Emergency Chargers

In short, I'm surprised that the charger didn't work. It is possible that it provides a voltage or current that is too low to charge with, but there's no way to assume that by the battery being used.

I was surprised to, and tried changing the battery, but that didn't seem to have an effect. I DID get the battery from a pack thats been in my desk for a while though, so maybe that battery happened to be dead as well? Seems unlikely but who knows...
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
I bought one of the ones described by the OP and got the same results. The droid wakes up, but doesn't indicate the battery is being charged. And the droid just dies if I leave it connected. Glad I only paid $3
I may have found a solution. I saw on this thread that the droid requires a short between the middle two pins to recognize that it's connected to a charger. I shorted them on my single AA charger (similar to the one the OP mentioned) and my droid indicated it was charging. Going to get my soldering stuff out of storage later and see if it really delivers any significant juice to my droid's battery.