Battery versus Dynamo

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Re: Battery versus Dynamo

Postby Brucey » Tue Oct 15, 2013 9:03 am

coast 2 coast wrote:Dynamo hubs add extra weight and extra rolling resistance, two concepts generally considered to be undesirable on a bike. They also add extra complexity and extra expense...


Weight? -Not much. A good system is about 0.5% of the all-up weight and a bad one is 1%.

Rolling resistance? -No; rolling resistance varies with load carried, dynamo drag doesn't. With the lights on a decent generator drags about 5-6W. By contrast IIRC each (touring) tyre drags several tens of W at ~20mph and aero drag is much, much more again. People invariably don't even notice if the lights are on or off. With the lights off, the drag is less than 1W with a good generator. I reckon my present generator drags less than 0.5W, lights off.

Complexity? -Surely this is only a problem if it adds unreliability? For example freewheels / freehub bodies are 'complex' and most people don't choose to service them because they are pretty reliable and can be replaced more easily than overhauled in the event of trouble. I have never heard of the electrical parts of a generator failing in normal service, and if you want to be able to service the bearings (in an entirely normal way, just a little more difficulty on the RHS) then get a shimano generator with copper windings in it (some have aluminium windings). In the worst case with a shimano generator you might have to unscrew the innards and replace them, just like a freewheel.... :roll:

Expense? -Really? How do you value your time? How much do you spend -in time and cash- on batteries, battery lights, and faffing about with them?

I have used a hub generator for over 30 years and have ridden tens of thousands of road miles on it. In the meantime I have used battery lights for other bikes, offroading etc and I have spend ten times as much money, and a hundred times more time on the battery lights for one tenth or less of the riding time. I have also suffered innumerable failures in the battery lights, none in generator systems in this time; not an unusual experience... If you are doing a reasonable mileage in the dark, for reliably seeing where you are going on the road a generator system is a vastly superior arrangement.
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Re: Battery versus Dynamo

Postby [XAP]Bob » Tue Oct 15, 2013 10:03 am

Just looking at weight again - a couple of hundred grammes for a "plain" front hub - 390g for a SON Delux (the one recently mentioned)

So that's a battery system running at under 200g to even start to compete on weight - oh, and you need a decent runtime out of it as well...

LiPo (a nice dense battery tech) runs at at sub 200Wh/kg that's a MAXIMUM of 40Wh - more likely 25Wh.
Lithium Ion will get closer to the 40 - but will last less than a year of regular charging.

So what will 25Wh get you, maybe a couple of days commuting - that almost makes it harder than charging every day...
What will the Dynamo get you? All week, all year, for years to come.

The battery is unserviceable, and has a known-short service life, even if it is rigorously cared for (with an expensive charger) every day.

In terms of drag the 5W which is pulled by a dynamo is a small proportion of the overall power put out. I can sustain 150W all day - I sustain more than that on the way into work - but lets look at that 150W figure (http://www.gribble.org/cycling/power_v_speed.html)
150W - 18.53mph (I took default values)
145W - 18.29mph

A loss of .24mph - or 25 seconds over 32.5 minutes for 10 miles.

Time to remove and set up batteries each day is almost certainly longer than those 25 seconds
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Re: Battery versus Dynamo

Postby mig » Tue Oct 15, 2013 10:55 am

through the generosity and skills of a forum member i too now have a generator system on my winter bicycle.

for all the points made above pro and con i'd like to add this : i took this bike for a test run one afternoon. during this time a came across a car rushing up a side street to join the main road not wanting to come to a stop (as they do.) i noticed how the driver looked once at me then again quite distinctly then came to a full stop in time. ordinarily i probably wouldn't have had a light on the bike at that time of day as the batteries would have run short for the darkness later. the generator system is there all the time, isn't likely to fail nor run out of power therefore i'm going to use it & therefore i'm that little bit safer on the road. it's that very aspect that attracts me and, by extension, my family, to it.
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Re: Battery versus Dynamo

Postby PBA » Tue Oct 15, 2013 2:41 pm

Wow! lots of dicussion on this one. I've done my best to read it all but forgive me if I've missed someone making the same points...

It's probably not fair to compare a dynamo system to the very top of the range battery lighting as they are clearly intended for different purposes.

The choice seems to me to be down to bicycle lifestyle and the kind of person you are:

Do you use lights often and would like them available whenver you want them? - Go dynamo.

