# How to calculate Li-Ion battery charging rate?



## LightForce (Apr 20, 2007)

Hi!

I have a charger circuit, which terminates charging process after 3 hours. It is permanently programmed by manufacturer and I can't simply turn it off. I want to synchronise charging termination with a moment, when battery reaches a full charge. So what C-rate should I choose? How to calculate it? I want to recharge three 18650 cells connected in parallel with total capacity of 6600 - 7800 mAh. Charger's current capability is a strong 4A.

If charging rate turn out to be too high for this pack, I can go for 2 x 3 hrs of charging time.

Cheers,

Damian


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## SilverFox (Apr 20, 2007)

Hello Damian,

Is the charge circuit designed for charging Li-Ion chemistry? Does it utilize a constant current/constant voltage charge algorithm? Does it clamp the voltage at 4.200 volts?

It looks like you have cells in the range of 2200 mAh to 2600 mAh, is this correct?

Tom


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## LightForce (Apr 20, 2007)

Hi Tom,

Don't worry, everything is OK. This unit is specialised, stand-alone Li-Ion charger with total voltage error lower than 0.75%. Internally set to 4.200 V voltage limit. It interrupts charging process and disconnects the cells, when current lows to C/10 rate, if time is shorter than 3 hrs.

These are LG ICR18650S2 or B2 cells.


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## SilverFox (Apr 20, 2007)

Hello Damian,

In that case, if you hook up 3 of the 2200 mAh cells in parallel, you end up with the equivalent of a 6600 mAh cell. Charging at around 0.7C will give you a complete charge, if the cells are completely empty, in around 3 hours. 0.7 * 6600 = 4.62 amps. Use your charger at the 4 amp rate and you should be good to go.

If the cells are only partially discharged, the charging time will be reduced. If that is the norm for you, you may be able to add a couple of more cells in parallel and, while the charge rate will be lower, they should end up fully charged in the 3 hours. You may have to play with this a little to get the proper number of cells to charge at one time.

Tom


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## LightForce (Apr 20, 2007)

Thanks Tom!

I decided to use a charger which recharges each 3.7V section of 14.8V pack independently in order to keep the pack perfectly balanced, without need of additional balancer setup.

I appreciate your knowledge about all the batteries and commitment here on CPF, great work:goodjob:


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## MrAl (Apr 20, 2007)

Hi,

I never recommend charging cells like this in parallel with a hefty charger and
here is why...

If i understand you correctly, your charger puts out 4 amps max, and
the cells are 18650 cells.

Since the cells are 18650 they have a max charge current of about 1 amp each.

Now if you charge one cell alone, the max safe current is 1 amp, so if you were
to try to charge it at 4 amps you would be risking damaging the cell or worse.
So you connect three cells in parallel. Ok, now each cell can take a max of 1 amp
but 4/3 equals 1.333 amps so as soon as you hook it up you are already charging
the cells at too high of a current.
But lets say they can take the 1.333 amps for now. What happens if one cell
charges up before the other two? What happens is that now that one cells
gets a low current like 100ma while the other two have to split 3.9 amps.
This means now two of the cells get 1.95 amps each, which is almost twice
the rated current for charging safely. But that's not the worst...
Once a second cell charges up there is only one cell left being charged.
Now if the first cell gets 100ma and the second cell gets 100ma that leaves
3.8 amps going to the last cell ! Clearly that's way too high unless you can
find cells that can take almost 4 amps continuously as there is no way to
estimate the time that one cell will be charging on its own.

To understand this a bit better, consider that each cell has it's own
characteristic, where its current draw is totally dependent on its
terminal voltage:
i=f(v)
where 
i is the charge current and
v is the terminal voltage and
f is it's characteristic function.
Note that every cell has its own function, so for three cells we have
three *different* functions:
f1, f2, and f3.
Using the functional equivalent for each cell, this gives rise to three
*different* current levels:
i1=f1(v)
i2=f2(v)
i3=f3(v)
where v is the same for all three cells.
This of course means the total current divides up as:
I=i1+i2+i3
but I is always a constant, so we replace I with 4:
i1+i2+i3=4
From this it's clear to see that if any two cells currents goes to
zero the remaining cell takes the full current until it too charges up.
After say i2 and i3 get close to zero we have approximately this:
i1=4
which means the first cell gets four amps!


It's better to charge these cells individually and not in parallel unless maybe
you set the current down to the limit of one cell (about 1 amp or so).


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## Supernam (Apr 20, 2007)

I believe that this would never actually happen, because say you have 2 cells that are unequal in initial charge. The higher charged cell will not necessarily finish charging before the lower charged cell because charge will flow more easily to the lower charged cell. That's why we can charge in parallel and still get a full charge in all the cells. If one cell finished charging before the others, then that one cell would have hit 4.2 volts making the whole charger switch to constant voltage mode, which means that current would start to fall. 

Silverfox, is this right?


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## SilverFox (Apr 20, 2007)

Hello Al,

The smallest 18650 cells that Damian has are 2200 mAh cells that are rated at a maximum charge rate of 1C. So, each cells maximum charge rate is 2.2 amps. Hook three cells in parallel and the maximum charge rate for the 3 cells is 6.6 amps.

When you parallel Li-Ion cells, they equalize in voltage and state of charge. Paralleling Li-Ion cells is one of the best ways to balance them. Please note that this does not work for Nickel chemistry, but works very well for Li-Ion chemistry.

When Li-Ion cells are connected in parallel, one cell can not charge up faster than the others. The state of charge is directly related to its voltage. If all the cells are at the same voltage, they are all at the same state of charge.

This holds true even if the cells are different capacities.

You can take a 1000 mAh cell that is at a resting open circuit voltage of 3.5 volts and parallel it with a 2600 mAh cell that is also at a resting open circuit voltage of 3.5 volts and hook them up to a charger and charge at 2 amps and they will both end up at 4.200 volts, fully charged, balanced, and without doing any damage to either of the cells due to high charging currents.

It may be possible to end up with cells that are slightly out of balance if you introduce additional resistance when paralleling the cells. I am opposed to using magnets for connections because of this. However, the additional resistance will also have an effect on the actual charging current, lowering it from what was selected on the charger.

I should also throw in that we are talking about cells that have at least 80% of their initial capacity. It may be possible to have an imbalanced situation develop if one cell has a substantially higher internal resistance than the others. However, I have connected cells with 0.050 ohms with a cell of 0.320 ohms and did not have any problems with high current going to the lower resistance cell. At the end of the charge the high resistance cell would not rest at 4.200 volts, but dropped down to 4.012 volts. Please note that the charge was terminated when the current dropped to below 0.050 mA. On a side note, this is where I believe you will run into problems with your charger set up. You would continue to trickle charge this cell and are dangerously close to the lithium plating occurrence area. I understand that your voltage is clamped at 4.2 volts, but with aged cells the current dwindles down at the minimum level for a long time.

Unfortunately, I can not find any documentation on the effects of extremely low current charging at a lower voltage with aged cells. Once you exceed 4.200 volts, you're in trouble, but below 4.200 volts, there isn't much documentation available.

The real question is if lithium plating is voltage dependent or available electrode dependent.

At any rate, parallel charging Li-Ion cells is safe. You end up with fully charged, balanced cells. To determine the charge rate, you add the capacities of the cells, divide it by the number of cells in parallel, and check to make sure that number does not exceed 1C for any of the cells. The one caution is to check the voltage of the cells you are connecting in parallel to make sure they are within 0.5 volts of each other. This minimizes the surge current when they are hooked up together.

