you mean something like this:Look it up on Ebay. There are some fun little things. Didn't bought it yet but very handy I bet.
http://www.ebay.com/itm/Bike-Charge...isure_Cycling_Bike_Lights&hash=item4cfe6924c6
I was hoping there was a USB charger you could run off the white ignition wire - do you know of one?