Has anyone used a USB power pack with a N900? I decided to get one because a couple of times I have been out and about with a flat battery and no way to charge it.

I bought one that looked like a real bargain on eBay, holds 4 AAA batteries and charges them from a USB socket. Then you flick a switch and it provides USB power through a different socket to a device such as a N900. The trouble is, it didn't work. I plugged it in to my N900 and it didn't charge it.

Has anyone found one that does work with the N900? I know that Nokia have one of their own (DC-11) but it's a bit pricey and I was looking for something a bit cheaper, plus that one doesn't seem to hold a lot of charge. I really like the idea of one that takes AAA or AA batteries, it's easy to carry a few spare ones.

Dave.