Any way to achieve 10 watt output from a dynamo?
 

I will be doing a lot of camping on my bicycle in the winter and am looking for a way to charge my laptop by a dynamo. My laptop is an ultra efficient Asus model and only uses around 8.5 watts per hour of use. Given that no solution is 100% efficient I gather I will need around 10 watts of power per hour to charge the laptop for each hour of peddling. I know I could charge it over a longer period of time however I want to aim for 1 hour of peddling equating to 1 hour of charge.

Is there anything out that that would suit my needs, or maybe something I could adapt / make? The highest power dynamo I seem to be able to find is the Busch and Muller Dymotec S12. This outputs 12 volts but only at 6watts. If I went for something in the 12 volt range would I be able to use a 12 volt car charger with the laptop to take it up to the 19 volts that the laptop charges at?

Any ideas would be much appreciated.

Not sure of the actual wattage out but hub dynamos reportedly put out in excess of 6 watts at higher speeds.

The off the shelf solution is the B&M E-Werk. It is designed to run off of a 6V dynamo, hub or wheel driven I believe, and charge many portable electronic devices.

http://www.peterwhitecycles.com/ewerk.asp

BTW this might better have been posted in the lighting and electronics forum.

Thanks for your reply. I am well aware of the E-Werk, however you still have to get juice into it. So my question is what dynamo will output 10w per hour? The 12 Volt B & M outputs 6w, but is there anything that would output 10w? I know I could go for less than 10w but am aiming for a system where one hour of cycling equates to one hour of charge.

Minor correction, power units such as W are not 'per hour', you just need a 10W power source.

Also,
1. Your notebook probably need that power to be at a specific voltage range (that your power source need to match). The label on the back of the computer should tell you the nominal input voltage (e.g. 20V DC).

2. Your notebook probably as a LI battery and a charge controller that controls the charging sequence that include several states (e.g. http://www.batteryuniversity.com/partone-12.htm ). These chargers are typically optimized for continuous power sources which is probably not the case with pedal based generators.

3. The charge controller within your notebook qualifies the input power source and determines the charge current. Depending on that controller, you may not be able to charge it at partial rate (e.g. charge for two hours at 5W for one hour of use at 8W).

That's my 2c

You make some interesting points. Any ideas where I could find the specs of the charge controller as I am conscious it may only start to charge at a certain number of amps etc. I know the Ewerk can output 1.5 amp so I would have thought that is ok? The only problem is the Ewerk outputs 13.3 v and not the 19v that my Asus Ul30 uses to charge. Other specs of my laptop are that it uses a 84wh battery pack and seems very efficient in that fact that the computer only uses 8.4 watts per hour - as it lasts for over 10 hour before the battery is totally depleted. As the power requirements are not that I great I think there must be a solution where one hour of peddling could equate to 1 hour of charge?

Not possible to obtain and light and cheap portable charging station where 1 hour charge = 1 hour use. The nominal draw from an AC notebook transformer is 35W. To get 1 hour charge for 1 hour use, you have to use the best battery technology with at least 40W input from a power source.

Having tried a couple bottle dynamos before going back to my Niterider Moab, I was amazed bt how much resistance the dynamo added when engaged! These were 12V/6W units, so if you were to find a 10W one, it would nearly double the required effort to power it. We're talking about 8amp at 12V to get 10W, which is a very healthy draw.

My solution for touring (though probably not winter tours) is to put a 20W (nominal) solar panel on top of my trailer, to charge an ATV battery, which I can then use with an inverter to power my laptop at night. Of course, I will have a charge controller between the panel and battery. I can also charge different units (rechargeable AAA's, Moab battery, notebook, etc.) during the day while riding.

Check out Oatley Electronics down under. I don't know of any specific item but they seem to have all kinds of diy power sources, including solar. Also, if you're into audio, they make a really cool tube based headphone amp that'll fit on a handlebar. It needs 9v, so you could power it by the dynamo with the proper voltage divider.

