My reference to 50W was an example - whatever the load is, there will be losses. Have a look at this graph from https://aerosensor.tech/aerodynamic-drag-in-cycling/ - the actual frictional losses when riding a bike are tiny:mik wrote: Thu May 18, 2023 10:02 amSee comment above. There is no intent to power a 50w device from a dynamo - this is intended to extend the use time of / reduce the size of the battery that will be needed. 50w estimation is hopefully higher than actual too.Jobbo wrote: Thu May 18, 2023 8:41 am It's a ridiculous idea; you don't get anything for nothing and you'd be better off sticking a powerbank in your pocket.
What that shows is that to maintain 15km/h (9mph) on a bike on the flat without wind, you're expending about 25W. At 20km/h (12.5mph) it's about 40W and at 25km/h (which is the cut-off for assistance on an e-bike, so I'd say is a fair speed to want to maintain) it's about 80W.
I've found indicative power consumption for dynamo hubs here: https://www.cyclingabout.com/how-much-d ... -you-down/ - there are four different hubs shown on the below graph which is without any load whatsoever:
Taking the best of those (the SON28) you're looking at 1.5-2W of losses just for having it fitted to the bike, without it doing anything at all otherwise. The article shows the SON28 uses about 7W when powering a USB charged and 11W when powering a light. Unfortunately they don't measure the power output of the charger or light, nor state the nominal output, but there will be some sort of loss; this suggests it may well be about 13% for a decent charger: https://arstechnica.com/civis/threads/u ... e.1483654/. So taking the USB charger and a speed of about 20-25km/h as being typical, deducting the base loss from the dynamo and assuming 87% efficiency you'd be getting ~4.8W out for a ~7W load. That would charge an iPhone 13 or 14 Pro Max in 3.5 hours. In that speed range the drag effect is about the same as cycling into a headwind of maybe 2-3 km/h. At a lower speed the effect would be more like a 10km/h headwind (based on the aero power line on the top graph).
TL:DR - do you like cycling into a headwind?
