To never run out of power, a very simple and I think the smallest solution is to equip your bike with a device like the Highway Hawk USB Charger Plug.
The charger plug transforms the power to 5V at 2A. It comes with 46″ of wire to connect to your 12 V battery and can easily be fitted to 7/8″ and 1″ handlebars. MSRP $35. Distributed in the USA by Aeromach Mfg. 800-990-9392