800w battery will not charge

I just bought an 800w off of ebay for 20 bucks to use as a backup to my Pre. When you plug in the charger, the red LED comes on and it says it's charging but the battery % continues to go down. I did some searching and found the battery charging update and installed it, but it didn't help. The battery itself seems to be OK, I tried it in my Pre and it works just fine.

Any ideas?

The USB port in the phone could be damaged. Connect it to a computer and see if the phone is correctly recognized and you can successfully synchronize. I had 2 in a row where the USB port was somehow damaged and had to replace them.