Tom


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## SilverFox (Apr 20, 2007)

Hello Supernam,

No.

When you parallel the cells, they equalize in voltage and state of charge. In the worst case, with one cell completely full and the other completely empty, this equalization process takes about 30 minutes (assuming 2200 mAh cells). Usually it only takes about 5 minutes.

Tom


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## LuxLuthor (Apr 20, 2007)

Always great to keep getting additional details about parallel charging of Li-Ion cells.


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## MrAl (Apr 20, 2007)

Hi again,


Tom, what you are saying in effect is that EVERY li-ion cell has the same
voltage current characteristic, and i find this very hard to believe, especially
with two cells that are different base capacity, and there is an easy way
to illustrate this...

Suppose we have two cells (simpler than considering three) in parallel and
one of the cells is defective such that it is almost an open circuit. Guess
which cell gets all the current until it reaches near 4.2 volts? It doesnt
matter what the defective cell voltage is because it will never draw much
current, therefore the good cell will always get the bulk of the max current.
This of course means there must be at least *some* degree of cell
matching, and that match must last until the entire pack dies. This has got
to be impossible.

To say that two cells of different capacity share the current equally is also
just not going to work out. If you put a cell that is 1Ahr in parallel with
a cell that is 2Ahr and even if they share current equally the 2Ahr cell gets
2 amps (probably ok considering the specs you quoted in your last post),
but the 1Ahr cell also gets 2 amps, which is probably too much for that one.
This doesnt work out even when we consider equal current distribution.
In real life it's not going to work out.

There is nothing mysterious about this circuit to me. It's simply just another
parallel circuit, and all the components are known to be variable so some
degree. I saw the same thing a long time ago with the paralleling of LEDs.
Yes, it does work sometimes...but then sometimes it doesnt...depending
on how well the LEDs are matched *and* how well they *stay* matched
over time and temperature. This could and does sometimes mean failure.
If the LEDs are especially durable, they might last, but this isnt something
that should be taken for granted.

Could this be why all the computer packs have been burning up recently?

A good idea to check for an imbalance of cells is to put a resistor in series
with each cell in order to measure the charge current for each cell.
This is rather easy to do with home built chargers, but with store bought
ones it may be difficult.

Another idea would be to check the temperature rise of each cell as it charges.
If the temperature rise acts to raise the characteristic voltage of a cell it may
help to regulate charge current. To understand this better a resistor in series
with each cell to measure current and a temperature probe on each case to
measure temperature. The cell current and temperature could then be
compared to see how the effective series resistance changes with temperature.
I dont have any info in front of my at this time to indicate if this change in R
is positive or negative with temperature, but i would bet it's positive, meaning it
will aid the parallel charging process...but just how good it helps remains to be
seen...if it does at all. Anyone up to performing an experiment?
Of course the question then arises as to whether or not we want to subject
the cell to a temperature rise in the first place, and how high it gets.


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## SilverFox (Apr 20, 2007)

Hello Al,

Hmm, I am not sure I understand your logic...



MrAl said:


> To say that two cells of different capacity share the current equally is also
> just not going to work out. If you put a cell that is 1Ahr in parallel with
> a cell that is 2Ahr and even if they share current equally the 2Ahr cell gets
> 2 amps (probably ok considering the specs you quoted in your last post),
> ...



It almost sounds like you are suggesting that if you have a charging current of 2 amps available and are delivering it to 2 cells, each cell is receiving 2 amps. That just doesn't add up.

Each cell gets half of the current.

I believe the same goes for discharging. If you have two batteries in parallel and are drawing 2 amps from them, each cell sees 1 amp of load.

Are you really suggesting that charging is different from discharging as far as current division between cells goes?

Here is a test you can do to confirm this. Take two Li-Ion cells and discharge them. Make sure that they are within 0.5 volts of each other when measuring their resting open circuit voltage. Hook them up in parallel and set your charger for a 1C charge rate for one of the cells. If you are correct, your charge time should be around 1.5 hours. If I am correct, your charge time will be almost double that.

Tom


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## VidPro (Apr 21, 2007)

here is some logic along the lines of Mr. ALs concerns.

what if , lets say in the world of Murphey, after 400 charges or 3 years, 2 of the cells in the parellel set FAIL completly.
they open up and anode disconnect, now you would be pumping 6 amps into the last one in the bunch and because it to is Aged, and old and suseptable, you end up with a possible issue.

if the cell CAN charge in 2 hours, and you only NEED it in 6 hours, then charge this parellel set at 1/3c and remain much safer, and still achieve overnight charge.

because when you finnaly set up a great system like this, fully balanced series sets of good capacity by parelleling multiples, it is likly that you will use it, and use it, and use it, and years later, it will still work that you will completly forget to throw it away because its aged, and out of service. 

you still have your huge parelleled capacity, you still can charge it overnight, and top it off in a really short time, but you never have a "laptop fire"


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## LightForce (Apr 21, 2007)

Hi there

I've never thought, that I start such a heated discussion 

Taking an opportunity and not starting next unecessary thread I want to ask one question about this:





Is this correct setup? Each switching buck-mode charger module works from 4.7V input and charges cells with 4A current. Each 3.7V battery module is connected in series with separate 3.7, 7.4, 11.1 and 14.8V leads to the chargers. Will it work?

Nice Paint work, isn't it?

Thanks,
Damian


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## SilverFox (Apr 21, 2007)

Hello VidPro,

I believe I understand Al's concerns, I just don't agree with his assessment of how current flows in a parallel circuit involving batteries.

As I see it, Al has two concerns. The first is that he does not believe that Li-Ion cells equalize when they are connected in parallel, and thus he believes that it is possible for one cell to charge up faster than another cell in the same parallel circuit. The second is a warning that if a cell in the parallel pack fails during the charging process, it is possible to damage the other cells in parallel with it.

I have never had any problems with cells remaining unbalanced when paralleled, nor have I had any cells fail in an open condition. I have had a few short out, but that is a different story...

Theoretically, it is possible for a cell to go open, so I think we need to explore this. 

It would seem that this would favor charging more than two cells. If we take 3 cells with 2200 mAh capacity and hook them in parallel, we end up with a battery with a capacity of 6600 mAh. Charging at 0.7C would mean a current of 4.62 amps. If one cell went open, we would have 2.31 amps going into the other two cells. This is very close to a 1C charge, so everything would be OK.

Now, if two cells suddenly go open, that would leave one cell charging at a little over 2C. While this is not recommend for long cell life, it would greatly reduce the CV portion of the charge and the current would drop off during the CC portion.

This scenario becomes acute if we have more than 10 cells in parallel, are charging at 1C, and all but 1 cell go open at the same time. The people working with Li-Ion cells for Electric Vehicle use have discovered that Li-Ion cells, when empty, can handle up to 10C charge rates for a short period of time. This scenario would probably overheat the cell receiving the charge to where the PTC would shut things down, however I think it is highly unlikely that this would happen.

Let's look at some numbers. If we parallel 10 cells of 2200 mAh capacity, we end up with a 22 Ah battery. To charge at 1C we need to supply 22 amps. I don't believe there are any consumer chargers capable of 22 amps at 4.2 volts, so we would have to use a regulated power supply. 

If I were setting this large a pack up for charging, I would make sure I could check the current going to the various cells, or at least groups of cells. If a bunch of cells went open during the charge, I am sure it would be detected while attending to the charge.

Tom


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## MrAl (Apr 21, 2007)

Hi Tom,

Vidpro helped to explain my concerns very well.


