Charging Li-Ion with ripple current

[uk_caver]

typoFor charge maintenance on a charger, I guess a different restart-charge voltage might be used than in an intermittent drain application.
Presumably in a charger, a voltage is picked that a cell is unlikely to quickly discharge to after being charged up?

One scenario on a bike could be a stop-start pattern that results in charging being repeatedly triggered, so the cell is almost permanently in a state of being charged, possibly hovering around the retriggering point, or maybe continually being just topped up to cutoff point, then cycled back down to the retrigger point. Presumably in a case like that, the lower the retriggering voltage, the better for the cell, though at the expense of possibly reduced stationary runtime.
However, could it be that somewhat deeper cycling and/or a lower maximum voltage would actually end up over time with a greater [average or minimum] stationary runtime, compared to a system based round a less-well-treated cell which was kept more topped up but [hence] lost capacity faster? Much might depend on the pattern of cycling, and the frequency of light use.

 

[Bandgap]

Hello Steve,

No, but here is the direct link to Part 2.

Tom

Thanks Tom - have beed away for a few days.

Steve

 

[Bandgap]

It's about time I come out with the truth, my real intention:
The idea is to charge a Li-Ion cell from a bicycle dynamo.

Ah ha, my speciallist interest.

OK, I have not done this yet, but I have been charging batteries off bike dynamos for years and I have put a lot of research into this for my Mk5 bike light. - delayed this year by too much gardening so far!

- of course, I might have got it all wrong, but here goes.

You do need regulation - absolutely - don't be daft.

You do not need a capacitor.

Series regulation is tricky with dynamos because the voltage climbs high as the load reduces - a Shmidt can give 170V . AVOID unless you are an EE

The same applies to a buck regulator. - AVOID unless you are an EE.

The cleanest way to do it is with a simple shunt regulator, using an accurate voltage reference chip like the one I refered to in an earlier e-mail.
The circuit willl be smaller than a matchbox - loads smaller than a suitable capacitor.

The way dynamos work, the gaps between the cycles closs right up, so you get charging for most of the cycle - much more than a clipped sine wave.

ANY use?

If this is, I have also got some useful ( I claim ! ) stuff to say about charge termination and charge voltage.

Steve

 

[Bandgap]

I have just posted some stuff further up [now copied below in this email] that I claim will be useful - I am a bike dynamo nut.

A dynamo nut who is failing to get used to this new hierarchical posting structure.

in short - I think I know what you have to do.


Now copied in here--------------------------

It's about time I come out with the truth, my real intention:
The idea is to charge a Li-Ion cell from a bicycle dynamo.

Ah ha, my specialist interest.

OK, I have not done this yet, but I have been charging batteries off bike dynamos for years and I have put a lot of research into this for my Mk5 bike light. - delayed this year by too much gardening so far!

- of course, I might have got it all wrong, but here goes.

You do need regulation - absolutely - don't be daft.

You do not need a capacitor.

Series regulation is tricky with dynamos because the voltage climbs high as the load reduces - a Shmidt can give 170V . AVOID unless you are an EE

The same applies to a buck regulator. - AVOID unless you are an EE.

The cleanest way to do it is with a simple shunt regulator, using an accurate voltage reference chip like the one I referred to in an earlier e-mail.
The circuit will be smaller than a matchbox - loads smaller than a suitable capacitor.

The way dynamos work, the gaps between the cycles close right up, so you get charging for most of the cycle - much more than a clipped sine wave.

ANY use?

If this is, I have also got some useful ( I claim ! ) stuff to say about charge termination and charge voltage.

--------------------------------------------

For - charge termination - take a look at

www.electronicsweekly.com/Articles/2006/02/01/37528/Float+charging+lithium+ion+cells.htm

Something to note here is that these are all professional-grade cells.
I have no idea how this reads across to consumer cells, or cells from any other manufacturer.

DON'T FLOAT-CHARGE LIION CELLS UNLESS YOU KNOW WHAT YOU ARE DOING OR ARE GUIDED BY SOMEONE WHO DOES. YOU CANNOT ASSUME ALL LIION CELLS CAN BE FLOAT CHARGED.
- end of safety announcement.

