If it can be recharged via a USB port you can use this circuit with a dynamo.

The dynamo may be any standard bicycle type dynamo rated for 6-volts at 3-watts or 500mA.
The 4 diodes form a bridge rectifier to convert AC to DC. S2 electrically disconnects a hub dynamo and should be included with mechanically disconnected bottle and bottom bracket dynamos as the diodes can allow the batteries to drain very slowly discharging them after a week or so. The 4 Ni-MH batteries act as voltage and current regulators providing clean power for the USB port connector and devices connected to it. S1 should be open when recharging and powering whenever any devices are connected to the USB port. After dark you would simply disconnect any devices from the USB port and operate them from there internal batteries. Closing S1 allows the 1-watt headlight and LED taillight to be powered from the batteries with the dynamo supplying power to keep them recharged. This design had 120mA of excess current recharging the batteries while the LEDs are on so they continue to operate at stops or hill climbing. If traveling where your not stopping simply disengage the dynamo for 15 minutes of every hour to prevent overcharging the batteries. In the event you need to rapid recharge batteries simply disconnect any devices pluged into the USB port and open S1 allowing full 500mA to recharge the batteries which is the suggested current for rapid recharging of Ni-MH batteries.
I use a Palm TX and Bluetooth GPS unit and keep both charged and operating on my touring bike.