There are battery pack devices that take 4 AA batteries and charge usb devices from them. If you decide to get one, make sure it has a voltage regulator. I have the Tekkeon mp1550 ($20) which works well with nimh rechargeables. I think using alkalines in such a device though might not be that efficient, and perhaps a bit more than half the power will be wasted as heat. You could pre charge several sets of 2900 mah nimh batteries in a one hour charger, or buy some Sanyo Eneloop or Duracell precharged 2000-2100 mah low discharge rate nimh batteries.
There are usb crank chargers, and usb solar chargers, however I have not tried those. Another option is to buy a large capacity lion battery pack.
