Commuting - Battery vs. Generator

I was looking at generator systems recently. Some thoughts on battery vs. generator systems:

Advantages to battery systems:

(1) More power available. Generator hubs or sidewall units run with 3 watt lamps. Battery powered commercial units can run up to 40 watts. HID lights make even brighter apparent light possible with less than 20 watts. Typical halogen systems have 12 to 20 watt lamps, many times more power than generator lights.

(2) Less energy cost to the rider. A high-end generator hub, like a Schmidt, still brings some drag. The drag of a top end Schmidt hub with a 3w light on will be between 6 and 7 watts, depending on one's speed.

(Too see more, click here:

http://simon.trinhall.cam.ac.uk/bike/dynotest.html#drag)

This is a significant amount of drag. On flat ground, 7 watts drag will reduce speed about 1/3 of a mph at 18 mph. 7 watts drag at 16 mph reduces speed 2/5 mph.

To put this into some kind of comparison, expensive time-trial aero wheels reduce drag compared to standard spoked wheels by 12-13 watts at racing speeds. The performance cost of a generator at commuting speeds, then, is more than half that of the benefit provided by expensive high-end aero wheels at high speeds.

Sidewall generators are even less efficient, and slip in wet weather.

With a hub generator, even when the light is off, you pay a small cost, about a watt or two. That reduces speed a very small amount, about .1 to .04 mph.

With a battery system, you must carry additional weight in batteries. However, on flat ground you can well over _10_ pounds of batteries before you suffer a drag penalty equal to that of a high-end generator hub with the light on. That's a _lot_ of batteries. On a 5% slope, you could carry more than five pounds of batteries for the drag cost of high-end hub generator. That's still a lot.

Furthermore, if you're riding in the daytime you don't have to carry the light or batteries at all.

(3) Battery systems are more easily swapped from bike to bike. For most, you simply undo a handlebar mount attach it to the other bike, and put the battery in the other bike's bottle cage.

Generators have some advantages:

(1) Indefinite run time. Good hubs will last tens of thousands of miles. You can ride all through a single night, too, with no stopping, no battery purchases, no additional weight penalty in extra batteries. (Though with some battery systems, those that don't use rechargeable batteries, you could throw away batteries as you used them through the night, and/or buy new ones as you went.)

Sidewall generators might not last quite as long, but they will outlive their cost in batteries many times over.

(As far as continuous riding in a single night goes, it's worth mentioning that a 4 watt Princeton headlamp will run about 2.5 hours on 4AA batteries. A night's worth of batteries will weigh much less than 5 pounds, the drag cost of the generator. You do have to stop and change the batteries, though, which might give the overall speed advantage over a night to the generator. )

(2) Worry-free lighting. It's always available whenever you need it, with no problems of dead batteries.

(3) No batteries to buy. The generators are cheaper over the life of the unit if one rides at night regularly.

(4) More environmentally friendly.

My take on this overall:

3 watts isn't very much light, even when tightly aimed. I would prefer battery systems for most night riding for that reason alone. I like the flexibility of bike-to-bike transfer of the lights, too.

The first of these two reasons is the primary reason, I think, why generator lights don't catch on in the U.S. People who buy lights almost always want more light than generators can provide.

The drag of even high-end generators, too, is significant.

If I were riding a lot at night on reasonably well-lit roads, though, and was therefore less interested in producing all of the light I would need to see _by_, then I would look long at the generator systems. They would save a lot of money over the life of the system, and one wouldn't have to worry about dead batteries.

Neat information, Merriweather. Thanks.

It seems a lot of your experience is with the inexpensive generators used in the USA.

You should see the 12 volt bicycle generators they use in rural Africa and other developing countries. I was investigating purchasing bicycles wholesale in China last year and a manufacturer asked me if I wanted 6 volt or 12 volt generators on the bicycles.

"12 volt generator on a bicycle?!" I asked. Sure enough they sold them to countries where bicyclists really need lights at night to actually SEE things rather than just identify themselves to automobiles.

They must cause a lot of drag to be sure.

