[25]
  Additionally, there is a Battery Charging Specification (Version 1.2 –
  December 2010), which increases the power handling capability to 1.5 A
  but does not allow concurrent data transmission.[22] The Battery
  Charging Specification requires that the physical ports themselves be
  capable of handling 5 A of current[citation needed] but limits the
  maximum current drawn to 1.5 A.


In other words, it's worth it to try as many sources as you can find when looking for a good charging port.