A hub dynamo (SON for example) can put out 10-12 watts if you build the correct circuit to do some impedance matching. See the site below for details on providing power to LEDs for lighting. I've been using circuit #11 for a couple of years to power a string of 6 LEDs - works great. I don't know if there is any off-the-shelf stuff that would work.

http://www.pilom.com/BicycleElectron...moCircuits.htm

The computer does not use anything like 40w for 1 hour of charge. I don't know where you got this figure from? Even if the computer uses a charger that is 40w output, this just means that the battery will charge quicker. It certainly won't take all of 40w for one hour of charge. On some laptops they are very power hungry, but I have measured the power consumption on mine and it only equates to 8.4W used per hour. With brightness turned down etc I can get 12 hours of battery life, however more typically with wifi on, screen brightness normal etc I get an impressive 10 hours of use from a 84wh battery. So where do you get a figure of 35 - 40w needed to charge for one hour?

You've got a lot working against you here and a lot of confusion about units of power, voltate, current and time. First off a bicycle dynamoe is a variable voltage output where the final voltage depends on how fast you are going. And from what I've seen here they can produce in excess of 13 volts without too much grief or leg energy. So far so good. But is that an unloaded or loaded voltage? Many such generators will show one voltage when running with no load but when a load is hooked up the output voltage will drop in relation to how much current is being used. More information on the dynamoe's specifications would be required. Such as "so many volts at such and such a current draw" to know if the dynamoe is suitable or not.

Some confusion concering the units of power... Your laptop uses 8.5 watts to run. Not 8.5 per hour, it's 8.5 watts of a continuous nature that runs for an hour to equate to 8.5 watt-hours of consumption per hour. A watt is 1 volt running at 1 amp. The equation is watts = volts x amps So if your pack is a 9 volt output then the computer is drawing 8.5 watts then the current is 8.5/9= 944 millamps or .944 amp. Time does not enter into the equation other than to understand how long your battery pack will last. For example if your pack is a 4.5 amp-hour pack then it will last for 4.5/.944=4.77 hours. Another way to look at it would be that your pack has 4.5 amp hours at an output of 9 volts. Amps x volts being watts means that it can produce 4.5 x 9 = 40.5 watts for one hour or that it holds 40.5 watt-hours of energy. At an 8.5 watt draw that pack would last for 40.5/8.5= 4.76 hours. The small error in the last number comes from rounding off in the calculations but matches closely enough.

So now that some of these misconceptions are cleared lets look at how you could use your dynamoe to charge your computer. First off as noted by Kamsta you need to learn more about your notebook and how it charges from the external source. Look at the output of the power supply. If it outputs 12 volts at such and such a current then you will need to match that ability with the dynamoe so that you correctly power up the charge control circuitry in the notebook. Anything less than that and the circuitry won't be able to power up and achieve the correct control of the battery charge. It is highly likely that you will not be able to generate that much power and at best you will get some beeps and other warnings of a "brown out" condition existing. It would also not like being interrrupted by the frequent slow downs or stops. The charge circuitry will work best with a stable power source. That is NOT going to be a dynamoe output by any stretch of the imagination. It is likely that your notebook came with a car charge cord. Using that would be a better option. But again it will likely complain about all the "brown out" time where you are not moving along at a speed high enough to generate the voltage and current needed to "light up" the onboard charge control circuit.

So that means you would have to remove the battery from the notebook and hook it to some form of low voltage and low operating power tolerant controller. One that will tolerate being frequently switched on by the rising voltage and switched off when it drops below the minimum needed. In addition it will also need to be able to check the battery pack voltage during the "on" times and control this so that it does not attempt to charge it beyound the specified maximum voltage given for the fully charged state. It would need to be smart enough that if it sees that final charge voltage once that it remembers that and does not continuously try to keep charging the pack again and again each time the charge circuit wakes up as the bike stops and starts.

I was going to tell you that such a thing does not exist but apparently there are some options. I checked Google and it looks like others were on the same track. Here's a link to a news item that will help point you in the right direction.
http://www.bikeradar.com/news/articl...e-dynamo-26428

And here's another link to a write up with some more info on the factors concerning charging lithium ion cells from a dynamo.
http://minisystem.blogspot.com/2008/...-charging.html

Now the question would be to find something that is made that will do what you need. I'll leave it from here for you to search and find a suitable option. Just keep in mind the power requirements for YOUR battery. It would do you no good at all to hook your 9 volt pack to a cell phone charger that is looking for a 3.6 volt pack.

