I have
Amazon.com: Motorola P790 Portable Charger: Cell Phones & Service one of those. I haven't had to use it in an "emergency" situation, but I have tested it out a couple times. I got my droid down to 5%, then plugged it up to the P790. It got up to 70% by the time the light started flashing yellow (which means it's battery is under 20%). It hit 90% by the time it was through, I would guess around 95%. I did use the phone, got a few phone calls, and a few texts during the charging. I went from 10 - 100% on a different test.
If you do get it, an adapter is required
Amazon.com: Mot Mini-USB to Micro-USB Char: Cell Phones & Service . If you don't have a USB to mini cable for something you already own, you will need to buy one of those as well. Shopping around may be able to save money on the cables or the charger. I don't remember where I got the charger from, but I remember I paid $15 for it and the cable, shipped. I had quite a few USB to mini usb cables lying around my house.
I have a few complaints with the setup, all of them are rather minor.
- The light is green when full, flashes green when charging/discharging, and flashes yellow when below 20% full. The green and yellow don't look that different to me.
- When you push the switch to push out the male, charging end, it doesn't lock. So in order to attach the cord (or a phone if you don't need an adapter), you have to hold the switch.
- The cord when plugged into the droid one of them has to be upside down. The way the cable is, you have to either plug the USB cable in and leave the charger upside down, or leave the droid upside down.
- You can't attach the cord to the battery pack all the time. When you charge the battery pack, the male end retracts into the phone.
As I said those are minor problems, it is small, inexpensive, and works. It probably won't charge the droid from 0 - 100, but it should give you enough juice to charge it until you get to a better power source. YMMV
