I'm a newbie at all of this... and on a budget. Can you point us in the right directions amongst these ubiquitous multitudes ?
Something like the
Dahon Reecharge or
B&M E-Werk. On a budget they aren't, though. There are designs for homebrew equivalents on the web, of varying levels of cleverness (read: efficiency), which will work out a lot cheaper, but require a bit of electronics-fu.
USB battery chargers are much easier to come by. You can probably find them in your local supermarket. The problem with those is that they tend to be limited to charging a pair of cells in series at fairly conservative rates.
One of the better options I've found is the
SCH600F charger. It has 4 independent charging channels, and will run on USB-spec power. It's a bit cheap and plasticy, and there's a big 'refresh' button that will need to be molly-guarded or disconnected internally to avoid accidental discharge, but it appears to work.
I conversed with the folks at Pedalpower about this and they were talking about taking 11 hours to charge 2 x 1000mAh AAA unless using one of they Li-Poly batteries, which adds a level of complexity and cost I am keen to avoid.
That seems awfully conservative, or awfully inefficient. I'm not sure which.
The problem is, there aren't any really nice solutions to this, and the commercial products are inherently niche and therefore expensive. That's a large part of the reason I'm designing my own: it'll still be hideously expensive, but at least it'll do what I want it to do.