A good compromise are the Japanese 6 volt generator lights. I don't know what amount of light they put out, but they are bright and you can really see where you are going with them. They are better than the USA toys and they don't cause much more drag.

Although I have moved away from generator lights in recent years, I like them and appreciated always having reliable light without a flickerin battery or other trouble.

The main reason I moved away from generator lights is that you don't have light when you stop at an intersection which is when you need them most.

Mike,

You're right, I was considering only 6.* volt generators, the sort that are connected to 3 watt bulbs. These aren't all inexpensive, though; the Schmidt hubs I discussed are nearly $200, without a lamp and without any wheel work you'd want. These are at the high end of hub generators. The drag figures I was discussing are for those high end hub generators.

I do recall hearing about the 12 volt lights, though. I've never seen them, or heard much about them. If they power a 6 watt lamp, though, the drag will be significant-- between 15 and 20 watts! On the other hand, having a 6w lamp with the other benefits of generator lights might be precisely what many cyclists would prefer.

In case this might lean you back toward generators, there are lamps now available that have a capacitor and an LED light. This light comes on when the halogen light goes out, and will remain on for a couple of minutes even when the bike is not moving. So, at intersections or making a left turn you can be seen. I don't know off of the top of my head where they can be found.

www.peterwhitecycles.com perhaps?

Cheers,

I haven't had a generator for 25 years. Mine was the old sidewall generator.

I like the idea of generators. They are self-sustaining.

But for now, I enjoy my rechargable lights.

Hi,
was at my LBS today, he showed me
a new light, it mounts on the brake and has a generator built in. Just unclip, and you have light. I could (will) live with the
small amount of drag it provided. I thought it was cool.

Late, do you recall the brand of that light you saw at the LBS?

I have been interested in dynamo lights but wanted brighter lights. Perhaps if someone came out with a light like the new Cateye 5-LED light or perhaps a low wattage light using the new HID electronics to get 10-12 W equivalent light from a 3 watt draw.

I have seen reviews of the LightSpin dynamo using Hella FF lights that sounds pretty good, but so far I don't think either is available in the US. LightSpin is supposed to come out with a unit that includes a NiMH battery that will supposedly provide up to 45 minutes of light.

One of you guys in England had one of the original LightSpins. Has anyone gotten one of the new ones with the battery? Anyone using the Hella FF lights?

Hi Rainman,
my LBS says it's a Union 9853 Halogen Generator set.
Hope that helps. I don't know about output, it looks like the same light a lot of the Euro generaors use, but I don't really know.

Originally posted by Merriwether

I do recall hearing about the 12 volt lights, though. I've never seen them, or heard much about them. If they power a 6 watt lamp, though, the drag will be significant-- between 15 and 20 watts! On the other hand, having a 6w lamp with the other benefits of generator lights might be precisely what many cyclists would prefer.


Tell you what, I can get some of the 12V Chinese generator lights pretty easily with my connections.

I'll get a couple, try them out, and let you guys know how they work.

Your comments about the high-end hub generators are incorrect. I use a Schmitt for randoneuring and all night rides. Unless you get down to very low speeds, such as climbing at 6 mph or less, you cannot detect the drag. Riding at normal speeds 15-20 mph, I can switch the unit on and off while coasting and cannot tell the difference. I ride with the unit on the bike all the time, including some rather fast sporty club rides. It does not slow me down at all.

Dave

Originally posted by hayneda
Your comments about the high-end hub generators are incorrect. I use a Schmitt for randoneuring and all night rides. Unless you get down to very low speeds, such as climbing at 6 mph or less, you cannot detect the drag. Riding at normal speeds 15-20 mph, I can switch the unit on and off while coasting and cannot tell the difference. I ride with the unit on the bike all the time, including some rather fast sporty club rides. It does not slow me down at all.

Dave

Dave, the drag figures I posted are consistent with their being imperceptible. But the drag is still significant.

Dynamo manufacturers will tell you themselves the drag of dynamo hubs is equivalent to climbing 7 feet per mile. This doesn't sound like much-- almost like flat ground, right?

