Complete bicycle electric system: dynamo, batteries, lights, accessories

[alexlockhart]

A frequent frustration for me when on long bicycle tours is the difficulty of keeping my batteries charged for my bike lights, cellphone, camera, laptop, and GPS. Now that I've found that hub dynamos can produce far more than the 6V/3W they're rated for, and now that I have money to invest in an electrics system for my touring bike, I've decided to build a system that does what a car electrics system does: generate electricity from motion, store that electricity in batteries, and use it for lights and other things. I drew up a quick-n-dirty block diagram in MS Paint to help me understand how it will all work, and thought I should include it here. It's at the end, but you may want to skip down and reference it as you read my long and complex explanation of the system.

The power in the system will be generated by a Schmidt SON hub dynamo. All of its power will go to charge a Li-Ion battery pack. The pack will have twelve 18650 cells (wired in 6S2P, output voltage range of 18-25.2V), since that's as many as I can fit in the water bottle they'll be housed in. I plan to charge them by sending the varying AC output of the dynamo to an automatic switching voltage doubler/fullwave rectifier circuit to give me at least 15V at low and high speed, and feeding that into a switch-mode buck regulator which will output 12V. That regulated 12V will power two battery chargers designed to charge a 3S Li-Ion pack from 12V, with their inputs (from the regulator) wired in parallel, and their outputs (to the battery pack) wired in series, so that their total output voltage to the 6S pack will be the 25.2V it needs.

My front light will be 4 Cree P4s controlled by a bFlex, hooked straight to the battery pack. I'll also use a switch-mode buck regulator to output 12V from the batteries, and use that to run 12V car battery chargers for my cellphone, camera, laptop, and GPS batteries.

I've already bought the parts for my front light, but they're not all delivered yet, so I'll assemble that when they come. I also have 4 adjustable switch-mode buck regulators rated for 60V input, which I mentioned above, and my Schmidt SON is on order. I haven't yet bought the Li-Ion batteries or chargers, and my design for that system is not yet finalized, so I'm looking for input.

I have some design constraints. Using 4 Crees for the front light requires a minimum 17V input to the bFlex, so I have to wire my batteries in at least 6S so that I can deliver that voltage when they are almost drained, at 3V/cell. That high battery voltage also means my buck regulator can always deliver 12V to my various other battery chargers. But I can't find Li-Ion battery chargers that can take 12V and charge a 6S pack, so that's why I'm planning to use two chargers designed to charge 3S packs from 12V, and wire their outputs in series.

Almost as an aside, my laptop takes 19.5V, so I will probably set one of my buck regulators to output 19.5V from my Li-Ion battery pack and plug that straight into the laptop, instead of buying a step-up converter designed to power my laptop in a car, since that would be unnecessarily stepping down to 12V and then back up to 19.5V. Also, the laptop power pack is rated to deliver 3.34A at 19.5V when plugged into AC wall power, so I could use one of my switch-mode buck regulators to step that down to the 12V that my Li-Ion battery chargers need, thus enabling me to charge my Li-Ion pack from AC wall power.

One potential problem I see is wiring the output of the Li-Ion chargers in series to charge a 6S pack with two 3S chargers. Does anyone have experience with this? Will the charge regulation be affected?

As I've said, I'm asking for your input, both problems you see and ideas you have. I don't have any experience working with dynamos or building lights, and have only limited experience designing and building electric circuits, but I've been reading and understanding a lot (mostly from the excellent posts here), and I think I have a working design. The details of the light design and building deserve their own thread, and I'll probably need to start a new thread for adapting Martin's voltage doubler/fullwave rectifier circuit to power battery chargers through a regulator, too. Right now I'm just trying to get this electrical system design out of my head and into the real world, and I need your help.

Alex

 

[georges80]

I've got a few concerns/comments.

Taking the dynamo output and stepping it down to then step it up has just doubled your losses (assuming similar efficiencies in the buck and the charger).

I really DON'T think that wiring the two chargers in series is a good idea.

