Battery still discharging when plugged into an external battery charger?!?

[augie0041]

I purchased one of those external AA battery chargers for my Touch Pro. When I plug it into my phone, the lights around the keypad light up indicating it is charging.
I travel alot and need this for my long flights. I had the phone charged at 100% before my flight. I turned on 'Airplane mode' and plugged in my external charger for a few hours until I wanted to play with it on the plane. It was down to 70%!!
I watched a 3 hour divx movie in TCPMP with the brightness as low as possible, volume all the way up (because I had to, it was a plane)and having it plugged into the external charger the whole time. The phone technically never died, but with about 20 minutes left in the movie, I kept on getting the battery low message (which interrupted my movie each time.) It was obvious the external battery charger was doing something...as the second I pulled the plug out, the phone immediately shut off from low battery.
About 30 minutes later, I decided to just try out PocketMusic and listen to some mp3s with the screen off and the battery charger still plugged in. No issues at all, actually after 2 hours of that, my battery was back up to 50% charged.
What is going on?