Had a 9.6V, 1.3-ah battery from a electric drill laying about. Ran test wires from it to my Nuvi 750 cig adapter and, lo+behold, it worked!

So today, I picked up 2 wires w/ alligator clips on each end, and a cig lighter from an auto parts store ($10). Ran just fine for 6.5 hrs off the setup. Now, all I need is a way to attatch it all to the bike! (For the GPS, I'll get a dead cycle computer, epoxy the detatchable part to the bottom of my GPS, and then just slide it on and off.)

Anyone else give this a shot? Similar battery packs are around $25 a pop.

Here is the circuit I use to recharge my GPS and cell phone.
http://i143.photobucket.com/albums/r154/n4zou/DynamoUSB-1.jpg
Batteries regulate and filter power for USB powered device plunged into the USB connector. The dynamo can not produce enough power to operate both the LED headlight and a USB device so switch S1 prevents that from happening. It also disconnects the batteries from the bride rectifier preventing slow discharge when the bike is not being used. You may switch to the headlight with a USB device plugged into the USB connector when entering a tunnel. Batteries must be solder tab type with no possibility of open or poor connection. If this happened unregulated power would flow into the USB device possibility damaging it.