I use a standard bicycle dynamo to recharge a GPS unit and cell phone. Both are designed to be recharged by plugging into a computer USB port. Here is the circuit.
Batteries must be solder tab type with no possibility of poor or open connections.
An open or poor connection will allow unregulated power from the dynamo to flow into any device connected to the USB port possibility damaging it.
Instead of using complicated semiconductor parts soldered to custom made circuit boards I just use 4 Ni-MH batteries. They work very well limiting voltage output of the dynamo and absorb excess current by using it for recharging. The batteries internal impedance will increase as output voltage of the dynamo approaches the design voltage of the batteries. 4.8 Vdc is the rated voltage for 4 fully recharged Ni-MH batteries. Recharging voltage needs to be slightly higher. At 5.2 volts the internal impedance of the batteries load the dynamo to the point it saturates and becomes impossible for it to produce more than 5.2Vdc. This is perfect for recharging these batteries and is well within the requirements of a standard computer USB port. A standard computer port is also rated at 500mA which happens to be the same as a standard bicycle dynamo. The batteries also make it possible to switch over to the headlight circuit leaving the USB device plugged into the USB port. The circuit above is actually two separate circuits. The batteries do not power the headlight emitters. The switch allows you to use the dynamo to power the headlight or the USB power system. The center off position electrically disengages a hub dynamo and prevents slow discharge of the batteries across the bridge rectifier diodes when the bike is going to be parked for more than several hours. My headlight is a two emitter system but may be replaced by any dynamo type headlight. More information about dynamo powered emitters can be found here.
http://pilom.com/BicycleElectronics/...lectronics.htm
Recharging a device is simple. Simply plug it into the USB connector, select the USB power circuit using the switch, engage the dynamo if your using a bottle or BB type dynamo, and start riding. Disconnect the device every half hour or so and check the battery indicator on the device for charge level. Pretty soon you'll know how long it will take to recharge your device. When riding at night you should use the headlight circuit only and secure any USB devices safely in a pannier or bag. Riding at night needs your full attention.
I've tried solar panels. There a waste of time, money, and weight for the amount of power you can get from them.