Black Version 

There are many battery packs available from many manufacturers, in many form factors, for a wide range of prices, but the problem is that most of them may as well be packaged air for all the good they will do you.  Most of the ones sold in convenience stores aren't rechargeable.  Many of these "one-use" devices do let you change the batteries, so they can be "made" to be rechargeable by putting in AA sized NiMH rechargeable batteries, but the capacity is still very limited.  The cheaper rechargeable models typically have a small capacity, 1400-2000 mah (or less!), and most consumers don't know enough to ask about things like this.  Sadly, the fact is that many of these smaller batteries don't even store enough power to even charge your smart-phone once, but they still cost 1500 to 2500 yen or more.

The Qi 5400 easily has more than twice the capacity of the majority of competitors.  It isn't the most stylish thing in the world, however, given its shape, and the fact that it only comes in black in white.  (You can add "Hello Kitty"(tm) stickers and glitter on your own...)

The Panasonic Mobile Pack 5400, on the other hand, packs serious power in a small package.  In fact, it can fully recharge many smart phones from 0 to full power - multiple times!  Of course, it can be used to power USB fans, etc., as well.  The output power can be controlled by the on/off switch on the unit, and an LED shows the status:

In order to charge the unit, there is a micro-USB connector, which can be connected to a computer or dedicated USB charger (which comes bundled with the unit in some packages).  While charging, the LED will glow Orange or Green to indicate the charging status, finally turning off when the charge is complete.  The unit also contains the proper electronics to charge via the wireless Qi standard, and Panasonic sells a matching charging pad.  The best part about the Qi standard is that it is compatible with multiple vendors, so you can use a non-Panasonic charging stand as well.  At any rate, the good thing about this battery pack is that you have the choice:  You can be a Qi early adopter, or stick with basic USB charging.