So you may not need to terminate and re-start.
I am going to float-charge - at 4.0 or 4.1 - Liion or LiPo - but not inside a building, and I am going to give my lighting system plenty of facility and room to ventilate safely if the cell bursts.
I have an idea to put a solar charger and Liion cells on a pole in the garden and let it float at 4.1 or 4.2 for a few months to test the idea where it can do no harm.

As a last thought, I just browsed across these that look interesting.
www.flyelectric.ukgateway.net/lithium-a123.htm

Steve

 

[Martin]

Presumably, once charging has been terminated, you'd want to hold off recommencing it until the cell voltage drops to an appropriate level?
What would an appropriate recommence-charging voltage be, when charging and discharging are unpredictable, as they could be on a bicycle light?

Well, exactly this was my problem with NiMH. The ever-changing charge current from the dynamo would have made it difficult to detect the voltage dip that indicates EOC. And once the bike stops and then moves on with a fully-charged cell, the dip will not come again and the cell gets overcharged.
I figured that Li-Ion is easier, it's a simple CCCV charge. The dynamo takes care of the CC, I just have to limit the voltage. When the bike stops, the current ceases. When it restarts, charging continues until the cell voltage exceeds a certain level, then the charge current is interrupted (or it just ceases by the nature of a CV charge). I assumed these repeated charge attempts don't harm a fully-charged cell as long as a certain cell voltage is not exceeded.

..delayed this year by too much gardening so far!

Sadly the exact same thing with all my hobby projects.

You do not need a capacitor.
..
The way dynamos work, the gaps between the cycles close right up, so you get charging for most of the cycle - much more than a clipped sine wave.

My Shimano DH-3D71 + bridge rectifier delivers something very close to a rectified sine wave (ABS(sin(t)) into a resistive load.
The current into an 18650 Li-Ion follows the same function, from 0 to 830mA peak. It averages at 500mA. The voltage across the cell varies b/w its open circuit voltage and open circuit voltage + 140mV => an AC part of 140mVpp.
Adding a smoothing capacitor of 10mF, the charge current follows more or less a sinusodial curve with a lower peak of 280mA and an upper peak of 700mA. The voltage across the cell varies b/w its open circuit voltage +50mV and its open circuit voltage + 120mV => an AC part of 70mVpp.
Still I don't need a capacitor as regulation is essential and it will just clip any excess voltage.

Series regulation is tricky with dynamos because the voltage climbs high as the load reduces - a Shmidt can give 170V . AVOID unless you are an EE

Absolutely. With my Shimano hub, I was able to direct-drive a flourescent tube. When trying to disconnect the dynamo or do any kind of regulation that reduces the input current, a number of components will have to be rated for a rather high voltage and that will typically punish me with higher cost and higher losses (Rds_on, Vfw). So the plan is to shunt any extra current.

The cleanest way to do it is with a simple shunt regulator, using an accurate voltage reference chip like the one I referred to in an earlier e-mail.

Right. It will work more efficiently with a smoothing capacitor (as MrAI explained), while it will still work without.

For - charge termination - take a look at
www.electronicsweekly.com/Articles/2006/02/01/37528/Float+charging+lithium+ion+cells.htm
..
I am going to float-charge - at 4.0 or 4.1 - Liion or LiPo - but not inside a building, and I am going to give my lighting system plenty of facility and room to ventilate safely if the cell bursts.
I have an idea to put a solar charger and Liion cells on a pole in the garden and let it float at 4.1 or 4.2 for a few months to test the idea where it can do no harm.

I assumed that float-charging is perfectly safe as long as I do not exceed a certain voltage. My reasoning is that the current is zero if the battery voltage equals the charge voltage, so the battery would not even know it's being float charged. I also assumed there's virtually no temperature-dependency of the charge voltage. Lots of assumptions.. Don't miss to publish the outcome of your experiments on CPF !


Thanks to all your detailed explanations, I can see a bit clearer at this stage of the thread:
My initial worry that ripple current adding to the DC charge current of a Li-Ion cell could push the cell beyond its limits and cause catastrophic failure, is pointless. This is because a Li-Ion cell will not fail catastrophically even when charged to 4.4 V. However, its useful life will be reduced so much, that it's a good idea to charge to 4.0 or 4.1 V only. With the constraint being cell life, even a large voltage ripple will not be able to create a dangerous condition.