QUOTE
It almost sounds like you are suggesting that if you have a charging
current of 2 amps available and are delivering it to 2 cells, each
cell is receiving 2 amps. That just doesn't add up.
END QUOTE

I thought we were talking about a charger that can put out 4 amps total?

Also, i was saying that each cell has its own characteristic. Part of this
characteristic is the internal (series) resistance. For two cells with
EXACT characteristics other than their internal resistance, and when this
internal resistance is different for each cell, when these are wired in
parallel and charged with a charger that can put out 4 amps, each cell
does not get 2 amps. One cell gets more current than the other.
Note that if we leave out the internal resistance we can model the two
cells as voltage sources. If both are at the same state of charge
(supposedly ideal conditions) and they both have the same characteristic
(also supposedly ideal) then both voltages will be the same. Now we
didnt say the internal resistance was going to be the same, in fact
lets say one cell (cell 1) has 0.05 ohms internal R and the other cell
(cell 2) has 0.1 ohms internal R. The charger is modeled as a constant
current current source that puts out 4 amps total. Now if the charge
equalizes, then both cells should get 2 amps each.
What actually happens however, because of the difference in series R
of the two cells is that the current can not divide equally (this is
similar to two unequal resistors in parallel), is that cell 1 will drawn
2.66 amps and cell 2 will draw 1.33 amps. How can i be so sure? I used
a simulation program and modeled all the devices as stated above.
Interestingly, when the terminal open circuit voltage of each of these
cells is measured they will both measure 4.0 volts. This is because
the internal resistance does not affect the open circuit voltage
measurement (im sure you know this already...just mentioning it for
completeness in this discussion).

I mentioned the LEDs in parallel before because that is a similar
situation, where you dont really know the character of all of
the LEDs and they will most likely be different.

Note that if the internal R of both cells above were the same, then
the current would share equally. I dont know any way to make sure
this is always going to be the case however, especially over long
periods of time.


One thing i should also mention here...
After i posted my previous post i read up a little on the protection
circuits sometimes employed in the cells. These circuits open
the cell up (open circuit it) under extreme conditions (im sure
you know this too). What i am wondering now is what happens if
we are charging two 'protected' cells and one gets a higher current
and the protection circuit 'opens up' and takes that cell out of
the circuit (temporarily) and so the other cell gets the full 4
amps, then that cell opens up, then the first cell circuit closes
and brings it back in so it gets 4 amps, then opens again, etc.
I see a situation where the protection circuits might be switching
on and off repeatedly in an attempt to protect the cells.
This could in fact help to regulate current to the cells, but you
have to realize that these protection circuits were not put in 
place to 'regulate' current. Instead they were put in place to
'protect' the cell. There is a big difference from a design 
standpoint. A circuit that 'protects' only has to meet certain
criteria that is often much simpler in concept than a circuit
that has to 'regulate', and of course this means 'regulation'
means taking much more into account than with simple 'protection'.
To design a protection circuit usually means a comparator that
detects some fault condition based on a voltage comparison, and
if detected, it opens the circuit. Just how long it stays open
is hard to say. Once the fault goes away, the circuit is then
closed. For an overcurrent this condition may come and go, and
just how fast it switches depends on the circuit.
A regulator, however, takes all the dynamics into account, so
that if it is desired to get 2 amps to the cell max, then the
regulator will no doubt handle this task well with no question.
I guess a good question then is did the designers take this into
account? Did they actually design a regulator or a simple
protection circuit? My guess goes to the protection circuit,
which is cheaper, and does actually protect the cell. It was not
made to protect the other cells in a pack however.
What else i dont like is that im not sure if most protection
circuits were made to be used in this manner.
After all, the cell will be constantly banged with a 4 amp pulse,
and close to the end voltage (4.2 volts) this is sure to push
the voltage above the max voltage spec of 4.200 volts
on a repeated basis.

We also have to consider that there are many packs out there
and they are probably being charged in parallel. The problem
is we dont know what the max charger current is. Is it equal
to 1C, or higher? If it is 1C then it doesnt matter if there
are 2 cells in parallel because the max current for any one
cell will always be 1C, even if one cell goes bad and opens up.

Now to get back to the original application...
If someone is going to use a 4 amp charger on three cells in parallel
and one cell has 0.05 ohms R and the other two have 0.3 ohms R
(quite a difference i know) then once cell could get as much
as 3 amps, while the other two share 1 amp between them (500ma each).
Again, all three cells open circuit voltage reads 4.0 volts.

What can be done about this?
The safest thing to do is charge at 1C.
The other possibility is to measure the current going to each
cell and if there is a big enough difference, add some series
R to the cell that draws the most current.
If we add 0.2 ohms to cell 1 in the three cell pack mentioned above,
the charge currents look like this:
cell 1: 1.5 amps
cell 2: 1.25 amps
cell 3: 1.25 amps

One thing we have going for us is that the cell with the lowest 
characteristic voltage (open circuit voltage) will see its voltage
rise to meet the other two cells, whereas the other two cells 
voltage will increase more slowly. This at least eventually
equalizes the characteristic voltages.


Finally, it would be a good idea to investigate the nature of
the protection circuits used for these cells to try to determine
its ability to regulate the charging current. Of course this
wont apply to unprotected cells, nor will it help in the case
where the protection circuits cutoff point happens to be a bit
higher than we would like it to be (im sure they dont build it
for exactly 2 amps on a 2Ah cell, but of course higher).

What i cant help but think about the recent recall is that perhaps
they were charging in parallel with a current that was too high,
and it eventually damaged one or more of the cells, although they
did blame the mass extinction (he he) on contamination, so its
hard to say for sure.

If anyone can find more info on the protection circuit, that would
be good too so maybe we can look into this a bit further.

Tom:
If you still dont agree on the flow of current in parallel battery
banks i can post a circuit somewhere and we can discuss it more
if you like...no problem. If i made a mistake in my analysis
i want to know about it, as to what caused it.

One big source of info is contained on the ieee site, which i
dont have access to at this time. The info is on a pseudo electrical
model of an Li-ion cell that supposedly can be used to determine
all sorts of nice information about the cells. If anyone can
get access to this, this would help everyone using Li-ion cells.


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## LuxLuthor (Apr 21, 2007)

This is a great exchange of questions. I now see that Al (among other things) is raising the question about what happens when there is different levels of resistance in the individual cells in parallel. I don't know if that has been examined like this before. I am not smart enough about all this to answer, but I do see the questions Al is asking.


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## SilverFox (Apr 21, 2007)

Hello Al,

So, your main concern has to do with the current supplied to each cell, and that current is dependent on the cells internal resistance.

First of all, lets get back to the original calculations.

Originally, we were charging 3 cells of 2200 mAh capacity, and came up with 4 amps as an acceptable charging rate.

Let's rework this for 2 cells of 2200 mAh capacity. When we parallel the two cells we end up with a battery of 4400 mAh capacity. Keeping our charge rate at 0.7C we would select a charging current (for 2 cells) of around 3 amps.

Your choice of cell internal resistance is about the limit of the range we can expect to see, so let's say that cell 1 has an internal resistance starting at 0.100 ohms and cell 2 has an internal resistance starting at 0.050 ohms. Please note that the internal resistance is dynamic throughout the charge and aged cells have a higher swing than newer cells.

As you have indicated cell 2 will start off charging at a higher rate, but it is still under the 1C recommendation. If I did the math correctly, cell 1 will be charging at around 1.0 amp and cell 2 will be charging at around 2.0 amps. Keep in mind that the constant current portion of the charge is only 20 - 30% of the total charge time. Once the cells get up to 4.2 volts, the current drops off.

