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Old Jan 21, 2004, 01:40 AM
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2raj's Avatar
Melbourne, Australia
Joined Jan 2003
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Question
Why does lithium cell-imbalance cause fire?

Can someone please explain by cell imbalance can cause a lipol start a fire?

Hopefully a better understanding of the cause will help us all to be safer.

Here is my theory...

Lets consider the imbalanced 3s1p pack depicted in the attached image. Cell 2 is not discharged as much as the other two. I suspect this is what is referred to as an imbalanced pack, voltage across all cells is not the same, right?

My understanding of electricity is that voltage of this pack is the average of the 3 cells, right?

So if we try to charge this pack, we will supply power until it reached 4.1v. So we will effectively cause Cell 3 go over 4.1 and balloon up, vent and start fire. Am I even close?

Cheers,
2raj.
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Old Jan 21, 2004, 02:33 AM
DNA
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In a series (3s) connected pack, the voltage of the Pack would be equal to
the sum of the individual cell voltages, or in your diagram it would be 11.4v.

To charge a 3s pack you would set your charger to charge to a total voltage
of 12.6v, as each cell needs to be charged to 4.2v. When the pack in your
diagram reaches 12.6v total, the voltage on each cell would be 4.1v, 4.4v, and
4.1v. Cell 2 (not cell 3) would be overcharged by 0.2v and may swell or vent.
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Old Jan 21, 2004, 11:28 AM
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The discussion is correct. The LiPo chemistry is very sensitive to overvoltage. Charging a cell to more than 4.2v is a recipe for trouble.

This is why parallel charging (several 4.2v chargers, plugging the cells together in series after charging for use in the aircraft) remains the safest approach to charging LiPo batteries.
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Old Jan 21, 2004, 12:19 PM
clever phrase here
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is there a definitive voltage number that a cell in series has to be off in balance to cause a cell failure?

does an imbalance always cause a fire?

in what situation, will a lipoly pack that is just sitting around cause a fire? i ask this because i've read some of you are "storing" the lipoly's in fire proof boxes, as well as charging them.

thanks.
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Old Jan 21, 2004, 12:22 PM
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Overcharging as a result of cells being out of balance will cause the cell to fail. They don't always explode or burn.

There is no specific voltage or % overcharge or whatever.

I store mine in a fire proof box, but I've never had one burn while charging or in use, so I don't think it's really needed. It just lets me sleep better considering my whole family is in the same home.
..a
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Old Jan 21, 2004, 12:54 PM
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Generally, if there's a voltage difference of 0.1 volt or more between the highest and lowest voltage cells in a pack, it would be wise to recharge all the cells individually, IMO.

- RD
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Old Jan 21, 2004, 01:09 PM
clever phrase here
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0.1 volts difference. not much room there...

please see picture below. the little circuit board thingies on those packs, are these the cell balancing circuitry? these are old cell phone batteries. and no, not the one on the far left.

mostly obsolete now, but do li-ion cells share the same imbalance failure conditions?
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Old Jan 21, 2004, 01:17 PM
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Yes li-ion cells can become unbalanced, just like lipolys do. The circuitry on those cells might keep them from getting unbalanced, I don't know.
As I understand it, the safety circuits also reduce the amount of current the cells can output.
So if you want to use them to fly most of our models, you have to remove the circuits.
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Old Jan 21, 2004, 03:46 PM
DNA
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The circuits on those li-ion packs are Not cell balancers. They are
cell monitors. They monitor the voltage of each cell. If one cell or the other goes over 4.2v, the whole charge is shut off. If a cell goes below 3v, again, it will not allow the pack to be charged. That's why there were SO many extra cell phone packs made that are now on the surplus market. If the cells go out of balance, the pack is worthless and would need to be replaced.
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Old Jan 21, 2004, 07:41 PM
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Quote:
Originally posted by Foam Crazy
.....but do li-ion cells share the same imbalance failure conditions?
The cells that we call Lithium Polymer cells are really Lithium Ion cells in a polymer and aluminum flexible case. The chemistry is the same and the failure modes are the same.