Do you want them only for some occasions and are wary of extra drag / carrying the extra weight when you don't? - Go battery.

I've used battery lighting for commuting and even when the system was reliable, my paranoia used to kick in and I'd carry spare batteries then when I changed to a rechargable system - smaller cheap lights instead! The ride was made less enjoyable by the nagging concern that my lights might one day give up en-route.

I switched to a dynamo hub (a cheaper Shimano one) but then changes in my job meant I no longer was able to commute. I've therefore not done the distance to say for sure, but it seems reliable and I have the (possibly ill founded) confidence that it is. Now when I go out in the dark, I don't worry about lights. I still have the little ones in the saddle bag just in case but I no longer worry that I might need them and I've given up making sure they are still there!

I don't notice any extra drag when the lights come on and I don't need the very brightest light. I do need to see where I'm going but when it's very dark, I'm happy to adjust my speed to give me a fraction longer to react.
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Re: Battery versus Dynamo

Postby mjr » Tue Oct 15, 2013 11:50 pm

Expense... I've probably spent too much time looking into this because I can't quite accept that magnetic dynamos aren't more developed, nor that there seems no good off the shelf tools to convert battery lights to dynamo power, but I'm still a long way from having wasted as much time on that plus upgrading my battery lights as it would have cost to replace the lot with a good hub dynamo and new good lights. So I stay with battery lights as I've already got the kit and the switchover costs seem too high.
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Re: Battery versus Dynamo

Postby [XAP]Bob » Wed Oct 16, 2013 7:52 am

Existing investment is a pretty good motivator.

Battery lights can often be moved between bikes, which can reduce cost - but that makes them more vulnerable to theft, or more of a pain taking them off etc.

Nice thing about permanent mounts is the alignment stays "right" very easily.
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Re: Battery versus Dynamo

Postby kwackers » Wed Oct 16, 2013 12:16 pm

mjr wrote:I can't quite accept that magnetic dynamos aren't more developed

More developed in what sense? They're efficient and coupled with LED lights near perfect imo.
mjr wrote:nor that there seems no good off the shelf tools to convert battery lights to dynamo power

That's easy. Battery lights come in all shapes and sizes, power, battery type etc etc. You'd need all the internals of the dynamo light anyway and all you'd end up keeping is the casing and the LED. You'd need several versions that would be fairly specific to a small range of lights and ultimately there'd be a tiny market which would push the price of your product to levels where people would simply buy a dynamo specific version.

I'm a fairly recent convert and a big fan of dynamos, but they can be expensive in a world where cheap bikes abound. If you do a lot of riding in the dark then I couldn't recommend them enough.
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Re: Battery versus Dynamo

Postby mjr » Wed Oct 16, 2013 12:38 pm

kwackers wrote:More developed in what sense? They're efficient and coupled with LED lights near perfect imo.

More developed in the sense that those I've seen (freelights, reelights, magtenlight) seem to work only with their coupled LED lights - or at least the dynamo outputs aren't spelt out. What happens if they break? OK, freelights are so cheap I wouldn't mind buying another set, but they seem like strictly be-seen lights (I've not found a good review yet) so I'd still need my battery front light, so I probably won't bother until my old AA-powered Basta rear light fails.
kwackers wrote:Battery lights come in all shapes and sizes, power, battery type etc etc. You'd need all the internals of the dynamo light anyway and all you'd end up keeping is the casing and the LED.

Considering my collection, most of them seem to be powered by pairs of AA or AAA cells. I've one that has three AAA and maybe one that has four AA, but it seems like if a dynamo add-on could deliver similar power in battery-shaped packages, it would convert a lot of them.
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Re: Battery versus Dynamo

Postby [XAP]Bob » Wed Oct 16, 2013 2:34 pm

mjr wrote:I can't quite accept that magnetic dynamos aren't more developed


Neither is the wheel - we've had only one significant advance since the circle was chosen as the shape, and Dunlop made that in the 1800s

What developments would you like to see?
I could see the desire for a 12V version - but B&M do one, and I don't think it sells in large numbers.
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Re: Battery versus Dynamo

Postby mjr » Wed Oct 16, 2013 3:06 pm

[XAP]Bob wrote:What developments would you like to see?

  • Documented power output details, similar to hub dynamo brochures like http://www.nabendynamo.de/service/service_en.html
  • Interchangeable lights and dynamos, similar to how many other dynamos seem to have settled at 6V 3W - but it'll probably be something else as I expect magnet systems produce less
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Re: Battery versus Dynamo

Postby [XAP]Bob » Wed Oct 16, 2013 4:05 pm

Oh, you mean magnic/reelight systems....

