You are the power

For years now I’ve been a vocal convert to dynamo lights. After all, why would you say no to never having to worry about charging your lights, and never running out of battery?

While there are interesting debates to be had about the advantages and disadvantages of various different dynamo headlights, that’s not what this post is about. This post is about having USB charging on your bike, and doing so for a fraction of what it’d normally cost.

Why on-bike USB charging?

Well, a better question might be to ask why not? In my case, it’s because I do off-grid cycle touring, wild camping along the same, with no (or limited) access to mains power. Your reasons may be different, but this post is about how to achieve this on a budget, and ignore why you may want to do so.

I have a Lumotec IQ2 Luxos light, with integrated USB-charging, but it’s started playing up and I need to replace it. The light I went for is a Fischer. At 70 lux it has exactly the same (normal) lux rating as my old light, but doesn’t have any USB charging. The Luxos is actually capable of 90 lux, but only for short periods of time, and normally outputs 70 lux.

However, the Fischer light also costs a fraction of the price, so this is a great trade-off. Seriously – we’re talking £30, versus £190 – that’s a huge difference!

An overview

Dynamos generate (supposedly) 6v of power, but on a fast descent they can generate a great deal more. That might sound like a bonus, but in reality it adds complications that we have to deal with.

Dynamos generate alternating current (AC) while USB devices require a very stable direct current (DC). AC current can be plotted as a sine wave, while a perfect USB (DC) current is plotted as a simple straight line. To get from AC to stable USB requires jumping through some hoops.

Fortunately, you don’t need to understand the electronics behind this, as other people already did the heavy lifting for us. All you need to do is purchase the components listed here, and assemble it all as instructed.


Well, this is pretty much a recipe for adding dynamo lights and USB-charging to your bike, and any decent recipe has a list of ingredients, right?

  1. A front wheel with a hub dynamo. Mine’s a Shimano dynamo hub, which is slightly less efficient than the top-of-the-range Son dynamos, but costs a fraction of the price. You can decide for yourself what hub dynamo you’ll buy.
  2. Optional: An overcharge protector. Mine’s a Shimano component. A word of warning here: these devices convert excess current into heat, and they can therefore get a bit hot. They require airflow to dissipate the heat generated. Depending on what converter you’re getting, you may, or may not need this.
  3. A dynamo-to-USB converter. There are many models available, but this one’s a decent price. As an added bonus, it contains a waterproof switch, to switch between lights, or charging, and has overcharge protection built in (up to 70v!)
  4. A USB-extension cable. All shall be revealed about this.
  5. Optional: Have someone 3D-print a USB stem cap for you.
  6. Optional: Make a mouldable glue weather cap for the USB port.
See also  Digital navigation revisited


If you purchased the converter I linked to above, there are several gotchas (to be fair, they apply to just about any converter). The first is the weatherproof claim. Yes, that enclosure is weatherproof, but only up to a point. The biggest weakness is the open and exposed USB port on the underside, and I guarantee that it’s but a matter of time before you either suffer water ingress, or the port badly rusts.

The unit is fairly large, and you have need to find somewhere to fit it to your bike. Remember, you want to be able to access the toggle switch, to switch between USB charging, or dynamo lights. This doesn’t need to be within immediate reach while riding, though that would help if it were the case.

Simply run the cable from the dynamo to the converter, and run the cable from the converter to the dynamo lights. I’d suggest running the cables flat against the body of the converter, then taping over them, so you’d have at least some strain relief.

Finally, the USB extension lead comes into play. Simply plug that into the unit, then wrap it up in electrical tape, taking care that the USB socket, and the plug of the extension, are completely taped over. Obviously, only do this after you fitted the dynamo power leads, and the wires to your dynamo lights. From that point onwards, you can simply replace the USB-extension lead when it’s rusted or damaged.

Pass-through charging

Unless it’s an emergency, try to not connect your gadget directly to the USB-out socket. Instead, use the converter to charge a power bank. Most power banks support what’s called pass-through charging, meaning they can be charged, while at the same time be charging your gadgets.

See also  But what about your privacy!!?

Sounds brilliant, doesn’t it? Don’t be fooled: pass-through charging is not your friend! In practice, what you’ll find is either the power bank will charge extremely slowly (while being discharged at a faster rate), or more often, the gadgets receiving charge from the power bank will be charging extremely slowly.

The way around this is simple: use at least two power banks. One power bank is being charged during the day, when you don’t need lights, while the other is charging your gadgets. Simply rotate the two on a daily basis. This is a FAR more efficient way of keeping kit charged up.

Water kills USB devices

It shouldn’t need saying, but you need a way to keep your power banks dry and protected from rain, or splashes from puddles. The easiest way is simply to carry the one being charged inside a bidon. Feed the wires through the spout, then seal it with hot glue. You’ll want to put some foam padding inside, to stop the power bank rattling around.

Being able to remove the power bank means that, when camping overnight, you can take the charged power bank into your tent or bivvy. Remember, cold seriously affects lithium-based batteries, so if it’s cold, keep them inside your sleeping bag overnight. The same goes for any gadgets you’re trying to charge up overnight, when it’s very cold.

If you used a stem cap USB adapter, ensure you buy a long enough USB extension lead to reach it, from wherever you mounted the converter.

If you wanted to, you could make a port cap for the USB port, using mouldable glue. Do not simply push the glue into the port, as all you’ll do is seal the port up, rendering it useless. Making a port cap should markedly extend the life of the USB extension cable.

See also  But what about your privacy!!?

Instead, use clingfilm, and place the mouldable glue inside the clingfilm (I’d advise a double layer!) That way, once the glue sets, you’d be able to remove the port covering plug you just created. Just remember that the glue will stick to anything not covered by clingfilm.

As an alternative to making a port cover, you can buy silicon dust covers. Just be aware that, while I’m sure these would help, I’ve no way of knowing how effective against rain they’d be.


Now you should have an on-bike USB-charging system, with replaceable components. Before a ride, insert a power bank into the water bottle, and plug the USB cable from the converter into the power bank, before closing the water bottle.

Next, decide if you want to use USB-charging, or dynamo lights, flick the switch on the converter to your preferred option, and start pedalling. All in, this should have cost you £60 or less (excluding the power banks). That’s an enormous saving on any of the options available from the likes of SJS Cycles (who actually normally have excellent prices).

If you still have to buy a front wheel with a dynamo hub, they tend to start off around the £70 mark. Add in around £25 for a decent dynamo tail light, and a full, new setup would set you back around £160. That how much you’d pay elsewhere just to add USB-charging to an existing dynamo setup!

Now don’t be selfish! Tell ALL your mates, and all the other cyclists about this, and share this post widely.

3 thoughts on “You are the power”

    • I’ve only ever heard good things about PedalCell, and they certainly make some big claim (with supporting data). Having said that, I’m afraid I have no personal experience of a PedalCell, so cannot comment any further on it. The Cinq 5 appears to essentially just be a power bank, but I’ve no idea how efficient its pass-through charging capability would be, and ultimately that’s what would make the difference.

  1. I went for a B&M usb-werk for £65 two years ago. It has a small cache battery to prevent frequent power cycles and I have it connected permanently inline with my AXA Blueline front light and rear.

    At higher speeds it can provide enough power for all functions but normally, with a quick button press on the back of the light to turn it off, I can charge my phone at lower speeds. However the first usb-werk failed within a year during a tour but I got it replaced under warranty.

    I don’t have the auto-on light though. I am jealous!


Leave a comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.