A lot of the problems with generating consistent power (typically for charging batteries) from a dynohub stem from the fact that it operates over a wide range of rpm. If a generator was mounted to mesh with a front chainring (for example, on the rearward side of the smallest one), the output would be proportional to cadence, and therefore much more consistent. Is there a reason this hasn't been done, apart from mounting and sealing issues?

Coasting.

There have been and are chain driven dynamos. Why, here's one now:

https://www.amazon.com/Lontano-3-9in.../dp/B01DR4NMCS

If you're doing a long, multi-day ride, you're probably spending less time coasting than at low wheel rpm (where a dynohub can't generate much power). You could also just keep pedalling!

As for those chain-driven things - the reviews say it all. I can't imagine it's possible to design anything good, given that the range of chain movement across the gear range.

can you elaborate on the shortcomings of hub dynamos regarding rpm?
The German standard requires the dynamo to bring the light up to full power at a fairly low speed.... 10kph or so, IIRC.

Of course, there are some dynamos being sold that don't meet the standard, or are designed for smaller diameter wheels, but anyone worried about low output at low speed wouldn't buy one of those.

Steve in Peoria

StVZO requires dynamos to provide 6V at 0.5A into a 12 ohm load when riding at 15 km/h which is 9.4 MPH.

Outside the mountains you're not going to have problems running a bicycle GPS. I started and finished rides at 99% on my Garmin Edge 800 until the battery got older - now it finishes at 97%.

Modern cell phones use too much power to keep them charged with the screen on.

Unless you are climbing steep hills, the hub runs at higher rpm than the crank. And on steep hills you likely are OK giving fewer Watts to the dynamo. So a crank dynamo would have to be physically larger for a given power output.

That chain dynamo from above can be small due to higher rpm. But as pointed out, has its own issues. Especially on FD drivetrains I assume.

A tire friction dynamo also runs at high rpm. But has tire wear, noise, friction and other issues.

In the end the hub dynamo still is the best of all bad options.

I'm thinking about charging a battery or USB device, not lighting. In this case, the wide range of wheel rpm means you won't get full charging capacity (eg, USB requires 5W) until you are moving fairly quickly.

If you took power from the chainring, you could gear back up to something more reasonable.

For charging stuff the best thing to do is use a cache battery with pass through charging (so you can charge the battery while the battery is charging your device). That smoothes out the power peaks and troughs. You want a dynamo that will supply the most power over the day.

What are you charging that requires a full 5 watts continuously? ​


Agree.

Indeed - this is the best thing to do with a dynohub. I'm just wondering about what would be best for battery / device charging, given a blank slate.

5W is the USB charging standard, so most phones (although I'm already behind here, as a lot of phones are moving to USB-C). It's not something I'm doing at the moment, I'm just wondering about this from a design perspective.

Last winter or spring, someone on this forum (I think on the Touring forum?) mentioned that the best batteries for this purpose were ones that were sold for use with solar power systems, as those were ones that could charge and discharge simultaneously. And I suspect that they also could take even a small amount of power and put it into the battery without a minimum threshold of power needed, but I am guessing on that. The person that suggested that also mentioned a battery made by Voltaic.

Last spring I bought a V44 (now out of production) that was made by Voltaic. I used it for a five week bike tour this past summer. I am quite happy with it for pass through cache battery purposes. I use an SP PV8 hub and Sinewave Revolution charger to supply the V44.

The voltaic batteries cost more than other brands, but after buying some batteries expressly for pass through purposes and then finding out that they do not simultaneously charge and discharge, I decided to pay extra to get one that should meet my needs.

But, I mostly use devices that will happily draw well under 5 watts to charge. My battery will charge a device that needs a full 5 watts or more, but the hub would not keep up, so the battery would slowly discharge. I think that on average while touring taking into account hills (slow up hills) and stop lights that I probably average around 2 watts of power from the hub that goes into the cache battery.

I know a guy that uses an Anker power bank, but he charges the power bank while riding during the day, charges his phone from it at night. I do not know if it will function as a pass through cache battery.

***

I think the current leader for USB charges that put out the most power from a hub is the Forumslader, I have never seen one but they have been mentioned on this forum and on the Touring forum.

What are you hoping to accomplish? A few people use dynamos on their bikes to charge their electronics, but it doesn't seem worth the expense unless you're going to ride many days with a lot of time away from a power outlet. I use an Anker external battery to keep my phone charged while I'm on the bike. I'm at a power outlet every night.

I'm just thinking about this as a design exercise. Given that current solutions for charging larger devices (a phone, battery pack, etc) are expensive and less than ideal due to the issues already outlined (wide range of operating rpm, relatively low power output available, considerations for existing lighting systems, etc), what would a more ideal solution look like? I had the thought that a separate generator, driven by a front chainring, would alleviate most of these issues. It would largely operate within a small rpm range, could be sized for more output power than dynohubs, and wouldn't need to share its output with lighting systems.

