I purchased the Duracell Instant Charger and it works fine too. But it can't be "recharged" during the ride. I'm going to use another external battery that has two replaceable AA NiMH batteries so I can carry an extra pair on this upcoming ride. I don't think I'll need them but it's reassuring to know that I won't run out of batteries when I use a little back-lighting at night.

BTW the Duracell battery is very small and light (and has a convenient on-off switch) so it may be the best way to go once I figure out how much battery life I need. I think the Duracell battery will about double the Garmin battery life but I have yet to test that.