Now, if we went for the 4 amp charging rate with cells starting at these internal resistances, we have a 2200 mAh cell charging at 2666, or at roughly 1.2C. Although this is a little high, it is still manageable. I don't think there would be any issues charging at this rate, but it is above the "recommended" rate.

0.100 ohms is about as high as you can get and still have a cell that is capable of delivering 80% of its initial capacity, so that is a good extreme to work with.

I took stock of the Li-Ion cells that I have been charging in parallel. The impedance of those cells runs between 0.056 ohms and 0.062 ohms. With these values, and with a 4 amp charge rate, I come up with charge rates ranging from 1.88 amps to 2.12 amps. Hardly any difference at all. 

I guess I should advise people that if they have "crap" Li-Ion cells, they should recycle them.  If they insist on trying to use them, they should charge them individually.

The notebook battery problem is an interesting one. I believe that the balance protection used in these packs checked the voltage of the serial stacks to make sure they were equal. This does not check the cells that are in series.

My original computer battery pack is a 4S2P set up using 1900 mAh cells. This works out to a 14.8 volt 3800 mAh battery pack. The charging rate is 3.5 amps. I also have a replacement pack that has upped the capacity to 4460 mAh, so it must have 2230 mAh cells in it.

Keeping each serial string in balance helps, but unless there is individual cell balancing, there still can be the possibility of a cell getting out of balance. 

I think the "issues" with the battery packs involved contamination in the electrolyte of the cells being aggravated by series cell imbalances. As a cell approaches or goes over 4.200 volts, it has the possibility of aggressively attacking the electrodes and plating metal lithium out. Add a good healthy dose of heat, and you are on your way to thermal runaway.

At any rate, I still maintain that parallel charging is safe, serial charging is where you run into problems, and individual charging is the best way to go.

Tom


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## LightForce (Apr 22, 2007)

Hi,

I don't want to go into details of your discussion, becouse my knowledge is way to little to compete with you. But guys.. Please, don't left my question without response.

Damian


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## SilverFox (Apr 22, 2007)

Hello Damian,

You may find this thread informative.

Here is another one.

Tom


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## MrAl (Apr 22, 2007)

Hi Tom,

In your example of two cells one with 0.05 (cell 1) and one with 0.10 (cell 2)
internal R charging in parallel with a charger that puts out a constant 4 amps...
now let the open circuit voltage of cell 1 be 3.9 volts instead of 4.0 volts,
while cell 2 volts is still 4.0 volts. See how much more current cell 1 draws,
and this is only a difference of 0.1 volt and 0.05 ohms, which still does not 
even represent worst case yet it is bad already.

The next thing to consider is how long can a cell take an overcurrent.
For example, lets say we have 3 amp through cell 1 and 1 amp through cell 2.
Cell 2 will be ok, but cell 1 will have a current that is higher than the max
rated current for that cell (2.2 amps). Of course the voltage will rise faster
than in cell 2 because of the fact that it is getting more current.
Also, if the cells are wired in parallel all the time there is a good chance
that the two voltages are close to each other.

QUOTE
At any rate, I still maintain that parallel charging is safe, serial charging is where you run into problems, and individual charging is the best way to go.
END QUOTE

All i really wanted to say was that individual charging is the best way to charge too,
and that parallel charging can be a problem if the cells are mismatched, and that it's
going to be a bit difficult to make sure the cells stay matched over time. If possible
i would measure the current getting to each cell before i left them charge for a long
period of time...and of course this means using series resistors, not simply measuring
the current with a meter for each cell. The series resistors increase charge time,
so they should be kept low. Interestingly though, i think 0.1 ohm resistors in series
with each cell would help balance the pack a little during charge.


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## VidPro (Apr 22, 2007)

LightForce said:


> Hi there
> 
> I've never thought, that I start such a heated discussion
> 
> ...


 
i think this is a GREAT setup, and will insure balancing of the pack which will make it last many time longer than one that is not balanced. except for the 4amps, why overdo it?

the only thing i have "run into" when putting 3 chargers on a single power supply is if the grounds or the positive connections conflict with eachother. some of the chargers just "pass through" the ground connection.
so just watch for that when you make this assembly.

you see if all the grounds are the same (or the hot) then you end up connecting the battery to itself. so check for that, and put any isolation in needed when you do it. like before tying all 3 chargers together check with either an ohm meter, or a volt meter to see if your intended results match your actual results before connection.

same thing with a series connection of curcuits, your intended results might not match what occurs in reality. 

on the discussion, its not "heated" LOL its just discussion, i hope. 
i agree with what everbody says to some extent, and even if the batteries are of slightly different capacity or resistance, this setup would cover for most everything EXCEPT failure, and failures are inevitable.


----------



## SilverFox (Apr 22, 2007)

Hello Al,

This is the crux of our disagreement...



MrAl said:


> In your example of two cells one with 0.05 (cell 1) and one with 0.10 (cell 2) internal R charging in parallel with a charger that puts out a constant 4 amps...
> now let the open circuit voltage of cell 1 be 3.9 volts instead of 4.0 volts,
> while cell 2 volts is still 4.0 volts. See how much more current cell 1 draws,
> and this is only a difference of 0.1 volt and 0.05 ohms, which still does not
> even represent worst case yet it is bad already.



When you have Li-Ion cells in parallel, one cell can not drop to 3.9 volts while the other remains at 4.0 volts. If you start with cells that are at unequal voltages, the higher voltage cell will charge the lower voltage cell until the cells have equalized in voltage and state of charge. 

Li-Ion chemistry has a very high charge efficiency, so the state of charge is directly related to the cells voltage. This relationship drops off when the cells are below around 25% charged, but is good above that.

This is also the reason you can parallel cells of unequal capacities and end up with fully charged and balanced cells. 

Help me understand how, with Li-Ion cells having a charge efficiency of 98+% and being connected in parallel, one cell can have a higher voltage than another. Every time I check cell voltage with my volt meter, I get equal voltage for each cell.



MrAl said:


> The next thing to consider is how long can a cell take an overcurrent.
> For example, lets say we have 3 amp through cell 1 and 1 amp through cell 2.
> Cell 2 will be ok, but cell 1 will have a current that is higher than the max
> rated current for that cell (2.2 amps). Of course the voltage will rise faster
> ...



Heat is one indication of problems within Li-Ion cells. Healthy cells start to heat up with charging rates of 3C or higher. The range of 1C to 3C is above the "recommended" charge rate, but it is a bit of a gray area. Not really sure what goes on in this range, but if you happen to bump into it, there does not seem to be any immediate problems.

Now, all bets are off if the cells are "crap."

You commented earlier on protection circuits. Most of the protected cells have the protection set at around 2C or higher. The Tenergy protected cells that I tested are set just above 1C, so this would add another thing to add to the check list. 

If you are charging Tenergy protected cells, you need to make sure to target 0.7C as the maximum charging rate, rather than 1C.

While we are on charging rates, I noticed that Saft and Emoli both choose 0.5C as a recommended rate for their C and D sized Li-Ion cells. The maximum rate is still 1C, but this recommended rate is lower than the 0.7C rate that is used for the 18650 cells.

I might also add that parallel charging is not breaking new ground. The RC people have been parallel charging cells, and series packs, since Li-Ion and Li-Poly cells have been available. To my knowledge, they have never had an incident involving parallel charging over tens of thousands of charges.

