Powerbank

[oskar23]

Which one is worth $ and doesn't lie about capacity?

[LeapFrogMasterRace]

Anker makes good ones depends on how many mAh you want.

[W-L]

Which one is worth $ and doesn't lie about capacity?

 

Anker or Aukey makes some good battery packs:

http://www.anker.com/External-Batteries/category-c1-s1

[Samfisher]

Careful about some aukey ones.  There's one that has quite a few bad reports of the casing completely melting off after a few charges and just dying.  Yes, it heats up so much that the plastic melts.