It's more than it may seem. No, it is not a directly detectable amount of drag. However, it is a drag equivalent to a ~7 watt power loss, for a typical rider. That amount of power loss will slow you per hour the amounts I've posted. It's more than half of the benefit of expensive aero wheels.

Check the figures yourself, if you're a physics geek.

Or go to www.analyticcycling.com play around for a bit.

Cheers,

Originally posted by mike


Tell you what, I can get some of the 12V Chinese generator lights pretty easily with my connections.

I'll get a couple, try them out, and let you guys know how they work.

I would be very interested to hear about these. Do let us know.

Thanks

The main reason I moved away from generator lights is that you don't have light when you stop at an intersection which is when you need them most.

i have never seen a unit that does this, but i read about one a few years ago and thought about building my own... the one i read about used big capacitors to store charge (like what a flash does) that then gave like 1-2 minutes of slowly dimming light when stopped.

another idea would be to actually power the light from a battery and use the generator to charge the battery so you have light while stopped and more light at slow speed. i have never seen one but it shouldn't be too hard to build - much like an motorcycle system with the alternator/battery just less power. as i see it there would be 2 options:
1) If the generator power roughly matched the battery then you have an unlimited duration light of about 3 watts that runs continuously even while stopped -- of course if you spent long periods stopped or riding slowly you would drain the battery and then only have a normal generator system, but acceptable since you'd have the same problem with a battery-only system
2) if the generator was weaker than the light then you would just be extending your light duration, but this could be significant -- if a 5W light with normal batteries lasted 2 hours, when it could be extended to near 5 hours with a 3W generator (if you were riding at speed most of the time) ---- of course, here you have the disadvantage of extra weight from batteries (although less than battery-only system) with the drag from the generator --- but for situations like commuting where reliable light is more important than speed it might be ideal.

Originally posted by nathank



another idea would be to actually power the light from a battery and use the generator to charge the battery so you have light while stopped and more light at slow speed. i have never seen one but it shouldn't be too hard to build - much like an motorcycle system with the alternator/battery just less power. as i see it there would be 2 options:

Nathan, I have seen plans for building the recharging generator as you described, but then I figured, "what's the use of lugging along all that extra hardware?" (battaries + generator + light) in addition to the extra load of the generator. I just switched to rechargeable batteries.

About generator lights that continue to shine when you stop:


If you visit Peter White Cycle's site (http://www.peterwhitecycles.com) and check under Lights –> Schmidt Dynohub page, you will notice that there are two Lumotec Plus lights offered with a standlight feature. Likewise, there is a generator-operated rear light that offers a similar feature.

There are, however, other less expensive options.

1. Rear blinker. With LED technology, battery taillights are quite interesting, with 30-50 hours per set of batteries. That solves the main issue I have with non-permanent lights. Besides, when you ride, it gives you a second (or third) red taillight.

2. White or amber blinker. Ditto. On busy commercial throughfares, I have more problems with pedestrians than with cars. The 5 W headlight with a blinker seems to attract more their attention than a 10 or 15 W headlight by itself.

3. The new Cateye EL-200 or EL-300 LED headlight. The 300 is more powerful, but the 200 has flashing ability. You not only have something for stops, but also some light to supplement the generator in long uphills, to light directly at potholes...

Regards,

I have 2 6v, 3watt lighting systems, one powered by a generator (Swiss-made quality model) and one from a lead-acid battery in a water bottle. I find that the generator is better for fit-and-forget style riding, where you do lots of small journeys, or for use when touring. The battery is better for regular commuting style journeys, and for riding up steep hills and muddy trails.
3watts is quite sufficient for on-road riding, esp when used with modern European bike-specific optics. The 10w off-road systems use generic sealed beam units which are not optimised for cycling, so waste a lot of their output. Modern computer-designed optics and standlight systems are so good and cheap, I sometimes wonder why people try to fabricate their own lamps from old flashlights etc.
Generators have come a long way since those 1970s Union models which clamp onto the stays. High-efficiency units with rubber rollers, fixed to a braze-on mounting tab solve most of the problems associated with those old units.