Firstly they aren't likely isolated - you are feeding them from the same nominal 12V input (common ground).

Secondly I very much doubt that the regulation will work with them wired in series. Li-ion chargers are quite complicated in their monitoring. They will attempt to charge at constant voltage (4.2 x n cells) but will also current limit. They will change from charging to not charging based on current flow. If you wire the chargers in series (you have the common ground, non-isolated issue above) and you now have 2 chargers trying to outsmart each other.

I also wouldn't want to run my cells down to 3V - that's definitely FLAT. I think a lot of the issues here are due to the 4 Cree requirement. If you went with 3 you would be able to use a common 14.4V li-ion pack and there are chargers that will happily charge those. You would still be able to charge most of your 12V stuff, since 14.4V is the sweet spot for that pack and by the time it gets to 12.8V output they would be fairly discharged anyway and you should be on your bike peddling

You would also likely be able to just rectify and smooth the SON output and drive it directly into the 14.4V chargers since it would be trying to boost up to 4.2 x 4 = 16.8V (nominal). To be totally safe you could run a regulator on the SON to limit the input voltage to the charger to less than 14V. Considering you are dropping/clipping only a couple or so volts from your SON it's probably as efficient as trying to buck down to 12V.

Anyhow, just some opinions (and we all have those)...

cheers,
george.

 

[n4zou]

Forget all the electronics, you don't need that stuff if your just recharging batteries. The batteries you connect to the dynamo and bridge rectifier will load the dynamo so the voltage from the dynamo will not exceed the rated voltage of the batteries used. It will pump 500mA current into the batteries if nothing else is connected to the charging circuit. I run a 1-watt LED headlight and use the following circuit.

This is a constant charge circuit so occasionally I disengage the dynamo from the tire to prevent overcharging the battery pack with the LED making light. The LED and it's driver consume about 350mA providing about 150mA recharging current when my speed is 10 MPH or higher. At 8 MPH the battery pack stops discharging into the LED headlight. During daylight hours if I need to charge something I just open S2 and connect the battery pack for whatever needs to be charged to the dynamo and bridge rectifier. I've found that a 9-volt battery pack is about all that my cheap bottle dynamo can handle. Most likely a good hub dynamo can produce higher under load voltage requirements. I am running a cheap ($7.20 USD) Schwinn dynamo.

S2 can be seen in the photo along with a terminal strip allowing changes to be made easily. The bridge rectifier can be seen under the switch on a separate terminal strip. It has just been replaced with another bridge rectifier made from 4 1N5818 diodes to increase efficiency. I also use a solar panel while touring for several days or even weeks.

 

[georges80]

Yeah, but the OP was talking about a SON dynamo - a lot more power/grunt than the tyre rubbing generators of old.

He's also talking about a li-ion pack - you DO need a smart charger to prevent your ride turning into a bomb.

Anyhow, good to see all these various thoughts/ideas for charging/running things. Haven't seen any wind driven generators being proposed yet

cheers,
george.

 

[diff_lock2]

He could use the ni-mh to stabilize the power for the lipos, like n4 said.

 

[alexlockhart]

Taking the dynamo output and stepping it down to then step it up has just doubled your losses (assuming similar efficiencies in the buck and the charger). . . You would also likely be able to just rectify and smooth the SON output and drive it directly into the 14.4V chargers since it would be trying to boost up to 4.2 x 4 = 16.8V (nominal). To be totally safe you could run a regulator on the SON to limit the input voltage to the charger to less than 14V. Considering you are dropping/clipping only a couple or so volts from your SON it's probably as efficient as trying to buck down to 12V.

Yes, stepping down and up will double my losses, but the switch-mode regulator is about 90% efficient, and I expect the battery charger is also, so it's not a lot of wasted power. Also, the 3 cell chargers I was planning to use don't step up, they step down - 13.8V (standard automotive "12V") in to 12.6V out. The 4-cell ones you talk about (which batteryspace does not carry - where would I find one?) do step up, from 13.8V to 16.8V. Still, I think the power curve of the dynamo is such that it would deliver over 14V into a high-ohm load (battery chargers) whenever I'm going over 12-15mph, which is often. So I think that a simple shunt regulator would be throwing away more power as heat on average over the course of my riding than double-regulating would. It would take some experimentation to get a better guess.

