I have hacked the automatic LED safety flasher LED device on my bicycle and disconnected the
LED and instead inserted a schottky bridge rectifier and a 4500uF/6.3V capacitor to the generator coil (150-ohm DC resistance) that gets a kick with every pass of four small magnets mounted on the spokes.
I attached two wires and I now have a generator output I want to use to charge a Li-Ion battery in my custom made light in front of my bike basket.

So, I will use this generator output to charge a 18650 Li-Ion 2200mAH battery to make an emergency headlight/ nighttime safety flasher instead.

I must pedal fairly at a good pace to get the battery to charge at all.

If I use the raw output of the generated DC voltage and connect it straight to the Li-Ion cell, then the voltage must rise to >= the battery voltage before the battery would get any charge.

If I instead use a very low-voltage boost converter (MCP1624) that starts operation at as little as .63V, it will boost the voltage up to 4.18V in small bursts and the slightest movement of the bike will generate some charge.

Which is more effective/efficient at charging?

 

Welcome to the board. Interesting thread. I'll watch it.

 

Getting the voltage high enough is one thing. You really need to do an energy calculation.
How much power does the headlight require?
How much power can your generator supply?

 

Getting the voltage high enough is one thing. You really need to do an energy calculation.
How much power does the headlight require?
How much power can your generator supply?

The question here is not to do with power usage but power harvesting. The LED flasher/headlight always uses more current than the small pulses can supply, but the battery can charge during the day for light use at night. When flashing at night, only slightly more current is used than would be supplied by slow pedaling.

 

experiment would reveal how much charging could be done in "X" amount of bike riding. we cannot know how much power your modified generator produces otherwise. it is untested.

 

Another thing to consider when charging the battery, the generator (pickup coil) has a copper-loss dc resistance of around 150 ohms, so how does this loss figure into the calculations. At .63V to a boost converter, I have to get a 1.5V pulse( taking in account the voltage drop of the schottky bridge) charging the 4500uf capacitor through 150 ohms resistance. Without a boost and 4500uf cap, I have to get about > 3.6 to 4.5 volts pulse amplitude from the generator (say 2.70 to 4.1V battery charge state) to the battery cell and current arrives through a 150 ohm generator source resistance.

Is there any difference?

 

Consider boosting voltage with a transformer before you rectify. This will give you more voltage to work with and reduce the percentage loss in the rectifier. A transformer is not as sexy as a DC-DC converter, but can be nearly as efficient with far more simplicity.

 

experiment would reveal how much charging could be done in "X" amount of bike riding. we cannot know how much power your modified generator produces otherwise. it is untested.

Kermit2, all my bike riding is somewhat chaotic, but this is not the point, I am talking about energy harvesting of whatever power is available to me by generation, and in any instance my biking sessions are sometimes far, sometimes slow, oh oh gotta stop for the light, and sometimes during the day, sometimes at night.

 

Consider boosting voltage with a transformer before you rectify. This will give you more voltage to work with and reduce the percentage loss in the rectifier. A transformer is not as sexy as a DC-DC converter, but can be nearly as efficient with far more simplicity.

I don't think adding the complexity of a transformer will help at all, just make things worse..here's why:
The output of the coil is in the form of pulses and they vary in amplitude and duration and rise time in accordance with speed. The transformer is far less efficient due to copper losses in the windings and core losses. At any other bike speed than that would render somewhat fixed pulsed voltages, the voltage transformed would be always too high or too low.

On the other hand, the boost converter ic uses only microamps to operate and the energy stored in the inductor in a boost converter is almost 100% available for harvest, with only the dc resistance giving some loss. The boost converter has an efficiency of 70 to 90%, depending on the ratio of the input voltage to output voltage so long as the input voltage >.9V.

The output of the transformer still would need to be rectified, that also introduces loss.
The output voltage would also need to be regulated(Consider: the boost regulator is easily set to a fixed output voltage.)
In any case, there is no chance(no room!) to fit a transformer into the tiny space available in the modified bike light I am using.

 

Sure, it's all about energy harvesting.

Have you determined how much energy you will be able to harvest from four magnets on bicycle spokes?

 

Sure, it's all about energy harvesting.

Have you determined how much energy you will be able to harvest from four magnets on bicycle spokes?

Yes, I have first breadboarded a tiny MCU prototype and it was wired to directly charge the battery from the bridge rectifier filter cap circuit. I did use a boost IC to create a steady +5V for the MCU. After a couple of hours of daytime biking the light would readout a gain in charge voltage and after a couple of hours of nighttime biking I would see the voltage drop from the starting reading before my nightly adventures.

Depending on the amount of nighttime and daytime biking and the amount of high power flashlight mode use, I noticed the battery would gradually run down and had to be removed..in about a month I had to take the poor thing into the house for charging, and this is consequences of a phenomenon called winter, with long nights and short sunlit days.

 

nobody has said anything agreeable with you yet...so, what was the question again?

 

I don't think adding the complexity of a transformer will help at all...

Complexity? It's a component with 4 wires and no moving parts. You will not find a simpler solution.

