I recently noticed a strange behaviour of my Lumia 800 when using my external battery pack. When the battery pack is connected, the phone acknowledges that it is on an external power source and displays the "charging" icon. However, the phone doesnt get charged, it slowly uses power. Diagnostics app confirms that: While on the external power source, the phone is actually discharging. This is either a bug or the phone is actually consuming more power than the battery pack can deliver, which would be really weird. There is definitely no defect in the battery back btw, on my HTC Trophy it works just fine.
Firmware is 1600.2483.8106.11500
The battery pack is http://www.amazon.de/Just-Mobile-Portabler-Ersatza
Lumia is a bit fussy about charging, in particularly if the battery is flat, and this is down to there being different USB charging specifications.
Typically a USB port is not supposed to take more than a 100mA until it has negotiated with the host device to deliver more, and typically the maximum is 500mA (although 1000 or 2000 is also known). because not all chargers are intelligent enough there are various standards of shorting out the data pins to indicate capability. However Apple uses a different approach, by indicating voltages on the data lines so if you have a device that is designed to work with an i device, then it is less likely to work with other devices. There is a good article in Wikipedia about it.
Thx for the insight, seems it's more complicated than I thought :-) Still hope Nokia is going to fix this. Especially with a handset that is not known for it's great battery performance the option to use an external battery pack is of great importance.
I'm about to leave for a short hike right now and while I always used to track the route with Outdoor Navigator on my HTC Trophy, I simply can't do that on the Lumia 800 because it drains the battery :-/