Tom


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## MrAl (Apr 22, 2007)

Hi Tom,

QUOTE
When you have Li-Ion cells in parallel, one cell can not drop to 3.9 volts while the other remains at 4.0 volts. If you start with cells that are at unequal voltages, the higher voltage cell will charge the lower voltage cell until the cells have equalized in voltage and state of charge. 
END QUOTE

Who said 'drop'? I didnt say 'drop', you did. I said 'be', as in: "let one cells
voltage 'be' 3.9 volts, and the other cell 'be' 4.0 volts.

You said that the two cells are first placed in parallel to 'balance out'. Well,
we already said the internal R of a cell can be 0.05 ohms (actually can be lower)
but lets say 0.05 for now... if one cell is 4.0 volts and the other is 3.5 volts that
means as soon as they are connected 5 amps will flow from one cell to the
other. This means one cell has to drain and the other has to charge at a higher
than desirable rate. Clearly you can see the problem here.

QUOTE
I might also add that parallel charging is not breaking new ground. The RC people have been parallel charging cells, and series packs, since Li-Ion and Li-Poly cells have been available. To my knowledge, they have never had an incident involving parallel charging over tens of thousands of charges.
END QUOTE
Well then why did you agree that it's safer to charge a single cell rather than
in parallel or in series? Clearly you see a potential problem here.

I guess if it is absolutely necessary to charge in parallel then you wont stop
people from doing it, but that doesnt mean it is a good way to charge them.
If i needed a high current pack perhaps i would consider charging in parallel.
Until then i wont.
BTW, asking the RC people what they do is certainly not the answer to all
problems. They do what they *have* to do. Also, do we have any good
data on what kind of charge currents they are using?

Also, does anyone really care if their cell gets charged at 2 times the rated
charge current? I wonder how many people really care anyway. They would
simply charge them and assume nothing is wrong. It is possible that the life
gets reduced by something like this, but many people would not care or notice
just as long as they can get some time use out of their pack, even if it's not
as long as it could be.

I realize that sometimes my criterion for a circuit is a bit more cautious then
most. This is because over the 30+ years i've worked with electronic parts
i've seen many fail very badly because they were not run to spec.

One thing is for sure though, and that is for any Li-ion pack that is wired in
parallel and that pack is charged by a current that is 2 times the rating of
one Li-ion cell, sooner or later at least one of the cells will be run over spec.
To show this is not true, you would have to show that two cells can age
the same way.


----------



## SilverFox (Apr 23, 2007)

Hello Al,

OK, it seems that I misunderstood your post. We are in agreement that once the cells are hooked up in parallel, the voltage of each cell will be equal.

Now, what happens when you go to parallel cells that are at different voltages. This was discussed in another thread, but let's go over it again here.

Yes, there can be an imbalance when you hook the cells up in parallel. Yes, there is a sudden pulse of current when the higher voltage cell charges the lower voltage cell. Yes, with a 0.5 amp difference in the voltage between packs, that current pulse is around 5 amps.

However, the large current pulse lasts for less than half a second. The drop in current is exponential and it will quickly (within seconds) settle down to below 3 amps and continue to drop as the voltages and states of charge equalize. 

Is this a concern? No. Is this a safety issue? No.

Li-Ion cells can easily handle high pulses of current without problems. The actual value for the "allowable" pulse depends on the cell. Large C and D size cells are less tolerant and can only handle pulses in the 4C to 10C range. This means that if you have a 3700 mAh C cell, it is only good for a pulse current of 14.8 amps. As you can see, our 5 amp pulse is less than that.

18650 cells can handle pulses from 10C to 25C, and when you change the anode material you are good for even higher pulse loads. Let's go with 10C. We have been discussing 2200 mAh cells, so 10C would be 22 amps. Once again we are well below the pulse load capability of the cell.

Now, if these loads are continuous, we will have problems, but Li-Ion cells are very tolerant of high pulse loads.

Li-Ion cells can handle high charge rates when they are discharged, but the charge rate must be greatly reduced when the cell reaches around 60% of full charge. The Electric Vehicle people have studied this in an effort to facilitate a fast charge for a Li-Ion battery pack. Unfortunately, the tail end of the charge is what takes the most time, and fast charging initially doesn't reduce the total charging time that much. You can quickly get to around 80% charge, but after that it takes time to complete the charge.

Fortunately, in our situation, if we happen to parallel a fully charged cell with an empty cell, we will equal out at 50%.

Please understand that in no way is this recommended. We are simply looking at worst case here. If someone screws up and parallels an empty cell with a full one, it is good to know that it will not blow up. It is probably not the best for the health of the cell being charged up, but does not lead to a catastrophic event.

It is far better if the cells are closer than 0.5 volts apart. Best is if they are equal, but if they are 0.05 volts different the surge pulse is greatly reduced and becomes a non issue.

I expect people to remove the cells that they are using in their lights and charge them up. They should be reasonably close in voltage if they have been working properly in their lights. If the light takes multiple cells, the best way to balance all of the cells is to charge them in parallel.

I agree that the safest way to charge a cell is to charge it individually. However, if you are using multiple cells, they need to be balanced with each other. Individual charging does not insure balance at the end of the charge, but parallel charging does. When the cells are new, individual charging will bring the cells to a full charge and they should be well balanced. However, as the cells age, they may not all end up at the same voltage when removed from the charger. In this case, parallel charging will bring them closer to balance than individual charging will.

The problem with parallel charging is that there are more things to connect, and that means that there is an increased chance that something could be hooked up improperly. For this reason, individual charging is safer.

Series charging is where the problems can arise. However, if you stay within the specifications of the cells, you can go through a lot of cycles before the cells fall out of balance. A recent study of notebook computer batteries revealed that the typical 4S2P pack will stay in balance for over 500 cycles before things start to break down. Yes, that means that over those 500 cycles, the cells age equally.

The reason I bring the RC people into the discussion is that when I ask my friend why he charges in parallel, he tells me that his helicopter cost upward of $3000 and his battery packs run around $500 each and he is interested in getting the best performance he can from is battery packs, and does not want to crash his helicopter. I think he, and other RC pilots, are very interested in the health of their battery packs.

Also, a 6600 mAh, 22.2 volt 12 cell battery has to be kept in balance or bad things can happen.

The RC people have found that they get good life from their packs by charging at 0.7C - 1C. When they are in a hurry, they go up to 2C, but only when using active balancing or charging in parallel. A 1C charge takes just over 1 hour to charge to 90% full. A 0.7C charge takes just under 1.5 hours. A 2C charge takes around 40 minutes to get to the 90% full mark, and a 3C charge rate takes about the same as a 2C rate. At 1C, it takes an additional roughly 30 minutes to get all the way to 100% full.

They can confirm that doing a complete charge at 2C results in fewer charge/discharge cycles.

When we were using Li-Ion batteries in single cell lights, the best solution was to individually charge the cells. Now that we have lights that take a 3S3P battery pack, we need to charge this pack safely and make sure that all the cells are balanced. The best way to do this is to charge in parallel.

Another way to charge several cells at once is to get a balancing charger, or add balancers to your charging set up. Then you can charge the cells in series and balance them as you go. This works great as long as you get the cell count correct and the balancers do not malfunction. It also requires an increased investment in hardware, and once again you have to make sure everything is hooked up properly.

My prediction is that people will push things to the limits and then complain when their cells don't perform well. It is my hope that you and I and others provide them with enough information that they take basic safety precautions and we don't end up with someone starting a fire in their house.

People who have a light that uses 9 cells need to plan out their charging solution. If they only want to use the light once a week, they can charge the cells individually, or if they have a bunch of chargers, it will take less time to charge. On the other hand, if you charge the 9 cells in sets of 3 paralleled together, and if your charger is capable of 4 amps, you can have all your cells ready to go in around 6 hours. If we throw out one more handful of options, we can also spend some extra money and get a hobby charger and balancers and charge the pack in series, or series/parallel, and be ready to go in around an hour and a half. 