I'll trust your judgement about wiring the chargers in series, until someone can convince me otherwise. I think you're right that they will just fight each other, since they're not isolated. Now I know I need to find some way to charge the battery pack as a single unit, from a single charger, powered by 12VDC. Any suggestions?

I also agree that 3V/cell is much flatter than I want to run them, but the protection circuit doesn't cut them out until 2.4V/cell, so I just picked 3V as a nominal "dead" voltage. I expect that the battery pack will be near full charge most of the time, since it'll be trickle-charged as I ride very often, and I may be able to fully charge it overnight from AC occasionally. I plan to use the new voltage sensor LED on the bFlex to indicate state of charge, and if it gets low, I'll limit my consumption accordingly.

Yes, most of my problems are being caused by my light design using 4 Crees. I designed that for optimum beam pattern, not maximum brightness, and didn't realize I would run into these battery charging issues. I expect I'll find that the most brightness I'll need will be at the 500mA or 750mA max setting on the bFlex, which will help my light be more efficient. If I can't figure out some way to charge my 6S battery pack, I might experiment with a 3 Cree light and 4 Li-Ion battery pack. That would, as you point out, still easily give me regulated 12V for my other chargers, but then I'd have to step down to 12V and up to 19.5V for my laptop, instead of just stepping down directly. Also, I don't know where to get a 4 cell Li-Ion charger that runs on 12V, anymore than I know where to get a 6 cell 12V charger.

Forget all the electronics, you don't need that stuff if your just recharging batteries. The batteries you connect to the dynamo and bridge rectifier will load the dynamo so the voltage from the dynamo will not exceed the rated voltage of the batteries used. It will pump 500mA current into the batteries if nothing else is connected to the charging circuit.

As George noted, I certainly do need a dedicated smart charger for Li-Ions. Your use of robust and safe NiMHs allows you to use your clever direct-charge method, but you can't use that power for charging other batteries, at least not without some more complex electronics. Also, your circuit limits the maximum voltage of your battery pack - it would not work with 12xNiMH at 14.4V for a 3-Cree light. Another difference is that my Li-Ion pack has about 100Wh, which is 5-10 times as much power as your 2 NiMHs, even if you're using 2 D cells, and my hub dynamo can provide much more power to charge my bigger pack, which will be necessary to power a 10W headlight, 1.5W taillight, 20W laptop, etc. A solar panel would help, but I expect I can get all the power I'll need from a hub dynamo, making the solar panel extra weight.

Haven't seen any wind driven generators being proposed yet

No, and you won't, at least not until wind resistance ceases to consume 90% of a cyclist's power. Water power, though...maybe we could make a water mill using the water in our water bottles?

Thanks for all the helpful suggestions. I'm still looking and designing, so keep the comments coming!

Alex

 

[georges80]

For chargers I saw this on ebay (quick search) item 120164385142

Seems like the kind of thing you need if you went 4 cell. Runs on 12V-16V input so you wouldn't need to shunt much on your SON.

I'd guess that most laptop chargers that run on 12V nominal will do just fine even with 16V input (straight for a 4 cell li-ion pack). Any sane engineer that designs something for automotive 12V will know that they need to handle at least 14.4V (charging battery).

i.e. I wouldn't fixate on regulating everything to exactly 12V or 13.8V etc.

I also just did an experiment (for crazy Darell) that allows one bFlex to control a second (or third) bFlex. That allows the bFlex drivers to run in parallel. Requires removing 2 IC's off the 'slave' bFlex drivers and wiring 2 thin wires (control signals) between the master bFlex and the slave bFlex(es).

Requires some fine soldering - but opens up the option to control multiple LEDs rather than having them all in series...

My test was 6 LEDs (3 per bFlex) running off 12V input. All very happy.

cheers,
george.

