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Old May 10, 2012, 03:20 PM
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Is this behavior a sign of a bad lipo cell?

Hi guys,

Received new ZIPPY lipos (HK) and did some analysis on them.

With one of the batteries (2s 1600mah) , during a discharge cycle, I noticed that both cells were dropping voltage equally until 3.6v, then, one of the cell started to drop faster than the other, to the point where one reached 3.2v while the other was still above 3.5v. I stopped the discharge cycle at that point.

Then I did a charge/balance cycle and both cells got balanced after a fwew minutes, then charging continued normally, without one cell "charging" faster than the other one.

Tested this 3 times and same thing happens all the time...

Would that be a sign of a bad cell? I did read about bad cells dropping voltage faster than good cells but not too sure at which voltage difference it becomes an actual sign that the cell is bad.

In my case, I stopped the discharge cycle because one cell just dropped below 3.2 (stopped at 3.19) and the other was a 3.5v, so there was a 0.3 v difference.

My charger has a LVC set ay 6.0v, which IMO is far too low, so I haven'T dared letting the discharge cycle continue until the v-cutoff...

Thoughts?
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Old May 10, 2012, 04:14 PM
Space Coast USA
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I'm sure cheapo packs are not made from selected cells matched for capacity. Therefore 1 or more cells may empty before the others which is why it is important not to discharge the cells below 3.7V (static) or 3.3V(under load).

The important piece of info you left out was how much capacity were the packs showing when charged? Take the pack down to 3.7V(static) and then charge. If the mah in is greater than 1280mah you are OK. This is a pack that should definitely be flown on the clock and not rely on the LVC to tell you when to land.
So, after you know the mah on charging you'll know more about the pack.
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Old May 10, 2012, 04:28 PM
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Originally Posted by hoppy View Post
I'm sure cheapo packs are not made from selected cells matched for capacity. Therefore 1 or more cells may empty before the others which is why it is important not to discharge the cells below 3.7V (static) or 3.3V(under load).

The important piece of info you left out was how much capacity were the packs showing when charged? Take the pack down to 3.7V(static) and then charge. If the mah in is greater than 1280mah you are OK. This is a pack that should definitely be flown on the clock and not rely on the LVC to tell you when to land.
So, after you know the mah on charging you'll know more about the pack.
Did a charge cycle and it ended up with both cells showing 4.20 and capacity showing 1570mah on the charger.

Those are 1600mah packs so I guess they're ok.

Thanks for the info!
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Old May 10, 2012, 05:15 PM
Jack
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Originally Posted by Fred Methot View Post
Did a charge cycle and it ended up with both cells showing 4.20 and capacity showing 1570mah on the charger.

Those are 1600mah packs so I guess they're ok.

Thanks for the info!
What voltage did you take them down to? It is unusual to be able to get 98% of the capacity out of a cell that is accurately rated for capacity. Usually 80% or so is a more realistic expectation. That would have you getting less than 1300 out of them.

Jack
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Old May 10, 2012, 09:52 PM
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Originally Posted by jackerbes View Post
What voltage did you take them down to? It is unusual to be able to get 98% of the capacity out of a cell that is accurately rated for capacity. Usually 80% or so is a more realistic expectation. That would have you getting less than 1300 out of them.

Jack
It does sound odd indeed.... I'm doing what hoppy suggested right now...

Discharge to 3.7 (static)
Charge
Check mah

[...]
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Old May 10, 2012, 10:10 PM
Jack
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A typical low rate discharge to 3.7V should leave you 10-15% or more, I thought maybe you took them all down to 3.0V or less (a bad thing to do).

Maybe you got lucky and some under-rated packs or mis-marked packs of a larger capacity.

Jack
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Old May 10, 2012, 10:23 PM
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Originally Posted by jackerbes View Post
A typical low rate discharge to 3.7V should leave you 10-15% or more, I thought maybe you took them all down to 3.0V or less (a bad thing to do).

Maybe you got lucky and some under-rated packs or mis-marked packs of a larger capacity.

Jack
Well, I monitored the discharge cycle from the individual cell voltage screen on the charger and stopped it once one got to 3.19v, the other was higher though, 3.5v if I recall correctly.

thanks for posting!
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Old May 10, 2012, 10:54 PM
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Quote:
Originally Posted by jackerbes View Post
A typical low rate discharge to 3.7V should leave you 10-15% or more, I thought maybe you took them all down to 3.0V or less (a bad thing to do).

Maybe you got lucky and some under-rated packs or mis-marked packs of a larger capacity.

Jack
I'm figuring 3.7V is 80% discharged from my tests, thus the 1280mah goal. I think the 1570mah he reported was from the pack discharged to 3.2/3.5V.
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Old May 11, 2012, 06:46 AM
Jack
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What hoppy said on the 3.7V and 80% cutoff! That is great advice.

When I was getting started with LiPOs I found a couple of his posts with the testing info he mentions and started using the 3.7V per cell, as measured under load, for my low voltage cutoffs and ending flights. And I still have nearly every LiPO pack I have ever bought.

