Charging Li-Ion with ripple current

[Martin]

If I were to charge a Li-Ion cell with a charger that terminates at 4.2 V but has 200mVpp ripple, would that do something bad to my cell ?

I mean, towards the end of charge, there will be an average voltage of 4.2 V with a superposed sinusodial hum of 200mVpp, so that I get a voltage that changes b/w 4.1 and 4.3 V periodically.

Will the chemistry integrate this hum and see 4.2 V or will it notice the 4.3 V peaks ?

 

[wasBlinded]

I think if left on the charger it would almost as damaging as charging to 4.3v. Could you set it for a nominal 4.1v end of charge voltage? That would probably be easier on the cell.

 

[SilverFox]

Hello Martin,

Interesting question...

I would be tempted to try it and see what happens.

Tom

 

[MrAl]

Hi there,

In building my best Li-ion charger i wanted to be able to use it with ANY
DC wall wart over say 6 volts and under say 30 volts and still charge
the same and not generate ANY heat (well almost not any). This required
a switcher, and as we all know, switchers do not always put out a perfectly
smooth DC voltage. It ends up being a bit choppie unlike a linear circuit.
A close look at this choppie DC voltage shows it's constantly moving up and
down (roughtly 50kHz) and is never constant.
What i did was looked at the spec of the Li-ion and saw that it can take
a max voltage of 4.250 volts, so i used enough filtering to limit the spikes
to about 50 millivolts (0.050 volts) peak to peak. This means the voltage
can be set to 4.200 volts nominal and it will wander up to 4.225 volts and
down to 4.175 volts. This wont hurt the cell, but i still set it a little bit
lower to make up for any meter tolerance.

Point is, if you simply limit the peak to 4.250 volts you wont hurt the cell.
Since your ripple is 200mv peak to peak, that means if you set the nominal
output to 4.1 it will vary from 4.0 to 4.2 volts, which wont hurt the cell.
Better yet however is to add some more filtering. If you add some more
electrolytic caps to the output you can reduce the ripple to 100mv or even
50mv like i have.

BTW i have been using this charger for quite some time now, charging both
small and larger Li-ion cells.

 

[Martin]

I'm not asking this question to get help with a constellation that I actually have, this is more a theoretical thing:

How cheesy can a Li-Ion charger be ?
What will be inside tomorrow's $ 2.69 Li-Ion charger ?

Does it actually need a smoothing capacitor after the transformer-rectifier ? The rechargeable battery will do some smoothing, the transformer limits the current, like some real cheap NiMH chargers.

Because it is Li-Ion, no way to get around circuitry to switch off (or shunt) at 4.2 V, but if that can be 4.2 V avg and not peak, that smoothing capacitor can really go, making it cheaper, smaller..

I'm tempted to try it but would rather like to hear some more ideas as I yet have to get my wife's consent for this sort of explosive experiment.

 

[Bandgap]

Does it actually need a smoothing capacitor after the transformer-rectifier ? The rechargeable battery will do some smoothing, the transformer limits the current, like some real cheap NiMH chargers.

Be careful.
Liion cells are nothing like as robust and over-charge-tolerant as NiCd, or even NiMH.
Read Newbie's dire warnings on overcharging Liions - he is wise.

In my view, you always need something a lot more sophisticated than a transformer and rectifier, with or without capacitor.

The problem is, Liion cells need current limiting and an accurate charge voltage - generally 4.2V* within 1% or so.

Mains voltage can vary at least 10%, so unless you pick a transformer that keeps the charger output below 4.2V when the mains is at its highest, there is a risk of cell damage and maybe even fire or explosion because sometimes the output of the transformer-rectifier will get above 4.2V.
Most of the time, such a 'safe' transformer would under-charge the cell.

All Liion makers (that I know of) recommend a current-limited, regulated constant-voltage charge cycle.
- with automatic charge termination in almost all situations.

A few notes -
Relying on the protection circuit in protected Liion cells is not a good strategy.
This is a ONLY safety circuit and not a charger - it limits voltage to over 4.2V - reducing cell cycle life.

Under charging - to less than 4.2V - is safe and can result in longer cycle life. However, less run-time will be available from that cell on that cycle.