Chuck
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Old Jan 21, 2004, 08:25 PM
clever phrase here
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good input guys.

now... wouldn't those protection circuits then be useful in avoiding over-charging disasters? in case of a cell imbalance, one of the cells in a series will flip the power off switch. the pack will not be fully charged, so during use, you will begin to see gradual, diminishing power loss, which in turn would make you check into the cell imbalance possibilities.

if you build the pack like this, it sounds to me like you'd have an almost fool-proof protection against lipo fires coming from cell imbalance. no? so, if and when the pack balance is off, and you noticed this because of gradual decline in power delivery, then you undo the shrink wrap and maybe this circuit board would have a few small connection points where you could individually check and recharge the cells.

somebody must have thought of this already... why would this not work? too costly? too bulky? cell fail percentage too low?
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Old Jan 21, 2004, 09:22 PM
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Melbourne, Australia
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Thanks to all who have contributed. I see how an imbalanced pack can have cells with voltage dropping 2.5 or going over 4.2 in normal charge or discharge.

All the suggestions about protection circuitry here is hard to implement as you'll need to have high current rating circuitry, which will make the solution bulky and expensive. Imagine having an ESC size circuit for each cell.

I believe the Duralite packs use regulators for their cells. That is probably why they only have RX battery packs (low discharge rate.)

A solution would be to have connections from each cell going to the ESC. ESC monitors each cell and when any imbalance is detected ESC cuts off the motor but not the servos. After all ESC already has high current control circuitry. (Is anyone from Castle Creations here?)

A similar concept can work with chargers. But I think it would be more practical charge each cell separately.

I have already started looking for some surplus mobile phone chargers and will be repacking my batteries so I can charge each cell separately.
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Old Jan 22, 2004, 12:08 AM
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everything was expensive in the beginning...
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Old Jan 22, 2004, 04:11 AM
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True, even lithium ion batteries were expensive for notebooks and camcorders, until Nokia started ordering them in millions!
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Old Jan 22, 2004, 05:37 AM
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Quote:
Originally posted by 2raj
Thanks to all who have contributed. I see how an imbalanced pack can have cells with voltage dropping 2.5 or going over 4.2 in normal charge or discharge.

All the suggestions about protection circuitry here is hard to implement as you'll need to have high current rating circuitry, which will make the solution bulky and expensive. Imagine having an ESC size circuit for each cell.
[/I]
Well Suzanne has designed a board that does exactly that. Using a switching shunt regulator to monitor each cell (or parallel suite of cells) so that if the voltage goes over 4.2v, the charge current is bled past the cells to make sure they don't get over voltage. Because charge currents are lower, the circuit doesn't have to do too much in the way of power dissipation.

That doesn't stop the possibility of e.g. a shorted back going up in a crash, or motor failure. But it does protect the pack in its most dangerous phase - charging - because typically that is done when the pack is in the vicinity (homes, people,car etc etc).

See threads on her circuit. It adds a little weight, so is probably not ideal for small packs as in parkflyers, but in terms of protecting a big investment in a big kilowatt output pack, and the nightmare scenario of a big fire from such a pack, its worth looking into.

It also automatically self balances a pack, making it less likley to fail from one cell getting too low in voltage.

There is a LOT of FUD (fear uncertainty and doubt - IBM speak) being spread around by various people, and some pack manufacturers and cell suppliers are using it as a lever to increase their sales. Beware of either succumbing to marketspik, paranoia or indeed complacency.

Lipo is a bit dangerous, and needs care. But its not an atomic bomb armed and ready to destroy your neighborhood. 99% of people using em have had no problems beyond maybe a puffed up cell and a ruined pack, and if you don't run them too flat, or draw more current than about 5-6C for extended periods they are going to last, stay balanced, until they simply lose sufficient capacity to be thrown away.
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