 

[alexlockhart]

Thanks for the eBay link. I've never been much good at searching for odd things on eBay, and after a few tries with no luck earlier, I gave up. The charger you found would work, but it's about 5 times larger and heavier than I want. The 3 cell ones I was going to get from batteryspace are much smaller and lighter; I was going to strip them down to bare circuit boards and pop them in my water bottle with a bit of foam. That's the kind of charger I'm looking for: either just a little PCB to begin with, or something easily stripped down to just a little board.

I did some more searching on eBay, specifically for 6 cell chargers, and found item 170150622037, which is a combo of a charger and balancer, and is big and heavy, and super-expensive. It would do what I want, but it's overkill, and I'd hate to lug it around on the bike. That's all I found for 6 cell chargers.

I'm really hoping to find a way to charge 6 cells in series, since that would solve all my problems (at least for now, with this project). It doesn't need to be a fast (high-amperage) charger, since I'll be getting limited power from the dynamo anyway.

I'd guess that most laptop chargers that run on 12V nominal will do just fine even with 16V input (straight for a 4 cell li-ion pack). Any sane engineer that designs something for automotive 12V will know that they need to handle at least 14.4V (charging battery).

i.e. I wouldn't fixate on regulating everything to exactly 12V or 13.8V etc.

I have often thought this way when dealing with electronics, and have cavalierly over-volted things, usually only by 10-20%, and this has led to lots of magic smoke wafting through my room. I'm now much more cautious of exceeding voltage specs on things I need to use (and don't have spares). Things designed to run from automotive electricity should be much more tolerant of higher-than-design voltages, though, as that is often noisy and poorly-regulated, and in older cars, can spike to 20V. I'm not fixating on some magic number of volts, but I do want to design a "bulletproof" system, as I'll be relying on it while I'm out in the mountains of Mexico this winter, far from civilization and safety. So I don't want to run any of my components unprotected or close to their limits, and my voltage regulators are cheap, small, and efficient safety.

Nice to hear about your experiments with master/slave bFlexes - that's a really neat option. I'm not about to spring for another one, though, unless I can't figure out any other way (or any cheaper way) to make this work.

Alex

 

[PeLu]

I certainly do need a dedicated smart charger for Li-Ions.

Depends how you define 'smart'.
I already posted my idea of just charging the Li-Ions to 4.2V/cell and then disconnect them. You will sacrify maybe 15% of their capacity (maybe a little bit more) but gain overall efficiency. Your SON will either work at full 0.5A or idle. No mode in between where you have to waste more energy.
It also will give you more tolerance for ripple.
I would also stay with 3 or 4 cells in series and adapt the other circuits.

... a SON dynamo - a lot more power... than the tyre rubbing generators of old.

Why?

 

[n4zou]

Depends how you define 'smart'.
I already posted my idea of just charging the Li-Ions to 4.2V/cell and then disconnect them. You will sacrify maybe 15% of their capacity (maybe a little bit more) but gain overall efficiency. Your SON will either work at full 0.5A or idle. No mode in between where you have to waste more energy.
It also will give you more tolerance for ripple.
I would also stay with 3 or 4 cells in series and adapt the other circuits.
Why?

Typical Hub dynamos use 26 poles and magnets. BB and Bottle type dynamos use a single round magnet and 4 poles. You have much higher power density with a hub dynamo. Most have Zener diodes that limit output for standard 6V-3W lighting systems. Opening the hub and clipping the diodes allows higher power output. This also adds drag so you'll be pedaling more to overcome it. That's still not as bad as a BB or bottle dynamo.

 

[n4zou]

He could use the ni-mh to stabilize the power for the lipos, like n4 said.

Correct! Use the Ni-MH batteries to charge the other batteries. Just consider them to be voltage and current regulators.

