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Old Jul 31, 2004, 05:34 AM
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53859 Niederkassel, Germany
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Question
LiPo - extreme cell imbalance

Some dark clouds appeared in my "Shiny New Lipo World".

Flying my Carbon Falcon a couple of days ago (PJS 300, 2xKokam1200HD, GWS 9x4.7" prop), I noticed some rather rapid loss of power just a couple of minutes into the flight.

I charged the battery, and it showed full after less than 400mAh of charge. Strange, but logically sound, as the flight was rather short.

Time for a capacity check:

Cell #1 - 849mAh at 2A discharge rate
Zelle #2 - 269(!) mAh

The cells are less than one year old, have been treated carefully, and have less than 20 cycles. As the model occassionly lands on the pack, I assumed some internal "mechanical" damage.

As I bought 4 cells at the time, the identical twin pack - with only 5 cycles in an Alfa Mustang - showed the same massive imbalance:

984mAh versus 414mAh

Fully charged, both cells show identical voltage (+/- 1mV).

Any similar experiences?

Jürgen
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Old Jul 31, 2004, 05:55 AM
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Quote:
Originally Posted by Jurgen Heilig
... 984mAh versus 414mAh

Fully charged, both cells show identical voltage (+/- 1mV)...

Jürgen
Jurgen,

Were the cells were independently charged to this voltage?

- RD
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Old Jul 31, 2004, 08:40 AM
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Yes. Jürgen
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Old Jul 31, 2004, 09:14 AM
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Here is a picture of the two cells with drastically reduced capacity. No visible damage, just a strange "thickness distribution". Normal or unusual?

Jürgen
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Old Jul 31, 2004, 05:12 PM
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Jurgen,

No, I've never seen cells that look like that, with such pronounced waves in the surface of the cells.

Since all the interior laminates are supposed to be of uniform thickness and in vacuum contact with each other, there must be either swelling of laminate(s), or irregular delamination. Either is abnormal and could account for the loss of capacity.

If the cells hold voltage, that would indicate the seals have not failed and there is no internal shorting.

- RD
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Old Aug 01, 2004, 02:31 PM
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Nobody else with similar problems? Jürgen
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Old Aug 01, 2004, 06:10 PM
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No, but I haven't flown a lot this year due to uninteresing (to this board) reasons.

Mechanical distortion is always bad news and is almost certainly associated with the low capacity as RD sez.

I'd suspect mechanical damage of some sort - either landing on them or being squeezed by shrink wrap or something.

Internal pressure leads to ballooning - not what you have there at all. Looks like someone has stood them on edge, then stood ON them.
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Old Aug 02, 2004, 12:02 AM
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These cells have always been like this, i.e. there is no mechanical damage. I only started to wonder about the strange thickness distribution, when the capacity check provided poor results.

Jürgen
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Old Aug 02, 2004, 03:18 AM
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Quote:
Originally Posted by Jurgen Heilig
Cell #1 - 849mAh at 2A discharge rate
Zelle #2 - 269(!) mAh
Well, the problem is rather obvious, Cell #1 is English and #2 is German. Never mix cells of different origin in the same pack, that is a basic rule!!

<Serious mode>
I just bought a 3s pack and the middle cells shows similar problems. I will try to charge them independantly to see if there is any similaritys with your problems.


// Fredrik Wergeland
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Old Aug 02, 2004, 04:23 AM
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Jurgen: Were the cells like that when you recieved them?
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Old Aug 02, 2004, 11:06 AM
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Here is a picture of the same cells when they were brand new (09/2003 - unfortunately from the other side).

Jürgen
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Old Aug 02, 2004, 11:13 AM
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Another very strange thing: When I separated the two packs and made a new pack from the "good" cells, the first discharge cycle gave me about 1000mAh at 2A discharge rate. Not bad, considering that the cells showed only 849mAh / 984mAh when tested individually. Li-Polymer batteries suffering from "Lazy battery effect", or what?

Jürgen
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Old Aug 02, 2004, 11:36 AM
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Wouldn't series connecting cells with different capacities lead to an unbalanced pack? Did those 2 cells end up at the same voltage after the '1000mah discharge' charge cycle?

Interesting data - It will be interesting to follow these two cells.
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Old Aug 02, 2004, 01:04 PM
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849 and 984 mah is an awfully profound difference in capacity for two cells in a series pack. Those cells should be charged individually.

I'm surprised that one of the cells didn't overcharge when they were configured 2s. After all, with the imbalance you described, if you charged 2s, you could have had a cell at 3.8v and the other at 4.6v before the charger would back off. I would expect that much of an imbalance to puff a cell.

Did you charge the grossly imbalanced pack as 2s?

Jason
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Old Aug 02, 2004, 01:23 PM
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Ok, here is how I did it: I soldered the two cells (849mAh / 984mAh - both individually discharged) in series and measured the voltage. the difference in voltage was 5mV.
I then hooked them up to my Kokam LIPO-402, 1A charge rate, and closely monitored the voltages during charge. The difference slowly became bigger - up to 10mV - and when the first cell reached 4.20V, I stopped the charge. The cell with the lower voltage was then topped up as a single cell.

Jürgen
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