Also I'd suggest that you don't get all bent out of shape looking for a dynamo that will provide 10 watts of charge power. At this point just finding something that will work at all would be a good thing. From there try to optimise the ratio if you can do it using the knowledge that you pick up along the way.

Actually he was right. No modern power supply for a computer will use much less than that. This is because the power supplies are intended to charge the pack in only an hour or two from fully dead. Anything less would be intolerable to most of the buying public. If your power supply was intended to charge at a lower rate that only just matched the operating need then it would be not be able to run the computer while charging the pack at the same time. So yeah, your power supply is going to have about a 35-40 watt input to provide about a 30 to 35 watt output to the input of the computer. The difference being lost as heat from the outside of the supply due to the internal losses involved with the voltage conversion. And it NEEDS that much power to charge your pack in only an hour. And in fact to charge an 84 watt-hr pack in only an hour it would need to supply 84 watts for an hour, not a mere 35 -40 watts. A 35-40 watt supply would require a little over 2 hours to fully charge a dead pack. And that much longer if the computer was on so that it draws away 8.4 watts to run the computer instead of supplying the battery pack.

I am well aware of that but I did not say anything about wanting to charge the laptop from the dynamo in an hour! That would just be silly! What I said is I want one hour of pedalling to equate to one hour of charge. I think the poster misunderstood what I was trying to say.

Then you were misunderstanding on that part. If the supply supplies 40 watts worth of power the charge cuicuitry in the computer will happliy use it to charge the battery as fast as the supply will allow. It also points out that the charge circuitry in the computer likely won't be happy with an intermittent trickle of power from a dynamo and just charge over a longer time. It's possible that it would but that would require some careful testing to monitor the charge rate at the battery pack terminals.

In any event I hope the other material I provided sheds some light on the issue and helps you with your search for a solution.

This sounds like the way to go.

You are making the mistake of confusing an hour of computer use with an hour of charging time. Normally an hour of charging will give two or three hours of use, and then you have to add on inefficiency in the charger and the cell.

All the bicycle lighting dynamos (no "e", by the way) that I have seen do not really match the technical description of a dynamo, rather, they are alternators, producing alternating current at a frequency that varies with the speed of rotation. So add in the losses of a rectifier circuit, and it becomes even less practical.

Well I looked at the links and those National Semiconductor LM3622 chips look interesting. I am going to research more into these. Thanks for the links

If you're truly serious about all this something to consider is that most dynamos are going to be optimized to run a little light. That means low voltage and relatively high current. But in your case you will require a higher voltage and relatively low current. To get there if you cannot find a suitable dynamo you may have to look at rewinding an existing unit with more turns of a finer guage wire. I'm not sure if you want to go to that extent or not. It can turn the project into something pretty serious.

I would also only consider the wheel hub units since the tire sidewall runners produce a noticable friction all their own. Not to mention the howl that the toothed wheel produces.

I'm also thinking that sucking away about 10 watts of rider power is going to make the bike feel like you're riding up hill constantly or that the hills are just that little bit steeper. I know that the rim style dynamos were famous for making the rider feel like they were pedalling through mud when snapped over onto the sidewall. I was never sure if it was due to the mechanical aspects or the actual electrical drag. But everyone likes the hub dynamos so much that I can only assume that the drag produced to light the road is minimal. But it may be a lot more noticable at a 10 watt power level.

If I optimized a dynamo in this way do you know whether it would be realistic to expect 15v at fairly low speeds? Do you know of anyone who has modded a dynamo in this way? I really don't know how this would equate in real figures? What sort of voltage would I expect from a standard 6v dynamo if I used finer guage wire, but doubled the turns?

You can wire it for any amount of voltage... but the amount of power you can supply (the wattage) will remain the limiting factor. If you wire it for very high voltage you will only be able to provide low amps (current). If you wire it for very low voltage you can have very high current.

The human body is a very feeble machine and 10W will be a very large load for you to carry. I'm not saying your plan is a bad one, jut that you should realize thatit's gonna be difficult a the best of times.

And if your laptop uses ~10 watts of power (rounding up from 8.4W), and you figure out how to charge it at 10W, it will take as long to charge as it does to discharge (~10 hours), assuming no losses (which is probably not a good assumption).