Steve

*sometimes 4.1V, it depends on the form of carbon used inside.
A few cells are designed for other voltages.

 

[uk_caver]

I'd assume that having ripple around wouldn't exactly make it easy to measure voltages accurately - workarounds to try and deal with ripple with precision might be more complicated than just having a smoother supply.

In the end, I suppose much depends on what someone bothers putting into mass-produced silicon. I suppose someone could make a chip that took a rough rectified input and charged a cell with the output, cutting off charge at the precise correct point, but would that (along with a regular transformer), work out cheaper than a direct swiching design with a consequently smaller transformer and possibly fewer heat issues.

 

[Martin]

.. but would that (along with a regular transformer), work out cheaper than a direct swiching design with a consequently smaller transformer and possibly fewer heat issues.

You are right, this is what we're seeing these days. The switchmode solutions have become so small and so cheap, they have left behind the traditional transformer.

Still I wonder: Is the chemistry having an integrating effect or will repeated 4.3 V peaks make it go boom ?
I think I talk to Newbie as Bandgap suggests.

 

[MrAl]

Hi again,

There is nothing wrong with smoothing the DC a bit more to aid in charging a
Li-ion cell. The trick, as has been mentioned, is to keep the peak lower than
the max voltage for an Li-ion cell (4.25 volts). The drawback is that for part
of the time the charge current isnt as high as it could be with a perfect DC
supply, and so charge time is increased. The higher the ripple (and assuming
the safe level of peak voltage is used as suggested) the longer the charge time,
because a higher ripple means the charge current will be lower for a longer
time. Smoothing the ripple (and keeping the peak voltage correct) means
the charging supply doesnt go as low during the wave dips, and this means
the cell charges up faster. How much difference in time? Well, with a supply
that varies from 4.000 to 4.200 it could take a lot longer to charge because
with simple rectifier type power supplies the peak only lasts for a fraction of
the total 120Hz cycle time. This means that smoothing the ripple out to
a very small peak to peak value (0.050mv or better yet 0.010mv) could have
a profound effect on the charge time. 200mv peak to peak probably isnt very
good at all, while 10mv peak to peak is very good.
It's not too hard to calculate the required capacitance to get the ripple down
to an acceptable level.
Also as noted previously, i have used a value of 50mv peak to peak and got
good results.

 

[Bandgap]

If you were not thinking too much about power efficiency, a low-cost possibility would be to use an internally current-limited wall wart type transformer and a shunt regulator.
The regulator could be three cheap components, but would dissipate some power (get hot).
Its voltage could be set to clip ripple peaks to 4.20 or whatever and forget the capacitor.

But really, IMHO there is no sensible point in making a Liion charger without electronic regulation.
And if you are going to have any silicon in it at all, you might as well have a proper regulator.

Take a look at the amazing chips from Power Integrations for example.
The LinkSwitch-TN at www.powerint.com can do the whole mains to battery thing without a transformer (providing you can't touch the battery during charging, this is OK. I wouldn't let a kid use it).
You would have to mess with the basic circuit a little to get the voltage precision up to the level required for Liion charging, but I suggest this is the way cheap chargers are likely to go - after all, the 12V car phone adapters you buy are not just a big resistor, which they could be, they are switching designs - and that must have proved to be the most cost-effective solution.
For example, the High-Side Buck – Optocoupler Feedback design with a precision 4.20V voltage sensor (The Nat Semi LM3620 would do - and it has 4.1 and 4.2V select pins).

An isolated design (where you can touch the battery during charging) would only be slightly more complicated.

I apologise if my concentration on regulation is hijacking the spirit of this posting, but I worry about explosions and fire in Liion charging and I am not sure there is an unregulated transformer-and-rectifier design that will provide a reasonable charge time combined with safe charging under all circumstances.
I will cease to hijack now!

Steve

 

[Calina]

On a theoretical point of view this is an interesting question but I am wondering if this is not just splitting hairs.

How much difference in capacity could it be between a cell charge at 4.1 V and 4.2V ? Is it worth the trouble or he risk?

 

[wasBlinded]

On a theoretical point of view this is an interesting question but I am wondering if this is not just splitting hairs.

