Energizer XP4001, or other portable charger?

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Hi, I had a quick question or two and was hoping I could get some opinions on a reasonably priced "portable charger".
Basically, I'm looking for something to carry that could provide some extended battery life to one or two A855, if needed. Likely it would be a low use item (almost an emergency backup for example if out on a long hike, etc) as I'm usually close enough to my car or an AC power outlet.
Anyway, I was looking at the Energizer XP4001, mainly because of the capability to charge two devices, if needed, simultaneously. Reading the specs, however, if you only charge one the output is 5V, 1500mA, while the droid wall charger lists 5V, 850mA.
Is the 1500mA if only one device is charged a potential problem?
What other options are people using that work well?
Thanks in advance

By the way, great forum, I rooted my Droid by mainly reading this forum and had no issues. Currently running LFY1.95, slayher 1100mHz with CPU tuner, and it is like a new phone.....so thank for that too!