Use the very best magnets you can get and place them as close to the coils as they can be.
Why 4 pairs? Why not 8 and put the coils in series pairs (placement critical)?
Not that I know what I'm talking about.

 

Can you increase the number of magnets?

Yeah, the original LED flashers were harvested from a discarded bike, the flashers are always supplied with two magnets that would be mounted 180-deg apart on the spokes. I salvaged yet another pair from a discarded bike and now there is no more room, could not add two more..
But that is not the question I am asking..

 

Use the very best magnets you can get and place them as close to the coils as they can be.
Why 4 pairs? Why not 8 and put the coils in series pairs (placement critical)?
Not that I know what I'm talking about.

I have some very strong niobium rare-earth magnets harvested from dead harddisks that were used in the head positioning mechanism..but creating a reliable mounting for them on the spokes seems too much trouble for my fabrication factory. Their extremely strong fields would cause the spokes to be momentarily pulled towards the pickup on passing and maybe cause spokes to break or rub the pickup after some time of use.

Because of the radial position of the magnets, there cannot be more than one pickup coil to fit. Adding another coil in series also doubles the generator 150-ohm source resistance, so only one pickup coil can be activated at once and having two coils would mean the pulse outputs would differ in phase, possibly even opposing each other and attempting to deliver charge current though another 150-ohm resistance and impedance of the non-generating coil. Same location reasons that two coils could not fit in parallel. The pickup coils mount on the axle threads and short of welding another pickup coil to the one already there, it is not likely I have the equipment to attach to attach another.

But that is not the question I am asking..

 

Haha, well OK. The transformer could be external to the lamp holder, and probably not much larger than a dice cube, but you seem to have your mind made up.

It is not a question of making my mind up, or even getting this circuit to work, it is in answering this question:

The question is whether is it more effective to boost (any voltage >.63V always yields a short charging pulse)the voltage for charging or just feed the raw dc to the battery, meaning the charged capacitor must have a voltage >3.3 to 4.6V, a voltage greater than the battery voltage to add any charge.

 

View attachment 80448 View attachment 80448 View attachment 80446 These are the original battery-less spoke mounted magnet, axle mounted pickup and flasher lights.

 

Can you increase the number of magnets?

Maybe she can just peddle harder.

 

How about just spend some money:

http://www.amazon.com/Human-Creations-Bicycle-Charger-Universal/dp/B008KET1V2

 

How about just spend some money:

http://www.amazon.com/Human-Creations-Bicycle-Charger-Universal/dp/B008KET1V2

Ok, if you think my hobby of electronics is all about just buying things.

I live in a neighborhood where violent crimes against people are rare, but there are people here who would steal the diaper off a baby if left unattended.

I spent years learning my electronics as a hobby and I really get a kick out of using my skills to make my own gadgets!

 

Ok, if you think my hobby of electronics is all about just buying things.

I live in a neighborhood where violent crimes against people are rare, but there are people here who would steal the diaper off a baby if left unattended.

I spent years learning my electronics as a hobby and I really get a kick out of using my skills to make my own gadgets!

OK!

 

Basic electrical theory says maximum power transfer occurs when impedance of a source matches the impedance of the load, so I'd guess that the DC-DC converter would act as a variable impedance matching device, allowing a more efficient transfer of power from the source coil to the battery and would be more effective at charging the battery than a direct connection would.

 

If your boost converter puts out a constant voltage once the input is >0.9V it will obviously enable charging at lower wheel revs than if connecting the generator+bridge directly to the battery. However at higher revs, in the absence of a converter, when the bridge output voltage exceeds the battery voltage the charging current would be greater than if you used the constant-voltage converter. So it looks like swings and roundabouts .

 

If your boost converter puts out a constant voltage once the input is >0.9V it will obviously enable charging at lower wheel revs than if connecting the generator+bridge directly to the battery. However at higher revs, in the absence of a converter, when the bridge output voltage exceeds the battery voltage the charging current would be greater than if you used the constant-voltage converter. So it looks like swings and roundabouts .