How much difference in capacity could it be between a cell charge at 4.1 V and 4.2V ? Is it worth the trouble or he risk?

Its about 10%. And about the same between 4.2 and 4.3 volts. Charging to 4.3 volts wouldn't mean the cell is going to explode, it would simply last many fewer cycles of cell life.

 

[Bandgap]

OK, I can't keep quiet...

Adding to what wasBlinded said....

According to a paper written by Khosrow Khy Vijeh of National Semiconductor in 2003.....

"Initial capacity increases by about 5% for each 1% increase in charge termination voltage."

He has a graph showing
170Wh/l at 4.0V
220 at 4.1
260 at 4.2
300 at 4.3
This graph s pretty close to a straight line at the voltages of interest.

However, the life reduction is incredible. The graph is a real ski jump.

2500 cycles at 4.1V
700 at 4.2
200 at 4.3
under 10* at 4.4

so 50mV in charge voltage has a big effect on life.

4.1 and 4.2 are either side of the lifetime curve 'knee'.
Interestingly, at 4.0V, his graphs show virtually limitless life.

The paper used to be available from planetanalog, but I can no longer find it.

Steve

*hard to tell, the curve has hit the bottom axis of the graph.

 

[SilverFox]

Hello Steve,

I think you may be reading the chart a little optimistically. Unfortunately, there are no grid lines on the chart, so it is open for interpretation.

My view is that charging to 4.2 volts gives you about 500 cycles, and charging to 4.1 volts gives you somewhere between 1500 - 2000 cycles.

From experience, charging to 4.4 volts gives you 4 cycles...

Here is the article.

Tom

 

[Bandgap]

I think you may be reading the chart a little optimistically. Unfortunately, there are no grid lines on the chart, so it is open for interpretation.

I am happy to go with that Tom.
A few years ago I discussed Liions with battery makers and at the time they seem to think laptops got about 600 cycles from their batteries.

However you read that graph, life suffers dreadfully with increasing charge voltage.

Thanks for the link. It is a fine article, I feel. Did you find a way of clicking and getting part 2?

Steve

 

[SilverFox]

Hello Steve,

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

Tom

 

[Martin]

Some excellent material that you guys have linked.

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. The current is limited to around 500mA by the nature of the generator. It's output frequency is 0 to 100 Hz, whereas below 8 Hz the voltage will not reach the critical 4.2 V.
Because the frequency can be so low, it is difficult to smooth the supply with reasonably-sized capacitors. Chopping the voltage up electronically doesn't make a difference, as there's simply nothing b/w two pole steps of the generator. So there is ripple. What can I do ?
1) I can terminate the charge when the positive peaks of the voltage across the cell start to touch 4.2 V.
2) Or I can terminate the charge when the average voltage reaches 4.2 V
3) Finally I can do as Steve mentioned, clip whatever exceeds 4.2 V and eventually shunt the full current with the regulator.
What I like to do is (2), the best charge performance, the least losses. I suspect this approach is safe but because this is not the typical way people charge Li-Ions, I'm not 100% sure.
As NewBie suggested, I have meanwhile contacted a number of battery makers (no answer yet).

 

[SilverFox]

Hello Martin,

I would suggest you follow Al's advice and limit the charge to 4.1 volts. If the ripple is significant, you may end up with cells charged slightly above that, but that is OK.

Tom

 

[uk_caver]

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?

With Lithium cells, is there some ideal depth of discharge?
If charging was only done to ~4.1V, would multiple shallow discharge/charge cycles be better than fewer deeper ones?
If charging was to ~4.2V, would the picture change, given that charging while the cell voltage is high seems to do the most damage?

 

[SilverFox]

Hello Uk caver,

4.05 volts is used as a trigger to start charging again when the cell has been charged to 4.200 volts. I am not sure what the best value would be when you charge to 4.1 volts.

The self discharge rate is so low that I don't recommend leaving cells on the charger. Charge them up and remove them from the charger in between use. If you are going to store the cells for an extended period of time (2 weeks or longer), you should store them at 40 - 60% charged.

What is very interesting is that you get the most cycle life if you charge to 3.92 volts, and discharge to 3.0 volts. You loose some capacity, but greatly increase the cycle life to something like well over 50,000 cycles.

Tom