Well, revisiting this topic renewed my interest in dynamo lights so I launched another search project and found a vendor of the LightSPIN dynamos in Canada (bikespecialties.com) so I ordered one. It's the original basic model with no standlight. Once I get it I will give trial on my every-morning-in-the-dark 9-mile commute and let you know what I think. I plan to use it with a Hella light. I will continue to use LED blinkies in the rear. It will be very interesting to see how the light compares to things like a Cateye Micro Halogen and Vistalight 10W. I find that sometimes a clearer light from a much lower power unit is more effective than the dull yellowish light from a higher power lamp. You know I will post a report.

I will be out of touch until after Thanksgiving so if you address me I'm not ignoring you.

Rainman,

I agree. I have a 3W Lumotec with Schmitt hub and its light is actually better than my 15W halogen rechargeable. The rechargeable does put out more light, but its not nearly as well foucused for road riding as the Lumotec.

Dave

A little personal experience with generators. I recently installed a Dymotec S6 (6 volt) generator on my bicycle with the Lumotec Plus headlamp. The headlamp has a capacitor and an led so the light stays on (for third party visibility purposes only) when you come to a stop. It takes about 3-4 minutes of riding to charge the capacitor, but then the led will stay lit for about five minutes. It is bright enough to make you very visible but isn't designed to give enough light to ride by. That isn't an issue though, because as soon as you start up your normal light comes back on. The German designed unit comes up to full brightness at about 6 mph and the lights have overcharge circuitry to prevent them from blowing out at higher speeds. Contrary to what seems to be a common opinion, there is plenty of light for riding at night. At 49, my eyes aren't what they used to be, but I have no problems at all with the amount of light this unit puts out.
Now, the subject of drag. . . Give me a break! If the unit is mounted correctly, you won't even notice it. The unit is almost completely silent and is near bullet-proof. I have nothing against those who choose to go with battery powered lights, but there really is a bunch of crap floating around from people who haven't used well-designed generator sets. They're a whole different product than most people realize.

Originally posted by LET

Now, the subject of drag. . . Give me a break! If the unit is mounted correctly, you won't even notice it. The unit is almost completely silent and is near bullet-proof. I have nothing against those who choose to go with battery powered lights, but there really is a bunch of crap floating around from people who haven't used well-designed generator sets. They're a whole different product than most people realize.

:roflmao: :roflmao:

Well, power's power. You can't get it to the light without taking it from somewhere. Take it from the rider, and he slows down.

If ever a generator manufacturer figures out how to get power from a cyclist without reducing his speed, I'll run out and buy a generator the next day. I won't use it to power a light, though. I'll use it to make the bike go faster...

You are of course correct, the power is coming from the cyclist. . . but this is commuting, not road-racing. Any time you hang a rack, or for heavens sake, a pannier with clothes inside, you're going to pick up some drag. Hanging a two pound battery on the bike slows it down too. . . but by such a small amount that it is inconsequential in the big picture. That's the point I was making about the drag from the generator. In my experience, based on actual usage, the drag issue is a non-item. It just isn't enough to notice.

I am interested in those who talk about commuting and speeds of 25-30 mph. Possible yes, but highly unlikey for most commuters. In the real world of using a bike to go to and from the office, with the attendant racks, bags, etc., the small amount of extra drag from a generator is a bit like worrying about the space taken up by a book of matches in a high school gymnasium. Sure they take up some space. . .

Originally posted by LET
You are of course correct, the power is coming from the cyclist. . . but this is commuting, not road-racing. Any time you hang a rack, or for heavens sake, a pannier with clothes inside, you're going to pick up some drag. Hanging a two pound battery on the bike slows it down too. . . but by such a small amount that it is inconsequential in the big picture. That's the point I was making about the drag from the generator. In my experience, based on actual usage, the drag issue is a non-item. It just isn't enough to notice.


Yes, well, I can understand someone saying that he doesn't mind the additional drag of a generator because of the other advantages. The drag cost may be more than most riders appreciate, however. In any event, it's worth being clear on what that cost is. And what is that cost? Again, if you would be otherwise traveling at 15-18mph, and if you lose seven watts power, you plus bike weigh somewhere around 175 lbs, and you're on flat ground, then you go anywhere from 1/3 to 2/5 mph slower.

