I have just acquired a new Tandem with a Shimano dynamo front hub (currently running the front light). Is there a way to use the generated power to charge my car satnav for navigation via a USB ?
I'd query the suitability of using a car satnav to navigate a bike!
Apart from routing, a bigger screen, colour maps, voice prompts all add to the power usage significantly.
What about inclement weather?
Then, there's mounting it (securely) to the bike!
After that, as others have said, you will need a converter between hub and gps.
A battery is a very good idea too (I charge a gps or a powerbank from my hub - nothing else). The problem is you will need a "pass through" powerbank for that - not cheap.
Simplest thing to do is use a "normal" powerbank to power the satnav (easily tested at home). A second powerbank from the hub gives spare capacity.
I'm not sure how the latest "smart" powerbanks interact with intermittent charging.
However, check wattage of hub or all this is moot.
A cheaper and definitely workable solution is to use your phone. Osmand is a clunky but good app. Pair it with cycle.travel to plan a route.
If you do give it a go with the Satnav please come back and update us! I'd love to know!