Iphone charging problems

Alright so I have this promaster extended battery charger that I use to charge my DSLR batterys with. The charger has a buit in battery that allow you to charge the battery on the go.

The charge has a usb port on the side for you to plug in your usb devices and charge them. The problem is that when i plug in my iphone it doesn't want to charge. Is there anything that i can to to make it charge through there ?

I have used that port on the charger to charge a Env-3 before but it won't work with my iphone, anyone have any ideas ?

Thanks alot.

Heres a link to the batter charger..... http://www.amazon.com/dp/B001453ETO/?tag=macforums0e4-20

The iPhone requires voltage on the data pins, which is unlike pretty much every other phone on the market. Normal USB power adapters like your Promaster provide power on the 5v pin, which is fine for every other device except the iPhone and certain iPods. It's a crafty attempt to make sure you can only use Apple approved products to charge your phone, but I digress...

Anyway, if you're handy with a soldering iron, you can make up a cable with a couple of resistors linking the 5v line to the data pins, which will make it charge the iPhone. Can't remember the voltages it needs offhand...I'll have a look now.