Charging a cell phone via USB?

I recommend you do the following to serve your needs which I fully support. One day I will install the same system for the same reason

6 volt AC generator from: 6 Volt Electrical Generator for Motorized Bicycle Light - eBay (item 400109304267 end time Jun-08-11 00:35:57 PDT)

Rectifier for those who can't build their own: 6V MINI-CHARGER FOR Motorized Bicycle Light LS-1 & LS-2 - eBay (item 400109384706 end time Jun-08-11 10:15:02 PDT)

6 volt 1.2 amp hour SLA battery 6 VOLT, 1.2 AH BATTERY | AllElectronics.com or you can use this one from Ebay which is 8x more money and comes in a nice plastic case 6 Volt Encased SLA Battery For Motorized Bike Lighting - eBay (item 400108733669 end time Jul-04-11 22:23:51 PDT)

I If you want, you can easily install a heat sink mounted 5v voltage regular to drop voltage down to 5volt and you are done.

You'll have a battery to charge your cell phone and a system that recharges that battery when you ride. If you ever get stuck far away from home, you'll never have to worry about your cell going dead provided you have plenty of gas.

I'll gladly build you a 5v regular for cost of parts and shipping using powerwerx plugs if you get this far and don't mind the expense.

Another option is a foldable solar panel which is what some hikers use for long hikes and what I had as a backup for my satellite phone when I was in the military.

Powerfilm F15-300N Foldable Solar Panel - 5 Watts Output is a small 5 watt product. There are 12 watt products available too that are slightly bigger.

A 6 volt SLA for emergency lighting works great on these motors for storage and smoothing out your current instead of coming directly off the white wire.

Charge the battery with the white wire. Use a bridge rectifier from radio shack it is like $2.50. The rectifier has four "lugs" the diagram is on the back of the package. One of the two AC lugs comes off your white wire, the other AC lug is your engine "ground". Hook the + DC lug to the + battery and the - to the negative battery. Battery charges when the engine is running.

I have seen two sizes for emergency lighting 6v SLA the cheapest of the two also happens to be the smallest 4.2 aH I believe it is slightly smaller than a regular 6v lantern battery. I suppse you could use 4 rechargable AA or AAA batteries to keep it small and compact, but my understanding is that the charging characteristics of any available rechargable battery does not work well with inconsistent and fluctuating output of the white wire unless you want to build a complex charging circuit. An SLA is quite forgiving in that aspect

The problem with running directly off of the white wire is that the output varies greatly with with the engine RPMs and that can't be good for small electronics, the constant change in current. Do a search for the output graph of the white wire, somebody mapped it out, and the easiest way to smooth it out I would think is with a battery.

That is the beauty of the SLA setup, simple and cheap. There is a post where somebody set the charging circuit for the battery with a zener diode to prevent overcharging, I believe that the conclusion was that it was overkill because the worst case scenario of overcharging is a slightly warm battery.

I don't think I would worry about a voltage regulator if you are just using it to charge your cell phone, the charging circuit in the phone itself should be able to handle the extra volt just fine.