A voltage-limitation is essential for charging Li-Ions. The regulator will not only deal with the gradual rise of the cell voltage as it's being charged, it will also clamp any voltage ripple that exceeds the set voltage. So if there's ripple, it's within safe limits. The charge performance can be improved by smoothing the supply, while this is optional.

The circuit could look like this, while I will probably use an off-the-shelf integrated voltage regulator for the op amp + voltage reference.

 

[Bandgap]

For what it is worth, a have a few more comments.

1st, glad I am not the only one wrestling with spring weeds.

next....

My Shimano DH-3D71 + bridge rectifier delivers something very close to a rectified sine wave (ABS(sin(t)) into a resistive load...

I was really talking of the voltage waveform. So few people have a clue about current and phase that I was steering clear of current.
In my measurements with the Schmidt, using an RMS current meter, the dynamo seems to deliver the same RMS current into leds or incandescent loads. - providing the output voltage demands was roughly similar - For me the voltage waveform varied a lot.

The url="http://i168.photobucket.com/albums/u200/Vankoff/bikecharge.jpg"]circuit could look like this[/url], while I will probably use an off-the-shelf integrated voltage regulator for the op amp + voltage reference.

That looks good to me.

Another National chip, the LM385-ADJ I think, lends itself to a simple but accurate shunt regulator. As far as I remember, you only have to add one NPN medium-power transistor and a resistor, plus the two potentiometer resistors - that can be high value.
Afaik it senses voltage between the feedback pin and its positive terminal. So you can dump its negative terminal current straight into the base of the NPN, and just use a base-emitter resistor to keep down the Vbe with quiescent current flowing.
I may (as usual, be wrong in all of this!) a diode is needed somewhere in the negative system to stop the tail current discharging the battery when there is no charge current.
I would post a circuit, but I don't know how.

Bob Pease does something similar in this circuit
http://www.electronicdesign.com/Articles/Index.cfm?AD=1&ArticleID=4251

Charging efficiency (electrons in to electrons stored) is almost 100% for Liion cells, whit is why a favour them for pedal-power use - plus the remaining charge is easy to estimate and they are light.

Just out of interest, what are you going to do with the cell, and what capacity cell do you feel safe to charge at 500mA?
I am agonising over the minimum cell that can reliably deliver 500mA. My intention is only to charge at 100mA max - stealing it from the headlight when needed.

Steve

 

[Martin]

Another National chip, the LM385-ADJ I think, lends itself to a simple but accurate shunt regulator. As far as I remember, you only have to add one NPN medium-power transistor and a resistor, plus the two potentiometer resistors - that can be high value.

LM385-ADJ ? Wasn't aware of the ADJ version, nice to know, Steve. So far I was looking at the TL431 plus PNP.

Just out of interest, what are you going to do with the cell, and what capacity cell do you feel safe to charge at 500mA?

At this time, I only want to be able to charge single cells for flashlights, GPS, cellular. During daytime, when not using the dynamo for light. I'm looking at single-cell applications of 1000 .. 2000 mAh. Nothing complicated.

In my dreams I thought abt backing up the dynamo light system with a battery, but then this is getting tricky: Charge during the day and use at night if the dynamo doesn't deliver enough energy ? Or should I charge the battery day and night, losing some light at night ? Instead of running 3 series-LEDs, I could run 2 series-LEDs plus a Li-Ion cell in series. And why not try to maximize light output by holding off the night-charging until the cell is below 30% ? I like that. Using two Li-Ion cells in series would probably be better than one. These 2 feed a step-up converter that maintains a constant current thru a 3-LED headlight, so if the dynamo drives a higher current thru the LEDs, the battery and converter won't engage at all. Well, just ideas that will not be built very fast.

 

[AndyTiedye]

Added to Bike Light Thread Links

 

[Martin]

Added to Bike Light Thread Links

Amazing your filter found this ! Tnx !

 

[Martin]

Received a call from Sanyo today. They said that it's not acceptable that the ripple voltage exceeds 4.2 V.
So it's confirmed, need to clip the peaks.