On another note; it takes me 4 straight hours of riding above 10 MPH with the LED light disconnected just to recharge my 2 Ni-MH batteries if I completely discharge them. If running with the LED on it takes 18 hours! Normally I just recharge them at home (unless I'm on a tour and never allow that to happen) if I do that. I just wonder how much time it's going to take to recharge all that stuff, and I don't want to be hauling around all that stuff anyway. I'm already hauling around too must of just basic necessities like a tent, sleeping bag, stove, fuel for the stove, water, and endless other small items. While touring I just use the free computers in public library's for e-mail and uploading photos so I don't need to take the Lap-Top, thanks to Al Gore and the extra taxes on my phone bill to pay for it.

 

[PeLu]

I asked why a SON should be more powerful than a bottle dynamo:

Typical Hub dynamos use 26 poles and magnets. BB and Bottle type dynamos use a single round magnet and 4 poles. You have much higher power density with a hub dynamo.

How many have you tested? If you look into the 'prayer wheel' you can see that several bottle dynamos deliver >0.65A and quite high voltages. (This was actually a draw back for an old style configuration). (Of course if you have one of the newer ones, you have to remove the limiting circuit, but none of mine has (all older than 5 years)).
The lower number of poles is more than compensated by the higher rpm.
The SON was not optimized to give the highest possible power.

 

[alexlockhart]

Hub dynamos vs. bottle dynamos:

There is nothing inherent about a hub or a bottle style dynamo that necessarily makes it produce higher power. Hub dynamos must have many more poles than bottle dynamos because they turn much slower for a given wheel speed, but the effect is similar - at a given wheel speed, a similar number of poles are moving past the rotor for hub or bottle dynamos, so they're able to produce similar amounts of power. All dynamos that I'm aware of (except the B&M S12) are designed to produce 6V and 3W at 8km/h, which is German StVZO. All electrical generators produce a maximum of a certain amount of current, and produce voltage based on their speed - higher rotational speed gives higher voltage in a linear fashion. Since dynamos must produce 6V and 3W at a certain speed, they will necessarily produce higher voltage at higher speeds. Most bottle dynamos have zener diodes to limit the voltage to incandescent lights so they don't blow the filament. I'm not aware of any hub dynamos with zener diodes - I know the Schmidt SON does not have one. Hub dynamos are usually designed to take advantage of the magnetic field saturation effect to limit the voltage they deliver into a given load. With the standard 12-ohm load presented by an incandescent 6V 3W bulb, the Schmidt reaches saturation at just over 6V, and does not produce any more voltage into that load regardless of the speed.

That's the design specs, but I'm planning to take advantage of a dynamo's natural scaling voltage up with speed to produce more power. At 24km/h (my average speed on tour), a Schmidt can produce about 18V into a 36ohm load, which is still about 0.5A, meaning about 9W. That's the kind of power I'm planning to harness to charge my battery pack. Since I'll be averaging 3 times the speed it's designed for, if I give it 3 times the load, it'll respond with 3 times the voltage, at the same amperage, giving 3 times the wattage.

The only thing about hub dynamos that give them the ability to produce more power than bottle dynamos is that at higher power levels, the resistance goes up correspondingly, and the interface between the bottle roller and the tire can't support the amount of resistance to produce much more than the designed 3W, so they often slip against the tire, especially if they're not perfectly aligned or it's raining. Hub dynamos, of course, can never slip, so they're able to produce much more power than bottle dynamos, albeit with increased resistance.

Back on topic...

Depends how you define 'smart'.
I already posted my idea of just charging the Li-Ions to 4.2V/cell and then disconnect them. You will sacrify maybe 15% of their capacity (maybe a little bit more) but gain overall efficiency.

I couldn't find your post about how to charge Li-Ions to 4.2V/cell and then disconnect them - is there a way to make that happen automatically? I need something automatic to safely charge my batteries, since I won't be able to pay attention to them while I'm riding. This is why I've been planning to use a dedicated charger circuit designed to do that for me.

Correct! Use the Ni-MH batteries to charge the other batteries. Just consider them to be voltage and current regulators.

Although it may work to use Ni-MH batteries as voltage and current regulators, there are a number of drawbacks. Whereas Li-Ions are 99.9% efficient, Ni-MHs are only about 80% - that is, 20% of the power put into them while charging gets wasted as heat and is not available on discharging. Also, I'd have to find or design some way to automatically disconnect them when they are fully charged, which is hard to monitor for Ni-MHs with an intermittent power source.