Yes, published figures would be good, but they would wildly vary based on exact magnet separation..
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Re: Battery versus Dynamo

Postby mjr » Wed Oct 16, 2013 5:44 pm

[XAP]Bob wrote:Oh, you mean magnic/reelight systems....

Yes, published figures would be good, but they would wildly vary based on exact magnet separation..

Yeah, we're rather lacking a collective name for the new wave of spoke-magnet dynamos. I forgot magnic earlier and I noticed the M-Wave Helios rear light today but there's very little online about it.
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Re: Battery versus Dynamo

Postby andrew_s » Wed Oct 16, 2013 11:54 pm

mjr wrote:Yeah, we're rather lacking a collective name for the new wave of spoke-magnet dynamos.

Spoke magnet light seems good for the Reelight/M-Wave lights.
Magnic lights work on a completely different principle, so shouldn't really be lumped in with the other two.

It would seem that the front Magnic lights are somewhere around 100 lumens each when well set up. Based on this, I would expect the rear to be similar in brightness to a Smart R1 rear (1W into a good white LED is ~ 100 lumen).
I should note that advertising claims that these non-dynamo lights "Work without increasing friction" should be taken with a large pinch of salt. Lack of friction is not at all the same thing as no drag - for example, eddy current brakes are used to stop trains, and they also have no friction. Dynohubs also have no friction other than that in the wheel bearings, the same as any front wheel.

And, completely off topic...
[XAP]Bob wrote:Neither is the wheel - we've had only one significant advance since the circle was chosen as the shape, and Dunlop made that in the 1800s

Not so.

Dunlop tried to patent the pneumatic tyre in 1887, but failed because it had already been patented by Robert Thompson in 1846.
George Cayley invented the tension spoke wheel in 1808, as an improvement to the cart wheel with solid wooden spokes, which was too heavy with the gliders he was experimenting with. William Stanley improved the wheel again in 1849 by substituting steel wire for the cords that Cayley had used.
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Re: Battery versus Dynamo

Postby [XAP]Bob » Thu Oct 17, 2013 8:07 am

I had wondered whether to include tension spokes - Ok, 2 improvements in the 1800s - but we seem to be going back to cartwheels now (CF, 3 spoke designs)
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Re: Battery versus Dynamo

Postby mjr » Thu Oct 17, 2013 11:10 am

Getting back to dynamos - as I mentioned before, I've done far too much looking into this and it seems hard to find a good dynamo retrofit bundle, so I might as well write down what I've found.

The only things sold as bundles look awful at around the £10 mark - wouldn't a good starter/conversion bundle be a good idea for a smart shop? Here's what would persuade me to switch:
  • Bright enough to see on country lanes - for comparison, I'm currently using an old BS Cateye with a 120 lumen LED upgrade bulb, which is basically the power pictured in viewtopic.php?f=18&t=71242 with a more focused A-shape beam
  • No more expensive than if I had to buy a complete set of new equivalent lights and decent NiMH batteries - I make that about £50.
  • I can actually set it up on a Revolution Streetfinder or similar - this shouldn't be a difficult requirement, except that luggage on the pannier rack would block lights wired to the rear stays and possibly the seat post.
  • Standlights front and back - I prefer them, plus drivers here are spectacularly unobservant and I don't wish to give them more excuses.
I'm think my price-limit-of-a-non-believer and the fact that I can't rebuild wheels myself rules out decent hub dynamos for now. It would be nice if I could use the same lamps if I get one later, but it's not a requirement.

Either my "bright enough" or standlight requirements rule out all the spoke or rim magnets I remember, which leaves bottle dynamos. So far, the reasonable options that I've found have been:
  • http://www.dutchbikebits.com/ - Axa HR + Basta Ray + Union UN-4268 + sundries which I think works out about £45 after delivery and conversion.
  • "Sport Direct" Secu Sport and Champs Elysees lights from the horrid Amazon with their own-brand bottle dynamo or a Union or a low-end Axa or Basta for about £35 or an Axa HR from another supplier £45
  • http://www.bike-discount.de/shop/k342/lights.html - Axa HR + Axa Pico 30 T Steady Auto LED headlight + Axa Ray Steady LED which I think works out about £45 after delivery and conversion.

Are there others that you would suggest?
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