That's the cost, for better or worse. That's just the way it is.

This reduction amounts to less than two minutes loss per hour. That's not directly detectable, and certainly not to recreational riders with changing conditions from ride to ride. So, it's no wonder people say their generators make no perceptible difference. But that's a significant drag cost anyway. Some riders might decide they'd rather not pay that penalty for 3w of light when there are battery lights around, as the weight of batteries doesn't cause drag anywhere near generators. That's the way I myself lean, but again I understand someone saying that he doesn't care so much about the drag as about the other generator benefits.

The thing is, if the drag is worth it, you would think those who think so could just come out and say that. In fact, some riders have done that. But you weren't quite able to that, apparently. Unless you'd have us believe that "there's a lot of crap floating around this board" means "I agree with the remarks about the drag cost, but I think the cost is worth it".

Whoa,
it's 'not perceptible' but it is 'significant'. You're contradicting yourself. My guess- -you don't think it's cool. I hate dead batteries, and the one I am getting weighs but a few ounces,is physically small, has this tiny amount of drag, and is,--IMO--,cool.

There is some drag, obviously, because one needs at least 3 W to produce 3 W of light. Some data may be found at Peter White Cycle's (http://www.peterwhitecycles.com/schmidt-lumotec.asp) website.

At 25 km/h (approx. 16 mph), a good quality regular front hub requires 0,7 W whereas the SON (Schimdt dynohub) requires 1 W. From other sources, I remember that the Shimano Nexus requires approx. 3 W of power. With light on, the SON requires 6.5 W (and can actually power 2 3-W lamps in series, or 6 W) and the Shimano Nexus requires approx. 7.5 W, and recent bottle generators require 10-15 W. Older 1960-1970 generators may require more than that, BTW.

A typical cyclist produces approx. 70-80 W, so the power consumption from the light is not very important relative to the power of the cyclist.


As for the battery headlight, typical batteries weigh more than a bottle generator. The added weight means more power needed for uphills.

Regards,

Originally posted by late
Whoa,
it's 'not perceptible' but it is 'significant'. You're contradicting yourself. My guess- -you don't think it's cool. I hate dead batteries, and the one I am getting weighs but a few ounces,is physically small, has this tiny amount of drag, and is,--IMO--,cool.

Actually, I think the generators are cool, especially the newer dynohubs. I admire their elegance. All I've been saying is that they have significant drag. I posted earlier a link to serious data on the drag for the hubs, and other generators too. Michel has some data above, too. The bottom line: the drag from very good hubs is about 7w to produce 3w of light.

As far as "perceptible" and "significant" go, I can see how it is confusing to use them without clarifying explicitly what I mean. By "perceptible" I mean obvious to a cyclist directly through his senses, without making reference to any instruments. By "significant", I mean substantial or non-negligible. The drag from the dynamo hubs is small enough that the rider can't notice it without instruments. I recognize that, so I believe that good hubs don't impact the direct experience of riding for those who have them. But the drag of hubs has a significant impact on one's speed anyway. I've explained what impact above.

Just to be clear, I'll say again that I've nothing against generators. I think they're a reasonable option for lighting for many riders. I think their ingenuity is fascinating. But surely those who agree with these things can agree with them even recognizing generators' disadvantages.

Originally posted by mgagnonlv

As for the battery headlight, typical batteries weigh more than a bottle generator. The added weight means more power needed for uphills.



Michel,

No, this isn't true. The extra weight of batteries won't require as much energy as the drag induced by the generator drains even on very steep climbs.

For example, suppose that a battery in question weighs _two and 1/5 pounds_ (1kg) more than a generator. This is an unrealistically large difference, of course. Even old lead acid batteries weigh in total about two pounds, so the difference between such batteries and even a bottle generator can't be over two pounds. But suppose the weight difference is as much as I assume, 2.2 pounds.

Assume a 75kg rider-bike with a high-end hub generator, and a 76kg rider-bike with a battery. Even on a 20% grade (!), ascended at 3mph, the difference made by the extra kilogram of weight will cost only an additional 3w in power to the battery carrying rider. But even a high-end hub generator costs an extra 7w with the light on. So the batteries require less energy even on very steep climbs.

