An update on this post is found here and a step by step guide is posted here

I love to go on holiday by bike. And as a tech nerd I can’t be bothered by plain paper-maps!
Having a GPS-mapping application like OziExplorer on your bike makes navigation very easy.

The problem however is with the rapid draining of the battery-powered GPS / PDA.
Especially mobile phones / PDA’s have a very short (battery) lifespan, often no more then a couple of hours when the display is always on and a GPS-application running. Normally I just dragged a lot of AA aka penlite batteries with me and powered the phone with a simple battery holder like this one:

Of course this isn’t the best solution and I was looking for better alternatives.
Then I stumbled upon this genius, hub-dynamo driven “Bike mounted USB Charger” on an English bicycle forum.
The idea is quite simple. You take a HUB-Dynamo (Shimano DH-3R30) (I ordered one from ebay for less then 40 euro).
The hub-dynamo generates an alternating current that needs to be rectified and the voltage output should be regulated to 5 volts for USB powered devices.

The hub-dynamo:

These are the parts you need, available at any electronic (parts) store.
D1 – D4     1N5818
C1             2200uF   16v
C2, C3       470uF     63v   Low ESR
C4             .47uF      Tantalum bead
C5             22uF       Tantalum bead
R1             47K         .25W

Solder the parts according to this schematic:

And there you go, you have the best energy- efficient charger, all done with green energy!

This is my effort. It still needs some good housing and connectors but it works perfectly:

For more detailed information visit the bike forum at http://yacf.co.uk/forum/index.php
Many thanks go to Simon Galgut for providing the electronic schematic.