The output of the coil is in the form of pulses and they vary in amplitude and duration and rise time in accordance with speed.

Right, and the output of the transformer would be the same but at, say, 10X the voltage.

The transformer is far less efficient due to copper losses in the windings and core losses. At any other bike speed than that would render somewhat fixed pulsed voltages, the voltage transformed would be always too high or too low. ...The boost converter has an efficiency of 70 to 90%

A transformer can beat 80% efficiency, although I admit I don't know how it would do in this application.

The output of the transformer still would need to be rectified, that also introduces loss.

Right, but by rectifying after the boost, the rectifier loss is reduced to a MUCH lower percentage of the total power.

The output voltage would also need to be regulated(Consider: the boost regulator is easily set to a fixed output voltage.)

Not necessarily. The battery will hold the voltage down. Your battery charging scheme may require current limiting to avoid overcharging but I suspect your generator is too small to over-current the battery.

In any case, there is no chance(no room!) to fit a transformer into the tiny space available in the modified bike light I am using.

Transformers are not all as big as wall warts, but this is a valid problem. What space constraints do you have?

 

nobody has said anything agreeable with you yet...so, what was the question again?

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.

 

Which is more effective/efficient at charging the battery?

As I understand it, each movement of a magnet past the generator(coil pickup) generates a pulse whose power and amplitude is proportional to rotational speed.
If the coil is in fact unloaded until the pulse amplitude reaches an amplitude that will force current through the bridge rectifier, then no energy is lost and the voltage pulse amplitude automatically(conservation of energy) must rise to meet the point where current starts to flow, so no advantage is gained by using a boost regulator that has an efficiency of 70 to 90%(depends on how small the boost ratio is.)

However, the pickup coil has about 150-ohms copper-loss resistance, always some power is lost no matter which way I try to charge.

My other thought is that any small rotational speed generates some voltage >.63V through the diode bridge and so any power that is stored in the 4500uf capacitor is immediately ready to be converted to a voltage spike by the MCP1624 and adds charge to the battery..so even coasting on the bike causes the battery to charge and fast movement charges at an increasingly efficient rate.

 

Complexity? It's a component with 4 wires and no moving parts. You will not find a simpler solution.
Right, and the output of the transformer would be the same but at, say, 10X the voltage.
A transformer can beat 80% efficiency, although I admit I don't know how it would do in this application.
Right, but by rectifying after the boost, the rectifier loss is reduced to a MUCH lower percentage of the total power.
Not necessarily. The battery will hold the voltage down. Your battery charging scheme may require current limiting to avoid overcharging but I suspect your generator is too small to over-current the battery.
Transformers are not all as big as wall warts, but this is a valid problem. What space constraints do you have?

Thanks for you input, but..

I bought a kinda standard front mounted battery powered bicycle safety flasher light and it has a very slim curved interior profile and there is almost no room for anything taller than the width of the battery and the rounded shape of the enclosure(lens) makes very little of the interior space available for any components that are taller than a 3 to 4 mm (near the side edges) and then height is limited and increases to the circular height of the two AA cells which were centered in the enclosure. I did make room for the 16850 Li-Ion battery and a small perfboard circuit by ripping out the AA battery holder and positioning plastic pieces.

In any case a custom transformer to fit the efficiency, varying frequency, voltage and pulse shape, not to mention size limitations..even the 150-ohm copper loss resistance of the pickup coil says no.. in any case this transformer would be a custom device, its efficiency would be proportional to size..this idea is totally out of question!

 

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.

 

again the generator output has not been quantified. We cannot say for sure which way to direct you. you get less losses with the direct to battery connection, but no charging until you exceed some speed we do not know. you also do not know when, how long, or how fast you ride.
My crystal ball is in my other jeans. sorry

 

Can you increase the number of magnets?

 

again the generator output has not been quantified. We cannot say for sure which way to direct you. you get less losses with the direct to battery connection, but no charging until you exceed some speed we do not know. you also do not know when, how long, or how fast you ride.
My crystal ball is in my other jeans. sorry

I am not manufacturing energy to sell to the city.. I am riding a bicycle, and like most everyone else, the speed and length of my trips will vary to the occasion. But this problem must have a reasonable answer.

I don't know the charging effect at slow speeds, or even on average speed, if there was such a thing. Charging is dependent on how fast, but certainly this is dependent the way I bike.

It remains a question that can be answered by considering whether converting any available voltage >.63V at the capacitor is more efficient/effective than letting just the unboosted capacitor voltage rise to >battery voltage to force charging current into the battery, all this when a 150 ohm series copper loss resistor of the generator is always in the circuit.

At high speeds I can see that the voltage generated would be sufficient to charge the battery whenever the rectified voltage was greater than the battery voltage and so it is sufficient to force current into the battery. But I also know that even a higher speeds, that there would be considerable energy in the capacitor left unused because the voltage is just below the threshold of charging, and this energy would be harvested by a boost converter.

But one must also consider what is going on with slow biking, maybe even consider the that slow speed biking is a considerable portion of the total time I spend biking and maybe even boosting small voltages would be an efficient way to keep the battery charged.