Here a summary of his info and links to his posts:

LIPO Capacity @ Voltage per cell (cell voltages, multiply by cell count for pack voltage, i.e. for 3S pack multiply x 3)

4.20v = 100%
4.03v = 76%
3.86v = 52%
3.83v = 42%
3.79v = 30%
3.70v = 11%
3.6?v = 0%

Capacity below 3.7V "resting" is not usable for flying, it's where the battery voltage dumps and damage begins.

Avg Resting V % Remaining Capacity (for 3S pack, divide by 3 for cell voltages)

11.0V-- 17% (3.67V per cell)
11.1 --- 26 (3.70V per cell)
11.2 --- 30 (3.73V per cell)
11.3 --- 37 (3.77V per cell)
11.4 --- 46 (3.80v per cell)
11.5 --- 50 (3.83V per cell)
11.6 --- 57 (3.87V per cell)
11.7 --- 63 (3.90V per cell)
11.8 --- 69 (3.93V per cell)
11.9 --- 77 (3.97V per cell)
12.0 --- 83 (4.00V per cell)

These values are the average of many discharges of different size/age/brand packs. As you can see, the values are not absolute. A new pack gives different results from a well used pack but the chart is still useful for the broad picture.

Thanks again, hoppy!

Jack
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Old May 11, 2012, 07:12 AM
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As others have posted this is normal with Lipolys which do not have closely matched cells capacity wise and why it is unwise to discharge them past the 80% of capacity point.

http://www.rcgroups.com/forums/showthread.php?t=1302258

Charles
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Old May 11, 2012, 10:03 AM
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Thanks for all the input guys.

Well I'm a bit confused about those Zippy 1600mah packs then.

I discharged the pack down to 3.7v (static), then ran a full charge.

The cycle completed, with the pack at 8.39v (4.2/4.19) but the mah went up to 1650mah (from 3.7v), which is when the mah cutoff kicked in and ended the charge cycle.

Mislabelled packs, how often is that happening?

Or maybe it's my charger that is behaving or not calibrated? That is my next step by the way (checking charger calibration...).

Fred
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Old May 11, 2012, 10:56 AM
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Some chargers are in fact off by as much as 20% perhaps more. I(f you have a in-line watt / aHm meter you can use it betrween the charger and batterie's main leads and do a comparision.

Depending on the discharge rate as a percentage of C many LiPolys will in fact deliver 95% or so of their capacity discharged to to 3.7 per cell. What one needs to remember is that the lower the discharge rate the more capacity will be delived doown to any give voltage cut off point.

A 20C 2000 discharged at 10A (5C) down to 3.3 per cell may well deliver 100% or more of stated capacity.

Discharge thge same one at 40A (20C) to 3.3 per cell and it may only deliver 1500 to 1700 mAh.

These days most do not seem to understand how to read a discharge graph but for those who can the Battery Graph Vault has reall LiPolys discharged at real rates with real mAh delivered down to X volts at different rates.


The link I provide above

http://www.rcgroups.com/forums/showthread.php?t=1302258

shows a 2200 discharged at 30 A down to 3.333 per cell and it delivered 2117 mAh
Charles
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Old May 11, 2012, 12:31 PM
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Originally Posted by everydayflyer View Post
Some chargers are in fact off by as much as 20% perhaps more. I(f you have a in-line watt / aHm meter you can use it betrween the charger and batterie's main leads and do a comparision.

Depending on the discharge rate as a percentage of C many LiPolys will in fact deliver 95% or so of their capacity discharged to to 3.7 per cell. What one needs to remember is that the lower the discharge rate the more capacity will be delived doown to any give voltage cut off point.

A 20C 2000 discharged at 10A (5C) down to 3.3 per cell may well deliver 100% or more of stated capacity.

Discharge thge same one at 40A (20C) to 3.3 per cell and it may only deliver 1500 to 1700 mAh.

These days most do not seem to understand how to read a discharge graph but for those who can the Battery Graph Vault has reall LiPolys discharged at real rates with real mAh delivered down to X volts at different rates.


The link I provide above

http://www.rcgroups.com/forums/showthread.php?t=1302258

shows a 2200 discharged at 30 A down to 3.333 per cell and it delivered 2117 mAh
Charles
Thanks Charles, very informative!

In the meantime, I calibrated the charger. It was off by -0.15v... I'm still a bit new to all this so I have no idea if 0.15v was making any difference, but anyhow, it's well calibrated now, tested it against 3 different packs/different brand, charger and multimeter matched perfectly.
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Old May 11, 2012, 01:29 PM
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Which way was your charger in error? High or low? On both the high and low ends?

If it's delivering that much capacity. I'd not worry about it, just be careful about your end voltage.
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Old May 11, 2012, 01:38 PM
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Which way was your charger in error? High or low? On both the high and low ends?

If it's delivering that much capacity. I'd not worry about it, just be careful about your end voltage.
The claibration of my charger is pretty basic (Accucel-6).

The procedure is:
Measure any battery pack using a voltmeter/multimeter - take note of value
Start the charger in calibration mode
Plug-in the same pack to the charger - charger will display voltage reading of pack
Adjust voltage with dec/inc buttons.

In my case, the charger was reading the voltage higher than it actually was (multimeter reading 8.20 - charger reading 8.35).
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