(If we double the speed to 6mph the additional cost of the battery weight is 6w, not quite equalling the generator's drag. But ascending a 20% grade at 6mph requires 460watts, an effort beyond most commutes.)

At grades less than 20%, the advantage of batteries will be even greater than 4w.

These results assume heavy batteries. But newer batteries compare well to generators in weight. For example, my light runs on a 1/2kg battery, nearly as light as the additional weight of a generator hub. That weight I calculate by subtracting a Dura Ace hub's weight from a Schmidt dynohub's weight. The difference between a Schmidt dynohub and a Dura Ace hub: 630grams (Schmidt) - 170grams (Dura Ace hub) = 460 grams.

That's a 40 gram difference, and there's no climb you can do on a bike that will require an extra 7w for that 40 grams.

Of course, a bottle generator will weight less than a hub generator-- but then again the drag from such a generator will be much more than 7 watts. Without going through the calculations I'll just cut to the bottom line: a rechargeable battery will require less energy even on very steep climbs than a bottle generator will. The drag from the generator will cost more than the weight of the battery.

In one way, these results aren't surprising. Generators require power from a rider. Batteries are a power source. From that standpoint it stands to reason that the total speed of a rider with a generator will be reduced.

Generators have some advantages over batteries. I mentioned some at the top of this thread. But one advantage they don't have is allowing a rider to go faster than he can do with batteries.

Cheers,

I find it hilarious that a few folks are worried about 6 or 7 W (light on) or even 1W of drag with light off (reducing speed around 1/3 mph) when we are all talking about commuting! I mean the average commuters that I know have: lights, fenders, rack, panniers, thick tough tires, etc. All this stuff adds a lot of weight to the bike. Commuting bikes are by no means performance machines. I know that the aero drag of my fenders greatly outweighs the drag of a dynohub.

Don't get me wrong, as I use both a dynohub and rechargeables--they both have good and bad points to them. But for commuting, worrying over their insignificant drag is downright silly.

Dave

I ride a Kettler Silverstar with Lumotec headlight and Union generator running against a Vredstein Trigger tire with dynamo track molded in. I can appreciate both sides here. There is an increase in drag with the generator running. I don't notice the drag while riding, but i can feel it while walking the bike.

If I assume a speed reduction from 15 mph to 14.7 mph and a 5.5 mile ride with 5 minutes spent at stopsigns and stoplights. I calculate a bit less than 27 extra seconds required. That is trivial/unnoticable in a commute. In a race, it is huge.

So yeah, the penalty is either impreceptible or huge, depending upon the activity.

Paul

Originally posted by hayneda
I find it hilarious that a few folks are worried about 6 or 7 W (light on) or even 1W of drag with light off (reducing speed around 1/3 mph) when we are all talking about commuting!

But for commuting, worrying over their insignificant drag is downright silly.

Dave

Let's put it this way. If my commuting/touring bike had something wrong with the hub so that I was slowed 1/3 mph I would pay to have it fixed. Most people would. If someone messed with my bike, draining me of 1/3 mph, I would be p*ssed. I wouldn't mess with my bike to slow it this much for no good reason, either.

So, it's hardly ridiculous to think that this kind of drag is significant. Most cyclists think it is (including you, I'd wager).

It's true that commuting bikes, with their racks, panniers, and fenders, aren't designed for speed-- but speed is _traded_ for other benefits. No one just throws 1/3 mph away for no reason. If they did, then you would have a case that 1/3 mph is "insignificant".

When it comes to generators, many people think the benefits of the generators are worth the costs. That's fine by me. I see generators' advantages, and I don't think it's unreasonable to use a generator. But the costs of generators are, indeed, _costs_. Perhaps the costs are outweighed by other benefits, but there's no reason to think that this means that the costs aren't really costs, or are "insignificant". Similarly, the drag is a reason one might point to in favor of batteries.

Pointing out a disadvantage of generators is not to impugn them, all things considered. It's just to talk about generators-- you know, bike stuff.