The main problems I see are: difficulty of mounting (it would have to mesh with the smallest chainring, on the unused portion towards the rear of the ring), the small amount of available space, and weatherproofing.

my experience is that people on multi-day rides usually coast as much as they can.

Thanks for the insight - I was making the assumption that people would generally keep up a steady cadence.

It's possible to build a dynamo that provides a lot of power, but it would suck too much kinetic energy from the rider.

That chain-based dynamo seems close enough to what you're after, especially if it is energy-efficient. But bear in mind that there is a practical limit on how much power to expect from any such system because a rider is probably not willing to pay a 20-watt kinetic energy price for charging. And given that charging is available nearly everywhere from a power outlet, you're solving a problem that not many people have. Most people are near an outlet every day. Even if you're away for two days, you can get by on two little external batteries. I carried two on one long ride I took. I ended up depleting the first but not using the second. I kept my phone's display on for more than six hours. By the end of the ride, my phone's charge was at 100%.

I just was looking into this for a tour we're planning next summer. I came to the conclusion that it's a lot easier to find a small but high power density USB-C adaptor and locate the occasional outlet to charge my battery. All of these dynamos put out ridiculously low power. It's a significant time to just charge your phone. And this was in looking at it as if money weren't an object, but just for feasibility. When you add in the cost of the system (dynamo wheel), then the choice is obvious - go with the battery and hunt for outlets.

The problem is going to be that (a) you need a LOT more power, maybe a consistent 10-15W output and (b) cyclists generate motion with surprisingly little power and as such, can't tolerate much of a power loss going to the tire. Even if you could get 100% efficiency, that 10-15W would probably be uncomfortable quickly in a number of situations. Ultimately, solar is probably going to be better power "bang" for the buck, I'd guess.

J

JohnJ80 exactly. People see dynamo hubs and think, "great, free power" until they do the math. I don't feel my dynamo hub's drag, but I'm only powering two lights. If it gave 10w of electrical power, I'm sure I would feel it, and it would probably be objectionable.

I have thought of going to battery lights for some rides. I don't have that many watts to spare

I looked at this further and a 20W solar panel can be had for about $75 and would probably do a lot better in providing more power. It’s probably weight neutral to the dynamo hub but even if it’s not, the watts used in pulling the additional 500g (panel i was looking at) would be largely negligible.

I generally think 27Wh would more than suffice for what I need for two of us per day and the solar panel could do this in under three hours at half it’s rated capacity.

I'd be very surprised if you can get even 10W from a panel that you drape over a bike. But I'm glad to be surprised.

Pretty sure it would exceed what a dynamo would put into a battery. And even at that, there’s going to be more than three hours a day when you’re not riding - doesn’t have to produce power when you’re on the bike and moving like a dynamo has to. The cost-benefit is likely considerably better. I’m thinking about getting one of these to test.

But either way, I think for modern electronics it just makes sense to get the right charger and look for an outlet. There are some crazy small but high power chargers around now based on some of this new GaN tech. I have a usb-c 45W charger I got off of Indiegogo than is slightly bigger than the 5W apple iPhone charger and smaller than the size of the Apple 18W charger. That would put my daily capacity back into the battery in something less than an hour. Perfect for touring with such a high power density.

Here’s the Indiegogo link. They have both a 65W and a 45W charger.

https://www.indiegogo.com/projects/s...5w-gan-adapter

There’s a 100W charger now over on Kickstarter that is about the size of a deck of cars with two USB-C PD ports and two USB-A high current ports. There are a number of batteries now coming out that support USB-C PD input at pretty high powers. Omnicharge has a 13,000mAh battery that is USB-C in at 30W, for example. Right charger with the right battery can mean fast charging at pretty high capacity.

Regardless if you select solar or dyno I'd recommend charging into a power bank rather than the phone directly.

In my experience, if you have a power source that is intermittent this will tend to cause the phone to turn its screen off and on repeatedly when the power "appears" or "disappears". This extra screen time can burn off a significant amount of that new power.

The power bank is also more flexible in providing a buffer, you can leave on your trip fully charged and also take advantage of power outlets when they are available.

Having power in the separate battery is useful too because smart phones sometimes get caught up in a background process or hunting for service and will burn a battery much quicker than normal. This always seems to come up more while travelling. Having the power bank keeps energy in reserve if you need to make an emergency call but your phone has wasted its power unexpectedly.

Those are fair points about the solar panel, JohnJ80. Those small powerful chargers are nifty, but the smaller they are, the bigger the fire hazard. I'm not saying I wouldn't use them, but I would be careful.