While touring I just use the free computers in public library's for e-mail and uploading photos so I don't need to take the Lap-Top, thanks to Al Gore and the extra taxes on my phone bill to pay for it.

On past tours, through populated areas of the USA, I've done the same thing. I left my laptop at home and my other gadget batteries would last the 2-3 days between finding a place to charge them. But in eastern Oregon, I had to seriously limit my camera and cellphone use, since I didn't know when I'd find a place to charge batteries. I expect the situation will be worse in the desert and mountains of Mexico this winter, and the public internet access will be much more limited, also. So it will be very helpful to have the ability to use my gadgets and keep my batteries charged when the most civilization I see for a week is a taco stand.

Alex

 

[PeLu]

There is nothing inherent about a hub or a bottle style dynamo that necessarily makes it produce higher power.

This is what I wanted to express. And, only better bottle dynamos have Zner diodes built in. I doubt if the cheap ones you can get in any supermarket around here, have them. Anyway. Several front ligts have Zeners built in.

Hub dynamos are usually designed to take advantage of the magnetic field saturation effect to limit the voltage they deliver into a given load. With the standard 12-ohm load presented by an incandescent 6V 3W bulb, the Schmidt reaches saturation at just over 6V, and does not produce any more voltage into that load regardless of the speed.

No, this is not true. None of the dynamos ever comes close to magnetic field saturation (How could it be when the current is more or less the same?). The current is limited by the increasing series reactance. This diffference is important, because the reactance could be compensated.

I couldn't find your post about how to charge Li-Ions to 4.2V/cell and then disconnect them - is there a way to make that happen automatically?

Sorry, was in another forum, got confused. It is quite easy to set a switch to disconnect at 4.2V/cell and connect again at maybe 3.7V/cell.
If you go down to 4.1V/cell, you do not need to worry about lifetime (but sacrify a little bit more).

 

[wmaurer]

Alex, I'm very interested in your solution to this problem, as I will be embarking on a pan-Americas cycle tour next year, and, like you, want to charge all my gadgets from a Schmidt SON hub. I think your knowledge of electronics is much greater than mine, and it sounds like you're making great progress towards a design.

I have spent some time searching for an off-the-shelf product that satisfies this purpose, and have found something called the TuneCharger which claims to collect as much power as possible from inherently unstable sources (dynohub, solar panel) in order to charge batteries in a 'smart' manner, i.e. not overcharge:
http://www.tunecharger.com/documents_088.htm
I believe that the latest version of the TuneCharger is will be ready for sale in the next few weeks, which has been sized for the dynohub, including rectifier and overvoltage protector.

I thought this would be of interest as it seems that we're both after a solution to a similar problem. I'll be keeping an eye on this thread to follow your progress.

Wayne

 

[PeLu]

found something called the TuneCharger

The 'problem' with it is, that it does not switch off as soon as the cells are fully charged. Or does the new version?
It is a maximum power tracker, but only maximizes the ohmic load.

 

[wmaurer]

According to posts on the TuneCharger yahoo e-group, the new version allows you to specify end-of-charge mode along with a max voltage, and the charger will stop.

 

[Steve K]

I have spent some time searching for an off-the-shelf product that satisfies this purpose, and have found something called the TuneCharger which claims to collect as much power as possible from inherently unstable sources (dynohub, solar panel) in order to charge batteries in a 'smart' manner, i.e. not overcharge:
http://www.tunecharger.com/documents_088.htm
I believe that the latest version of the TuneCharger is will be ready for sale in the next few weeks, which has been sized for the dynohub, including rectifier and overvoltage protector.

Wayne

Cool! I've fiddled with peak power trackers myself, and it's an interesting concept.

There's not a lot of info at the website (that I can read), but I did notice a mention of a maximum input voltage of 20v. Wayne mentions an overvoltage protector. Is this something added on to clamp the dynamo output to 20v? If so, it should be sized so that it can dissipate 10 watts or so.