If speed is the most important criteria, spend the money and charge in series with balancers. Just make sure to double and triple check all your settings and make sure you understand your charger very well.

If simplicity is what you are after and you also are interested in not spending all week charging, charge in parallel.

If bottom line price is the main concern, purchase enough chargers to charge all the cells individually and it will take as long as it takes. However, your selection is limited in this area, especially with C and D sized cells.

It is my hope that whatever method people choose that they pay attention to what they are doing and that they charge safely. All three methods are safe with individual charging the safest, followed by parallel charging, and finally series charging with balancers.

Tom


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## MrAl (Apr 23, 2007)

Hi Tom,

I think we agree for the most part that charging a single cell is safer than
charging in parallel. I dont even want to talk about series charging, unless
each cell voltage is monitored individually.
We might have some areas where we dont agree...

QUOTE
I agree that the safest way to charge a cell is to charge it
individually. However, if you are using multiple cells,
they need to be balanced with each other. Individual charging
does not insure balance at the end of the charge, but
parallel charging does. When the cells are new, individual
charging will bring the cells to a full charge and they
should be well balanced. However, as the cells age, they
may not all end up at the same voltage when removed from
the charger. In this case, parallel charging will bring
them closer to balance than individual charging will.
END QUOTE
Perhaps you can explain how parallel charging can balance two cells
while charging individually can not?

QUOTE
The problem with parallel charging is that there are more
things to connect, and that means that there is an increased
chance that something could be hooked up improperly. For
this reason, individual charging is safer.
END QUOTE
That's not a good reason because i think people are smart enough to
connect their cells properly, but yes it is still a reason. The 'other'
reason (which is really what we are talking about) is that there could
be more stress on one of the cells in the pack. This leads to less
life at best. I dont think you could disagree with this point. If you 
charge a cell at 1C (its rating) and it works ok then if you charge
at 2C (twice its rating) you at best reduce its life.

QUOTE
The reason I bring the RC people into the discussion is that
when I ask my friend why he charges in parallel, he tells
me that his helicopter cost upward of $3000 and his battery
packs run around $500 each and he is interested in getting
the best performance he can from is battery packs, and does
not want to crash his helicopter. I think he, and other RC
pilots, are very interested in the health of their battery
packs.
They can confirm that doing a complete charge at 2C results
in fewer charge/discharge cycles.
END QUOTE
I dont get it...on the one hand you agree that with unequal series
resistance the current will not share equally and so one cell gets
more current which reduces its life more than the others, yet on the
other hand you seem to say it doesnt matter. I think this is because
you are basing your idea of 'what hurts' on your estimation of just how
different the series resistance can be. Perhaps we didnt talk about this
enough yet. Here maybe you can tell me how you control your series
resistance in your parallel cells  Also, you can tell me how you
check your series resistance in a pack of three parallel cells so you
know when one cells series R reaches 0.5 ohms. I dont rule out
the pack being dropped once either and this causing a problem.
You also have to realize that it doesnt matter what their pack costs,
because they are not as concerned about pack 'health' as they are
about running the RC device in the first place. In other words,
health is not the #1 priority, it is #2. #1 is running the thing
in the first place. If they are concerned enough about the health
of the pack then they should charge at a low enough rate to 
ensure none of the cells gets charged at too high of a rate.

QUOTE
If simplicity is what you are after and you also are interested
in not spending all week charging, charge in parallel.
END QUOTE
Well, you could use more than one charger, but this would of course
lead to problems with big packs as nobody wants to buy 9 chargers.
In other words, only charge in parallel if you have to.
Even with 3 cells i can see many people not wanting to buy three chargers
or even waiting three times as long to get their cells charged up, but that
is no reason to say that charging in parallel is actually as safe as charging
one cell at a time.


----------



## SilverFox (Apr 23, 2007)

Hello Al,



MrAl said:


> Perhaps you can explain how parallel charging can balance two cells
> while charging individually can not?



Let me give you an actual example...

I have a headlamp that uses 3 Li-Ion cells. I just charged the cells individually and they ended up with voltages of 4.192, 4.168, 4.184. The cell at 4.192 volts has an impedance of 0.084 ohms, the one at 4.168 volts has an impedance of 0.089 ohms, and the one at 4.184 ohms has an impedance of 0.086 ohms.

As you can see, even though I charged the cells individually, they are not matched. 

You may ask why this happens, but it is a whole other topic that we won't get into right now. Suffice it to say that there can be differences in contact resistance in the charger, differences in charge termination from slot to slot, and as you can see, there is some difference in cell impedance.

I normally charge these cell in parallel, and they come out very close to being equally balanced. This time I connected them in parallel after charging and then ended up with voltages of 4.181, 4.180, and 4.181 after a few minutes of being connected in parallel.

Now the cells are matched.

Just to make sure I didn't rush things, I just took another voltage measurement after 30 minutes and the voltages are the same as previously reported. 



MrAl said:


> That's not a good reason because i think people are smart enough to
> connect their cells properly, but yes it is still a reason. The 'other'
> reason (which is really what we are talking about) is that there could
> be more stress on one of the cells in the pack. This leads to less
> ...



First a funny story...

Please understand that I mean absolutely no disrespect toward Robocop, and I hold him in the highest regard. He risks his life daily to make the world a better place for us to live in. However...

This morning he accidentally placed a Li-Ion cell in his individual cell charger backward. The cell has a black tip with a very large + on the positive end, and all that is required is to orient the + close to the LED indicator lights. I don't think it gets much simpler than that, and I know that Robocop isn't stupid. 

My point is that if an intelligent skilled person can accidentally hook up a single cell backward in an independent channel charger, the odds are greater of something going wrong when you have several steps involved in the set up for charging. 

To the rest of you point I would just like to add that simply using Li-Ion cells stresses them and results in reduced life.

A typical Li-Ion cell looses around 0.2% of its life with every full charge/discharge cycle under the best circumstances. What is not clearly understood is what causes accelerated degradation.

Here is another real world example. I just finished testing some "high current" Li-Ion cells. They performed very well at their maximum continuous current load as far as being able to hold voltage under load, but they suffered a large cycle degradation with each cycle. In 10 cycles they were below 80% of their in ital capacity and considered dead.

Reducing the maximum continuous load by 20% reduced the cycle life degradation by half. It took 20 cycles to get down to 80% of the initial capacity.

Re-rating the cell for a 1C maximum continuous current draw resulted in an acceptable cycle degradation of 0.5% per cycle. So much for the "high current" classification.

The second part of this testing was to explore faster charging rates. Comparing 0.5C, 1.0C, and 2.0C charging rates revealed no increase in the 0.5% per cycle degradation.

This indicates that these "high current" cells are tolerant of higher than normal charging rates, but can not handle high continuous current loads.

Please understand that this is not a universal finding. There are many different formulations of Li-Ion cells. Some are more sensitive to charge rate, others are more sensitive to discharge rate. Some can handle higher temperatures than others. Some store better than others. And so on.

Now, I have given you some "real world" experiences, but we also need to look at theoretical possibilities.

You are concerned that cells will end up with extremely different internal resistances and if they do, that there are issues that may result from the differences. We both know that if there are differences in internal resistance of the cells hooked up in parallel, there will be different current flowing through the cells.

