I use, and wholeheartedly recommend, the Shutter Precision SP8 series of dynamo hubs, whether it be for disc or rim brakes it kicks out a decent level of power from 8mph – which is good enough for all but the toughest of alpine ascents.
The dynamo will kick out a constant source of power, but even then you can still find yourself needing an extra boost when 2 gadgets need juice simultaneously.
Step up the portable USB powerbank – and in this instance Anker really do step up!
I’d never heard of the company before TCR in 2015, but splurged a whole £20 on the device as replacement for a previous unit which lacked its capacity. At 13000mAH it holds enough to charge phones and GPS units a couple of times.
The size is sensible, allowing it to be stored in a frame bag, safely stashed away from the elements, and whilst you can get bigger, and heavier, units this size seemed like a really useful compromise.
The unit can kick out from 2 outputs simultaneously, should dire-straights come upon you, and when it’s not being used from output it can sit in the bag being trickle-charged from the dynamo hub the whole day.
If you are planning on entering multiday races or events, then don’t leave home without one!
There is also a slightly larger unit at 16000mAH which promises similar performance with a slightly larger capacity.