A hub dynamo turns wheel movement into electricity, powering bicycle lights as you ride. The generator sits inside the hub, with a cable leading to the light. The arrangement is used on both everyday and touring bicycles.
SON hubs have nominal ratings of 6 V and 3 W. These figures do not promise constant output: actual performance depends on speed, wheel size, generator design and the connected load.
A phone needs a suitable charging converter to turn alternating current into stable direct current. A buffer battery can smooth the supply when speed changes or the bicycle stops.
Newer systems can distribute energy between lighting and charging. For example, Ladelux disables USB charging and high beam when its battery runs low while retaining low beam. This makes the complete system worth assessing, beyond the hub alone.