# Battery Test with Bad Cell



## Elithion (Oct 6, 2009)

Correction: 
High DC resistance: 4.2 times the others.
Low capacity: 70 % of others.

The resistance increase dwarfs the capacity loss.


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## major (Apr 4, 2008)

Elithion said:


> High DC resistance. Capacity looks unchanged.


Thanks for the comment. At zero seconds it is 300 mV lower and at 600 seconds it is 700 mV lower. DC resistance doesn't explain that, does it? And it did not get hotter than other cells during the discharge as measured with a handheld IR thermometer. So I was thinking it was a loss of capacity.


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## tomofreno (Mar 3, 2009)

Elithion said:


> Correction:
> High DC resistance: 3.2 times the others.
> Low capacity: 70 % of others.
> 
> The resistance increase dwarfs the capacity loss.


 So is this the kind of thing the Lithiumate SOH picks up Davide? Does it give some detail, or an "idiot light" indicating "poor health"?


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## Elithion (Oct 6, 2009)

tomofreno said:


> So is this the kind of thing the Lithiumate SOH picks up Davide? Does it give some detail, or an "idiot light" indicating "poor health"?


The Lithiumate Pro does evaluate State Of Health.
The Lithiumate Lite does not (it does internally, but does not display it: you want to keep things simple for its intended users).
The Lithiumotive evaluates it for each individual cell.

The BMS evaluates an SOH value based on measured resistance and capacity (with respect to nominal resistance and capacity). It displays it (in the GUI) and sends it out (on the CAN bus). It's really up to the user who may be curious on the state of the battery to look at it. The BMS doesn't make any decision based on the SOH.

The SOH value ranges from 100 % ( = brand new), slowly down to 50 % (= it would take two batteries in this condition connected in parallel to work as well as a brand new battery), and even lower down to 0 % ( = dead).


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## onegreenev (May 18, 2012)

Major,

What cells did you use? What chemistry? Nominal voltage? Full voltage? Empty voltage? 

Interesting graph. Top Balanced Im sure, right? Drained to what level? 100%, 90%. Charged to 100% or 90% or? 



> So I was thinking it was a loss of capacity.


 Seeing the graph I'd say yes. 

Not sure how how high resistance is deducted from that graph. Odd. 

Pete


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## headwaycoral (Nov 17, 2012)

there maybe three status :
1.the battery is loss of capacity 
2.large connecting resistance 
3.test place may not right .


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## major (Apr 4, 2008)

It is a Lithium Polymer pouch type of product. It was a lab rat; that is used for testing and evaluation and never in a regular service. Charged to 4.2V/c and for the most part limited to 2.5V/c on the low end. Some wires got crossed so-to-speak and took cell #3 to zero. Never tried anything except to top balance, manually, although some "experimental" approaches were tried in doing so. So it was abused, but not killed on purpose. I don't want to name names.

There was no connecting resistance problem during the test shown and it was tested in the right place for me


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## tomofreno (Mar 3, 2009)

Elithion said:


> ...The BMS evaluates an SOH value based on measured resistance and capacity (with respect to nominal resistance and capacity). It displays it (in the GUI) and sends it out (on the CAN bus). It's really up to the user who may be curious on the state of the battery to look at it. The BMS doesn't make any decision based on the SOH...


 Nice diagnostic, and with the CAN output could trigger a "needs service" warning for someone to bring their car in, or connect it to the internet so a service person can download the SOH. The kind of capability I would want if I converted vehicles for others, since they are not likely to pay attention to their pack's SOH. Sorry for the diversion major.


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## GerhardRP (Nov 17, 2009)

major said:


> I screw around with batteries enough that I inevitably damage one. Such as the case with this one. It is a 12 cell battery and I somehow drained flat cell #3. I tried to charge and balance it a number of times. I put it to the test yesterday. I am now convinced it is toast
> 
> Anyway, I though you guys might find the behavior interesting. The graph shows the CellLog data for the 12 cells with a 100 Amp constant discharge followed by a rest period (cool down) and then a charge cycle the following day at about 17 Amps. All cells were close together at 4.1 to 4.2 V/c at the start of the discharge but I forgot to start the stupid loggers in time so missed the first few minutes of the test. Hey, nobody's perfect  And I just keep making mistakes with this bad luck battery.


Interesting that the only cell to show a slope change on discharge is the bad guy at 3 volts. None have reached the high voltage knee, so they are conservatively charged.


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## major (Apr 4, 2008)

GerhardRP said:


> Interesting that the only cell to show a slope change on discharge is the bad guy at 3 volts. None have reached the high voltage knee, so they are conservatively charged.


I felt bad about screwing up the start of the test graphed in post #1 so I reran it today.










And blew up the discharge portion: 










Also, I did measure a 10-15°F higher temperature rise on the bad cell #3 over the others.


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## Elithion (Oct 6, 2009)




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## IamIan (Mar 29, 2009)

Elithion said:


>


Although I see the resistance is much higher ... for determining the magnitude of the difference ... I think it would be better to look at the dV point shown at the end the rest period when the 17Amps of charge is applied ... and not the dV that happens over the rest period.


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## Elithion (Oct 6, 2009)

IamIan said:


> Although I see the resistance is much higher ... for determining the magnitude of the difference ... I think it would be better to look at the dV point shown at the end the rest period when the 17Amps of charge is applied ... and not the dV that happens over the rest period.


Yup.
That will work too. It's just that the resolution of the data is so much worse there. Still, I did as you suggested, and I got a ratio of 4.77. Not too far off from the ratio of 4.24 that I got looking at the end of discharge.