In theory, we can speculate that there is a possibility that a cell can go open. In practice, I have never heard of a Li-Ion cell going open. It is theoretically possible, but capacity fade is listed as the most frequent failure mode. Looking at this worst case, if a cell goes open you end up charging the other cell(s) at a rate that is higher than recommended. This has the possibility of reducing the cycle life of the other cells. How much of a reduction depends on the chemical make up of the cell.

In practice we can look at the worst case for cells that we have on hand. I have shown that I keep my cells closely matched, but others may not be so diligent.

Let's look at the worst case I can come up with involving the cells that I have access to. 

The highest impedance cell that I have measures 0.320 ohms. This is a 26500 3200 mAh cell. It is not in service because it is unable to hold voltage under load, and it comes in at around 1000 mAh of capacity. If I were using this cell in a light, the runtime would be terrible and if it was a direct drive incandescent, the beam would be very yellow and not white at all. I would have several indicators that there is something wrong with my batteries, should I choose to pay attention to them.

On top of that, when I charge this cell individually to 4.200 volts, it settles down to 4.004 volts when I take if off of the charger. When a Li-Ion cell is at a resting voltage of 4 volts, it is at roughly an 80% state of charge. This is another indication that this cell is "crap." Li-Ion cells are considered dead when they drop below 80% of their initial capacity.

As you can see, I have several clues that something is wrong even before I take the time to check things out. At this point I should recycle this cell and replace it with a new one.

Now, let's suppose that I have a 2 cell light that I want to use this cell in. I would parallel charge it with another 26500 3200 mAh cell. I just happen to have another cell like that, so let's explore what happens when we parallel these cells together and charge them.

The "crap" cell measures 0.320 ohms of impedance and the other cell comes in at 0.085 ohms. Both cells are 3200 mAh of capacity, so we would expect to be charging at 0.7 times the combined capacity or 4.48 amps. Let's round this up to 4.5 amps. Hold on now, since we are exploring worst cases, let's go for 1C charging and charge at 6 amps.

With 6 amps of charging current the "crap" cell will start off charging at 1.26 amps, and the other cell would start off at 4.74 amps. This represents a charge rate on the better cell of 1.48C which is a little high. 

However, as soon as the charge is underway, the impedance of the "crap" cell drops to 0.170 ohms, while the better cell stays roughly the same. Now we have 4.02 amps going to the better cell and 1.98 amps going to the "crap" cell. Now we have dropped to 1.26C which is still a little high. 

If we had gone with the 0.7C charge, the better cell would be charged at around 3 amps which is within specifications.

The cells will be balanced at the end of the charge, but the "crap" cell will crap out before the better cell does and will not be able to hold up under load. At the end of the discharge cycle, the "crap" cell has the possibility of ending up in an over discharged condition even though it was balanced at the beginning of the discharge.

Please understand that I have not tried paralleling these two cells, so I am just going from theory here. I did check the impedance of the "crap" cell during charging, and have charged these cell individually.

OK, where does that leave us.

Individual charging is good and safe, however sometimes you can end up with unbalanced cells.

Parallel charging always ends up with balanced cells, but you have to be mindful of the condition of your cells. If you parallel cells with wide voltage differences, there is an current surge as the cells equalize. If there is a vast difference in the internal resistance of the cells, you run the possibility of charging cells with lower internal resistance at rates higher than what is specified. The effect of this higher charging rate on the cells condition depends on the make up of the cell. Some cells are not bothered at all, others show an increased degradation.

We have covered a lot of ground here, and have revealed some of the concerns and the benefits of parallel charging. It is discussions like this that help people understand that there are a lot of things to consider when using Li-Ion cells.

It is no wonder that the major manufacturers of Li-ion cells do not sell directly to the consumer. There is a reason that we have to get our cells through secondary channels.

When Li-Ion cells and chargers become available at the local store, the safety issues will have been worked out. Until then, it is up to people like you and I to explore the issues and bring up concerns.

I believe in individual cell charging, however when involved in multi cell applications, I prefer to charge in parallel. If a series pack has balancing taps, go for series charging with a balancer. If your series pack does not have balancers, charge in series with great caution, and preferably outside in a fireproof area.

Tom


----------



## MrAl (Apr 23, 2007)

Hello again Tom,


Thanks for the detailed reply. I think we can at least
agree that either way you charge Li-ion cells there are
concerns, and that charging in parallel has even more
concerns. I realize also that sometimes it might be
necessary to charge in parallel if you have a big pack
where you dont want to have to take it apart just to
charge it, or run wires to each cell. I ran into this
same dilemma when thinking about how to best charge one
of my NiMH packs.

Back to Li-ion cells now...

As far as the cells being more 'balanced' when charging
in parallel however, i think that depends on what you
call 'balanced'. If you think you need to get every cell
voltage to be within 1mv of each other to be balanced,
that might be a little extreme. For example, you gave
two sets of three voltage readings for three cells:

Set 1:
4.192, 4.168, and 4.184 volts

Set 2:
4.181, 4.180, and 4.181 volts

Now you say that Set 1 isnt balanced, while Set 2 is, but
lets take a closer look at Set 1...

The mean voltage of Set 1 is: 
4.18133333333333

The percent difference from the mean for the three cells
(in order given) is:
p1=-0.255102040816357
p2=0.318877551020369
p3=-0.0637755102041115
where each p is given in percent.

From inspection of the above data, we can see that the
percent difference from the mean is quite small. One
cell is about a quarter of one percent low, while another
cell is about a third of one percent high, while the last
cell is less than one tenth of one percent low.
Now it's also true that we can say that the second set
of cells (Set 2) are 'more' balanced, but i dont think
we can honestly deem the first set 'unbalanced'.
So am i really to call these (Set 1) three cells unbalanced?

I am pretty used to getting all my cells the same anyway
i guess, because i build all my own chargers and set them
all to terminate pretty close to 4.15 volts, but this is
probably an exception to the rule because most people
dont design and build their own chargers. Still, most
manufacturers know the 4.2 volt termination scheme, so
i wonder just how much variance there can be from charger
to charger (on the open market) with respect to termination
voltage setting.

Thus, i put forward the idea that parallel charging does not
balance the charge of cells any better than single cell 
charging, unless more than one charger is used and there is
a significant difference in termination voltage between at
least two of the chargers.

BTW, all my lights use only one cell, so i dont have any
of these concerns on a daily basis like some people might
have 

Also BTW, what kind of meter are you using to measure your
cell voltage? I can get those resolutions but only if i 
use the 'tare' function on one of my meters 

Also BTW more, i know that we dont agree on everything here
but it has still been a very interesting discussion, and i think
we uncover some facts that could help not only ourselves, but
many other CPF users as well. Because of this discussion
I am currently in the process of trying to get a
what-is-supposed-to-be a very good model for the Li-ion cell.
I am told that much information can be gleaned from this
new model which was made by professionals in the industry.
Supposedly you can deduce information like charging termination
voltage and other stuff of interest. I hope it's as good as they
say it is, but above all i hope i can get a copy. I will no doubt
pass the information along once i get it.


----------



## VidPro (Apr 24, 2007)

SILVERfox, hey i told ya about li-ions that had anode disconnected.
you didnt listen 
i had camera batteries that were in series, and they got out of balance and in eventually they anode disconnected uneventfully (like they are supposed to) and the entire pack became useless.
i dissasemble them and either toss them, or replace the cells, and start it all over again, the stupid thing has no balance so in eventuality one parellel side goes over the 4.4v while the other side is around 3.8v and the charger charges blind the set to 8.4. 

same thing when i got free, and really cheap laptop packs off of e-bay for the cells out of them, a cell in the Series set or a SET of parellel batteries in a series set will have been overcharged and opened up DUE specifically to the lack of seriees pack balance. (anode disconnected). the rest of the cells were quite usable still.

that is the biggest failure with improperly charged *series* packs, when they just STOP working altogether, vrses slowly decrease in capacity.

but of course the OP is balancing series pack :goodjob: , so if done right wont have that problem, but still i have seen it, and WAY to often, cause it costs us here $38-$99 ever time it happens.
these dumarses think they can make a 7.2v type *series* pack and just wing it at 8.4v and it just doesnt work over much TIME.


