I just purchased
one of these (Portable Emergency AA Battery Charger Extender suitable for the Garmin EDGE 500 - with Gomadic Brand TipExchange Technology) and tested it this weekend on my Garmin 500, which seems to be one of the more troublesome Garmins to recharge while riding. It worked as I'd hoped it would - though I didn't fully recharge the unit in this test. It wasn't that long of a ride. Seems like a neat, affordable ($20) solution for an occasional need to recharge.