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recently I stumbled over an interesting feature of a bike.

The gravel bike from Decathlon has an inbuilt USB hub where you can charge your battery packs while riding. Now I wonder if there is a way to DIY something like that. I already have a dynamo in my front axle for my lights. So now I wonder if I can modify that or maybe attach one of those old flip dynamos to the luggage rack. Of course, I don't want that my light goes off, if I plug a power bank in, so I am thinking about two separate systems.

If anyone has an idea, of what could work and what I would need for this, I would be glad to hear about it. I am also ready to provide further information if needed, I just didn't want to create a chaos of maybe unuseful information.

Link to a review of the bike: https://bikepacking.com/bikes/decathlon-riverside-touring-920-review/

In the picture, you can see the USB cable plugged into the hub on the bike. Example picture of Decathlon Riverside Touring 920

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    I also looked into this some time ago, but after realizing that the maximum you get out of a bicycle dynamo is 0.5A I dropped the idea. You get much more out of a small portable solar panel, up to 6A, even on cloudy days you most probably get more out of it than from the dynamo. Feb 16 at 12:41
  • There's a lot of info for DIY hand-cranked USB generators. It's the same concept and has to deal with the same issues related to voltage output to USB devices.
    – anjama
    Feb 16 at 13:21
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    @GeraldSchneider solar panels are extremely inconvenient for cycling though. A hub dynamo is just there and does its job whenever you're moving (which is presumably most of the time). Feb 17 at 10:08
  • @anjama ah, but this is bicycles.se and not electronics.se Feb 18 at 7:49

3 Answers 3

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Decathlon uses a third party part, that is available for sale if you want to fit it on your bike: Cycle2Charge

The category name for this kind of products is "headset cap USB charger" or "top cap USB charger".

For reference, cyclingabout.com has published a list of hub dynamo hub chargers: https://www.cyclingabout.com/list-of-hub-dynamo-power-supplies-for-usb-devices/

As you can see, there's a selection of products available. Cycle2Charge is among the most simple ones, the advanced ones have also a built-in battery that is used as buffer to provide a more stable output or to still continue to charge when not going fast enough.

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  • Not really DIY when you buy it from a shop :)
    – mattnz
    Feb 18 at 8:10
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    @mattnz I don't disagree with you ;) But it's such a niche product that it's not unreasonable to assume that it must be custom-made.
    – Rеnаud
    Feb 18 at 8:27
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The "Forumslader" (German site) is a semi-commercial semi-DIY dynamo charger. Schematics for older versions are available.

This very detailed article by Olaf Schultz, also in German, contains a theoretical section about the characteristics for dynamos - useful for designing your own circuits.

This topic has been discussed a lot in German forums. For some reason, powering stuff by bike dynamo seems to fascinate German tinkerers in particular. Searching around in German and using Google Translate should reveal a lot of information about this.

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  • I mean this gets a lot easier, due to me being german... Kind of funny imo
    – Hagenbeck
    Feb 16 at 10:41
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    @Hagenbeck In fact, you'll find quite a bunch of German products in the list I gave (the one used by the Decathlon bike of the question included).
    – Rеnаud
    Feb 16 at 11:52
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Most Dynamos are 6V, 3W output, you would need to know the specs for yours.

Amazon/Aliexpress etc have heaps of 6-24V input to USB output devices for a couple of dollars. You can also get a 0.9-5V input to 5V output USB device, but this will probably pass higher voltage on to the USB port so would require a voltage limit on the input.

I am far from certain these would work reliably and there is a real risk of device damage if they don't. However, if you are a tinkerer could be worth a crack.

I would start by plugging the unit into the dynamo with a USB voltage/current meter, and connect something like cheap USB fan. The with the bike on a workstand, dial up various speeds and see what is happening to the usb voltage.

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