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## GerhardRP (Nov 17, 2009)

Elithion said:


> Yup.
> That will work too. It's just that the resolution of the data is so much worse there. Still, I did as you suggested, and I got a ratio of 4.77. Not too far off from the ratio of 4.24 that I got looking at the end of discharge.


Let me add a third third analysis; In the graph, I have added a lines from full charge at 40 seconds to the recovered voltage referred back to the end of discharge time for the orange cell and the bad actor. I claim that this is the open circuit voltage vs. time. Those lines are parallel to their respective discharge curves. The voltage difference have the ratio of 2.5 which is equal to the resistance ratio. Davide's capacity ratio works.


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## major (Apr 4, 2008)

Just a note: Cell#3 is the bad guy. Colors are similar for different traces so a bit confusing. And the upper trace (red) is cell#2. It is either a bit higher voltage due to heating from the adjacent cell#3 or it is an instrument problem with the CellLog. I've noticed some crosstalk between adjacent channels on the CellLog when manually balancing. But can't complain about the $29 devices too much 

Thanks for the analysis. Hopefully members can see how a bad cell can affect their battery pack in operation.


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## GerhardRP (Nov 17, 2009)

major said:


> Just a note: Cell#3 is the bad guy. Colors are similar for different traces so a bit confusing. And the upper trace (red) is cell#2. It is either a bit higher voltage due to heating from the adjacent cell#3 or it is an instrument problem with the CellLog. I've noticed some crosstalk between adjacent channels on the CellLog when manually balancing. But can't complain about the $29 devices too much
> 
> Thanks for the analysis. Hopefully members can see how a bad cell can affect their battery pack in operation.


I meant orange vs bad, whatever they call themselves.
G


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## GerhardRP (Nov 17, 2009)

major said:


> I screw around with batteries enough that I inevitably damage one. Such as the case with this one. It is a 12 cell battery and I somehow drained flat cell #3. I tried to charge and balance it a number of times. I put it to the test yesterday. I am now convinced it is toast  <snip>


Thinking about this thread...can you tell us exactly what you did to the bad cell? Was it "just" flat, or was it reversed? Was it high rate or an accidental modest resistive load?
Gerhard


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## major (Apr 4, 2008)

GerhardRP said:


> Thinking about this thread...can you tell us exactly what you did to the bad cell? Was it "just" flat, or was it reversed? Was it high rate or an accidental modest resistive load?
> Gerhard


When I said "I" screwed up and killed it, I was being kind to those I work with and taking responsibility for not making the system foolproof such that some idiot couldn't plug something in backasswards 

Far as I can tell, cell #3 was shorted externally. This took it flat; meaning down to zero volts. The shorting path was of unknown resistance, but did overheat and burn insulation but apparently did not go to an open circuit. So I can't say the level of discharge to take it flat but there was no mechanism to reverse it. 

The surrounding cells appeared to remain at sound voltage levels and appear to be undamaged. I put a single cell charge on #3 and watched it for a number of months, not using the battery. It appeared to have higher than normal leakage and I would top it off to "top" balance the pack. 

So I just got tired of screwing with it and put it to the load test. Looks like #3 is worthless in a pack with good guys, so I'm crossing it off. I wouldn't trust it in anything more than a single cell battery and don't have much use for that.

I have had other cells come back from flatness, and some that never took any charge at all after the visit to flatville. Seems like a crapshoot you should avoid


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## kennybobby (Aug 10, 2012)

Howdy Jeff,

That's a great set of data, thanks for sharing.

Did you happen to charge the pack again after test 1 and before test 2? i noticed that the voltage seemed to be higher at the beginning of test 2 and wondered what could cause that to happen?

In test 1 it looks like you pulled over 17 A-hrs out of the pack, then charged the pack up and put 14.5 A-hrs back in. Then in test 2 pulled out 24.2Ah and put 23.7 Ah back in. Does that seem about right?

What was the rated capacity of the cells?


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## major (Apr 4, 2008)

kennybobby said:


> Did you happen to charge the pack again after test 1 and before test 2? i noticed that the voltage seemed to be higher at the beginning of test 2 and wondered what could cause that to happen?


Test #1: 


major said:


> All cells were close together at 4.1 to 4.2 V/c at the start of the discharge but I forgot to start the stupid loggers in time so missed the first few minutes of the test. Hey, nobody's perfect


And test #2: 


major said:


> I felt bad about screwing up the start of the test graphed in post #1 so I reran it today.





kennybobby said:


> In test 1 it looks like you pulled over 17 A-hrs out of the pack, then charged the pack up and put 14.5 A-hrs back in. Then in test 2 pulled out 24.2Ah and put 23.7 Ah back in. Does that seem about right?


About? Yeah, I guess. But I wasn't accurately recording current. On discharge my regulator holds pretty constant at 100A but I would probably put a +/-2% on it. On the charger, current varies, starting near 17.8A and then down to 17.4A and then into a taper to hold ending voltage, so that is tough to draw Ah conclusions. And then I was terminating the test when the cell#3 hit low voltage, so Ah on the good cells doesn't mean much. The cells were about 17Ah rated and there were 2P, so like 34-35Ah as tested, rated.


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## GerhardRP (Nov 17, 2009)

major said:


> <snip>The cells were about 17Ah rated and there were 2P, so like 34-35Ah as tested, rated.


Is it possible that one of the #3 buddies is completely dead and the other more-or-less OK?


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## major (Apr 4, 2008)

GerhardRP said:


> Is it possible that one of the #3 buddies is completely dead and the other more-or-less OK?


Cell #3 is actually two 17Ah cells in parallel. I cannot separate the two unless I remove them from the pack. It is possible that one of the two buddies in that parallel set of two is the bad guy and dragging down his friend along with him. I will probably disassemble that pack in the near future and can look into it then.


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