The generator puts out only a tiny current <20mA at a pedaling speed short of short breath, and always after a milliseconds at most of boost converter operation the Vin to the converter will drop to below .33V turn off voltage because all the energy stored has been delivered to the battery..no danger of bridge output voltage exceeding the battery voltage unless the battery is fully charged already..this condition is handled by the MCU that automatically discharges the battery to a safe voltage by turning on the 15 LEDs of the headlight at full power to dump the excess charge.

 

The question is whether is it more effective to boost (any voltage >.9V always yields a short charging pulse)the voltage for charging or just feed the raw dc to the battery, meaning the charged capacitor must have a voltage >3.3 to 4.6V, a voltage greater than the battery voltage to add any charge.

This depends on what you mean by effective, and it also depends how you ride. If you want maximum energy harvesting at low speed, you HAVE to use the DC-DC converter or you'll get nothing. If you ride most of the time at higher speed, then you have to weigh the small gain from the converter at low speed against the fact that the converter will siphon off maybe 10% in efficiency ALL the time, fast or slow. So it's making matters worse at high speed.

Consider the energy balance. At low speed - the speeds that are useless without the converter - the available power is probably not enough to be worth the cost and effort to harvest by boosting up in voltage. For instance if your generator is making 3V pulses, 20% or so of the energy captured is immediately lost in the rectifier (assuming ∆V across a schottky diode is only 0.3V). Then you'll lose another 10% in the converter. The remaining power might be trivial.

 

The generator puts out only a tiny current <20mA at a pedaling speed short of short breath...

This is hardly worth talking about. 3V across the 150Ω coil is 20mA, and just 60mW. A conventional bike light uses a 6W dynamo; two orders of magnitude more power.

 

This depends on what you mean by effective, and it also depends how you ride. If you want maximum energy harvesting at low speed, you HAVE to use the DC-DC converter or you'll get nothing. If you ride most of the time at higher speed, then you have to weigh the small gain from the converter at low speed against the fact that the converter will siphon off maybe 10% in efficiency ALL the time, fast or slow. So it's making matters worse at high speed.

Consider the energy balance. At low speed - the speeds that are useless without the converter - the available power is probably not enough to be worth the cost and effort to harvest by boosting up in voltage. For instance if your generator is making 3V pulses, 20% or so of the energy captured is immediately lost in the rectifier (assuming ∆V across a schottky diode is only 0.3V). Then you'll lose another 10% in the converter. The remaining power might be trivial.

Interesting comment, my thoughts:

The voltage drop by the schottky rectifier at currents is a kinda sawtooth-shaped pulse with a fast rise time and an exponential decay, so the average current is likely lower than the peak value you state, and so also the average voltage drop is less, but even if it were .3 V, that would mean only 10% of the voltage drop(10% of 3.3 V) was across the diode.

My scope confirms that the converter attempts to deliver, even if only in low duty cycle train of pulses, the maximum power available stored in the 4500uF capacitor. The power stored in the capacitor is then stored in the inductor in each converter cycle, and so even if the voltage is set to regulate at 4.18V, the inductive kick always ensures the voltage is always a peak value higher than the battery, and most of the the area under the pulse shape is always presented to the battery, and so some charging always takes place, even with some loses in the bridge, the converter MOSFET, the converter diode and the inductor delivering power.

The inductor has a resistance of .33 ohms, and the remainder of loss is due to schottky diode voltage drop and perhaps some ESR losses in discharging the capacitor, all with very small power being used by the MC1624.

Even if 30% of the power is somehow wasted by the converter circuit, 70% of the power available is still delivered to charge the battery. Is delivering 70% of otherwise available power trivial? A trivial amount of power delivered/drained over a non-trivial length of time will slowly charge or discharge a battery.

