Thanks for the follow-up. I took great care in creating a harness for the USB charger (soldered joints, etc) and checked output with a multimeter so I doubt that is the issue but I'll tear it apart and re-wire it this weekend.
Did you wire directly to the battery? I purchased the the BMW harness that fits to the GPS plug under the seat so I get power only when the ignition is on and for a few minutes after ignition is shut off. This is the only device connected to the harness and I have ample amperage.
It is definitely not the apple cable as it is the same one I'm using right now to charge my phone at work. My wife's original iPhone seemed to charge just fine using the apple cable and the usb charger on the bike after I found the problem.
I'll get back to you over the weekend when I find some time to re-check my wiring job.
Originally Posted by Burnszilla
I just hooked up the USB charger to an iPhone 3GS and 3G using two different cables and they both charged fine.
Some causes could be:
1. Positive and negative terminals installed incorrectly.
2. Bike battery is too low.
3. Faulty cable. (I've had many iphone/ipod cables fail. they would work when you jiggle them or bend the wire.)
4. Faulty USB charger? Try and charge a different device like an iPod or blackberry and see if it works.