Steve K.

 

[alexlockhart]

Hub dynamos are usually designed to take advantage of the magnetic field saturation effect to limit the voltage they deliver into a given load. With the standard 12-ohm load presented by an incandescent 6V 3W bulb, the Schmidt reaches saturation at just over 6V, and does not produce any more voltage into that load regardless of the speed.

No, this is not true. None of the dynamos ever comes close to magnetic field saturation (How could it be when the current is more or less the same?). The current is limited by the increasing series reactance. This diffference is important, because the reactance could be compensated.

Hmmm...now I'm in over my head. I have read a lot about dynamos and thought I had understood their ohmic behavior, being limited to a certain voltage by the load given them because of the magnetic field saturation. But you're saying it's the reactance which does the voltage limiting, which I don't understand, so I'll assume you're right. Regardless, I think my understanding of the operation of an ohmic device is correct; that is, it provides a voltage based on the load, and in the case of a dynamo, it has a maximum current level and will provide whatever voltage is required to overcome the load presented to it (assuming it's turning fast enough). So the Schmidt can provide a maximum of 590mA, and will scale up the maximum voltage it can produce at that current with speed, so if the load is scaled up with speed, the power will scale up linearly with speed. Correct me if I'm wrong, please.

I couldn't find your post about how to charge Li-Ions to 4.2V/cell and then disconnect them - is there a way to make that happen automatically?

Sorry, was in another forum, got confused. It is quite easy to set a switch to disconnect at 4.2V/cell and connect again at maybe 3.7V/cell.
If you go down to 4.1V/cell, you do not need to worry about lifetime (but sacrify a little bit more).

That switch - I assume that's a transistor in a circuit designed to do that automatically? I've seen homemade circuits like that, but since the commercial "smart" chargers are relatively cheap and small, and since I'm not much good at designing or building circuits, I'd planned to go the safe and easy way with the commercial ones.

I have spent some time searching for an off-the-shelf product that satisfies this purpose, and have found something called the TuneCharger which claims to collect as much power as possible from inherently unstable sources (dynohub, solar panel) in order to charge batteries in a 'smart' manner, i.e. not overcharge:
http://www.tunecharger.com/documents_088.htm
I believe that the latest version of the TuneCharger is will be ready for sale in the next few weeks, which has been sized for the dynohub, including rectifier and overvoltage protector.

HOLY ELECTRONIC WIZARDRY, BATMAN!!
This is exactly what I've been looking for, and will likely revolutionize my design. It's designed for lead-acid and nickel-based chemistries, and I don't see anything about how it determines end of charge, so I'm quite sure it can't charge Li-Ions directly, otherwise it would be mentioned. It maximizes the ohmic load, which will be extremely helpful to get power from the dynamo, and has some kind of regulation on the output, but I think that's also based on the output load, which would work fine for charging lead-acid and nickel batteries, but not Li-Ion. Still, I'm imagining this being my power processor, and using just this between the dynamo and Li-Ion charger. It can only process 8W, which I think can be produced at about 13mph, before it starts limiting the source output - i.e. not making full use of the available power. But they can be wired in parallel, so I may have to buy/build 2 of them so I can take full advantage of the dynamo power up to about 26mph, which I never exceed for long.

I'm in a conversation on the bikecurrent list which I think will result in being able to use the 3 cell charger I'd originally wanted, so I may be about to overcome my design hurdles.

I've posted this in the electronics section of bikeforums also, so I'm trying to keep up with 3 threads at once, and I'll try to post a synthesis/update for all 3 later today. I just got so excited when I saw the TuneCharger that I had to post here first. Thanks so much for the tip!

Alex

 

[wmaurer]

Alex, I seem to recall reading something about the TuneCharger working with Li-Ion batteries somewhere on the yahoo e-group. Maybe you could find the answer there (but please keep us updated here!):
http://tech.groups.yahoo.com/group/TuneCharger/
The inventor of TuneCharger monitors the group.