 

A bridge rectifier has 2 diodes in series at any time, so that doubles the voltage drop there.

Only you can judge whether a low mA stream charging the battery is worth it. Calculate out your mA trickle rate and compare that to the battery capacity in mA•hr and the current draw for your lighting load. For example if 10 hours of riding gives you 2 hours of light, and that is acceptable for your usage pattern, then you're good to go. But if you need to ride for days to get a few minutes of light, it's probably a waste of time.

 

It

Interesting comment, my thoughts:

The voltage drop by the schottky rectifier at currents is a kinda sawtooth-shaped pulse with a fast rise time and an exponential decay, so the average current is likely lower than the peak value you state, and so also the average voltage drop is less, but even if it were .3 V, that would mean only 10% of the voltage drop(10% of 3.3 V) was across the diode.

My scope confirms that the converter attempts to deliver, even if only in low duty cycle train of pulses, the maximum power available stored in the 1500uF capacitor. The power stored in the capacitor is then stored in the inductor, and so if the voltage is set to regulate at 4.18V, the inductive kick always ensures the voltage is always a peak value higher than the battery, and most of the the area under the pulse shape is always presented to the battery, and so some charging always takes place, even with some loses in the diode and inductor delivering power.

The inductor has a resistance of .33 ohms, and the remainder of loss is due to schottky diode voltage drop and perhaps some ESR losses in discharging the capacitor, all with very small power being used by the MC1624.

Even if 30% of the power is somehow wasted by the converter circuit, 70% of the power available is still delivered to charge the battery. Is delivering 70% of otherwise available power trivial? A trivial amount of power delivered/drained over a non-trivial length of time will slowly charge or discharge a battery.

The answer I am looking for then is highly dependent on the pattern of my bike speeds on each journey. I go fast and slow, stop and go, sometimes in the day, sometimes at night.

Without a converter, no energy is delivered to the battery if I am traveling less than at a speed that will generate a voltage higher than the battery voltage to force a realizable charging current. This means I must be pedaling fast most of the time..not my style.

With a converter, almost all of the stored energy in the 15oouf capacitor is given to the battery whenever the voltage generated and rectified reaches >.9V, and even with some rarer bursts of high speed biking, the efficiency of the converter increases to almost match charging efficiency of that obtained without using the converter.

So the answer is that the converter will always give me the best charging, with small packets of power delivered continuously at slow speeds and and efficiency of the converter will be increasing to match a converterless charging circuit at higher speeds.

 

Still a question remains, what is best?

What if I should completely remove any filter capacitor and converter after the schottky rectifier. Then the generator voltage can rise instantly to the battery voltage and so with every pulse some portion of the generated voltage pulse peak causes charging?

Even if I am not using a converter, with a storage capacitor there exists a long time constant as a result of of a varying generator pulsating voltage that is attempting to charge a 4500uf capacitor through a 150 ohm resistor, so it that it only slows the time to attain a voltage greater than the battery.

Or is it, since the 4500uf capacitor quickly reaches almost the voltage of the battery and so because no current flows out of the capacitor between charge pulses, it only takes the next short pulse from the coil to deliver then almost all of its energy, because it causes a voltage rise that forces all the power of the pulse to push current into the battery and charging occurs.

 

[responding to #37] I don't disagree. But suppose the generator and system fail to charge the battery significantly, so that in a year you have to take the battery out for charging 19 times instead of 20 without your generator system. Would you consider that a success?

I'm just saying, it's not clear you won't be disappointed.

 

What if I should completely remove any filter capacitor and converter after the schottky rectifier. Then the generator voltage can rise instantly to the battery voltage and so with every pulse some portion of the generated voltage pulse peak causes charging?

I think your comments above about your riding pattern suggest that the converter makes sense, to get at least some charging at low speed.

The capacitor smooths out the supply to the converter and I think that is a good reason to include it. It's not a negative.