Looking for an external battery pack to charge the iPhone while traveling. Apple sells a Kensington product in store but it's on backorder and it's $70. I've been to local stores and have seen a few generic models (Energizer, etc.) that work with mini-usb phones - wondering if I can adapt male mini-usb to female USB then plug the iPhone cord into that. But chances are I wouldn't get more than half a charge out of one of these anyway. Anyway, has anyone bought an external battery that they've had good experience? I need it to be compatible with the 3G and get me at least one full charge.