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## SilverFox (Apr 24, 2007)

Hello Al,

My goal for balance is to have all of the cells within 0.01 volts of each other.

With this criteria, cells 1 and 3 were in balance and cell 2 was slightly out.

I am using a Fluke 199 Scopemeter for voltage measurements.

I am very interested in what you find out about the model. If you need some raw data to compare to let me know. It is always fun to try to refine the model to represent real life. 

Tom


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## SilverFox (Apr 24, 2007)

Hello VidPro,

I can assure you that before your anode disconnects, the capacity of the battery pack will have faded to below 80% of its original capacity.

When you salvage batteries you are at a little bit of a disadvantage because you don't know the initial capacity of the cells in the pack. I salvaged some cells from a computer battery pack. When I checked them out they came in at around 700 mAh. I am not sure if Panasonic every made 18650 cells with 700 mAh of capacity, so I pretty much know that they were well on their way out. 

I used them a few times and they continued to get worse, and they could not hold voltage under load. I finally decided that they weren't worth the trouble and recycled them.

I am sure that there are cells with life left in them that you can get from discarded battery packs, but once you go below 80% capacity, the cells behave differently and many times not uniformly. This signals danger to me, so I tend to stay away from it.

If you applications involve moderate loads, they may work fine, but once you get under the 80% capacity limit strange things can crop up.

Tom


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## MrAl (Apr 25, 2007)

QUOTE

Hello Al,

My goal for balance is to have all of the cells within 0.01 volts of each other.

With this criteria, cells 1 and 3 were in balance and cell 2 was slightly out.

I am using a Fluke 199 Scopemeter for voltage measurements.

I am very interested in what you find out about the model. If you need some raw data to compare to let me know. It is always fun to try to refine the model to represent real life. 

Tom

END QUOTE

Hi Tom,

Well, do you realize that 0.01 volt is only a quarter of one percent of
the cells voltage? I was just trying to make the point that if we
define 'unbalanced' too rigidly we could say that no two cells are
ever balanced 
It seems to me that 1 percent or even 2 percent shouldnt be too bad.
A 0.05 series resistance and an imbalance of 0.04 volts (1 percent)
only gives rise to a current of 400ma if they are connected in parallel.
Not that i like this, but it seems acceptable.
For your cells of 0.080 ohms there would be about 250ma of current
flow.

It's of course up to you how you want to define your cells.


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## LuxLuthor (Apr 25, 2007)

I'm not in the league with Tom, Al, or VidPro....and yet I do want to share these two very real practical examples of what Tom was saying about 3 posts ago.

I have two of these 3D FM mag85 lights that each use 9 x 17500 Li-Ions in 3S3P setup. What I had to do was individually charge each battery before I could use the light again. That was untenable....so my answer was to buy 3 DSD chargers that had two slots, but gave a "whopping" 600 mA charging current divided between two slots...and no individual cell voltage termination (as far as I understood...vs. the Pila IBC charger which does sense termination voltage separately in each of its two slots). So even charging 6 cells in 3 DSD chargers at that low of a mA rate took forever. It improved slightly when I upgraded to a stronger Nokia AC transformer that I think put out 800 mA (400/slot).

Then we started getting into balance tap lead charging, using this Voltcraft NiMH cradle that we modifed, and described in this thread. Here is a picture I took from my post #2, final image. Now I can charge all 9 batteries at once, or even 18 batteries for both of my 3D lights by adding a second cradle in series, yielding a 6S balance taps setup with my Hyperion LBA10.





================================

My 2nd real life example was I bought this other FM 2D Maglite and 8 x 14670 Li-Ion cells. Starting with post #62 in Jan 2007, I started noticing that these brand new cells would not hold their voltage, and/or would not charge on the Pila IBC or DSD charger. I would get error lights on the charger, or some would apparently charge up to 4.17 but then quickly drop down to a resting voltage much lower...sometimes down to 3.6V with no use.

I was dumbfounded that all 8 could be bad, but when returned to AW in Hong Kong, eventually his factory determined that there were bubbles or other problems with the electrolytes that missed QC because they initially charged up. He replaced them, and at the time, my only concern was having quality batteries (that I paid for) to power the light. I was not aware as recent as January, 2007 of the Li-Ion fire/explosion risk, because I even tried charging all of them in the FM 2P4S pack with one of those universal chargers to see if that would work before shipping them all back to Hong Kong. They were protected cells, so I think there was some level of inherent safety with them, but I have no idea what might have developed if I kept screwing around charging them without knowing what I was doing.


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## SilverFox (Apr 26, 2007)

Hello Al,

While 0.01 volts is a very small difference, percentage wise, in voltage, you must understand that the capacity of a cell drops around 1% per every 0.008 volts difference.

In the example I gave, there is roughly a 3% difference in capacity between the highest and lowest voltage cells. If you are using the cells in a single cell application, there is no problem. However, if you use several of these cells in series, the balance issues begin to raise up.

You will also note that there was a difference in impedance between the cells as well. The indicates that under load, the higher impedance cells will lose more capacity to heating of the cells, than the lower impedance cells. 

Now we have cells starting out with differences in capacity and adding to that the fact that the higher impedance cells will be further hampered by cell heating. 

If you use cells properly and limit the discharge to a safe voltage, you should have no problems at all, however, if you push things to the limits, you may end up over discharging a cell.

My thought is to start with balanced cells and that eliminates one of the imbalances. That is why I strive for starting with cells within 0.01 volts of each other.

Tom


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## MrAl (Apr 27, 2007)

Hi again Tom,

Ok no problem.

I'll post any new info i get about the Li-ion model.
So far no news yet however.


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## Calina (Dec 3, 2007)

How could I have missed this thread?

Very interesting and highly informative.

Thanks to all.


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## VidPro (Dec 3, 2007)

SilverFox said:


> Hello VidPro,
> 
> I can assure you that before your anode disconnects, the capacity of the battery pack will have faded to below 80% of its original capacity.
> 
> ...


 
totally agree, higher self discharge, less capacity,etc. but remember my gas studies where i am trying to get a handle on cheap junk failing faster because of gas forming sooner than it should.
those cells should increase in pressure too, causing the same disconnect.

and indeed i was referring to when the strange things start happening, which is probably when the cell is headed for the dumpster anyways.
When the situation of FAST charging a parellel set comes up and its 1C for the whole SET, then its worth mentioning that the whole set might not even exists, when you go to charge it.
Cheap cells could fall out much sooner than good cells , making some definate number of cycles or age impossible to quantify. even one cheap cell out of a bunch that the rest are ok. When the cells have gotten cruddy, is when the cells heat more, and will drop out, so the fast charging can make a ALREADY bad situation worse. the problem is the bad situation or battery no doubt, but fast charging THEN it still compounds it.

depending on the cruddyness of the cell the gas can form way earlier than it does in other non cruddy cells, also it can depend on how its charged. I have no data on gas forming more or less from a fast charge than a slow one. i have microbes of data that show no disconnect or overheating, occured from slow charging, even bad ones.

so when everything goes bad, i would rather be slow charging, BUT i dont know if slow charging MAKES it bad to begin with.


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