# When is a cell discharged? (Lithium Iron Phosphate)



## kennybobby (Aug 10, 2012)

*How are you Balancing, at the top or bottom?*



corbin said:


> I have Thundersky Lithium Iron Phosphate cells (with Yitrrium). The discharge cutoff is 2.8 volts.
> 
> Does this mean once the cells hit 2.8 volts, even under normal discharge (say 1C), it should be considered 0% SOC (100% Depth of Discharge)?
> 
> ...


Yes for all practical purposes it is fully depleted at 2.8 while under load at C/3. The open circuit voltage will float back up after the load is released, but the cell does not contain any real energy to do anything after reaching cut-off while under load.

That one cell will sag even further lower and be permanently damaged if the pack continues to operate under load. i don't know the damage voltage--it is too expensive to determine. i would pull that cell and do a bench test to determine it's performance. 

If you have ever pulled the pack down to 30% then that one cell may have been pulled way below "0%" into the damage zone. That is the reason why you should bottom balance rather than top balance your cells/pack. 

At nominal 1C discharge i have found that below about 3.0 to 3.1 volts there is only about 5 minutes of very poor quality saggy power left--the last 5 or 10 % is not very useful power for practical driving purposes.


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## corbin (Apr 6, 2010)

*Re: How are you Balancing, at the top or bottom?*

Hi Kenny,



kennybobby said:


> Yes for all practical purposes it is fully depleted at 2.8 while under load at C/3. The open circuit voltage will float back up after the load is released, but the cell does not contain any real energy to do anything after reaching cut-off while under load.


Is there a spec or somewhere that says this is what is to be done?




kennybobby said:


> That one cell will sag even further lower and be permanently damaged if the pack continues to operate under load. i don't know the damage voltage--it is too expensive to determine. i would pull that cell and do a bench test to determine it's performance.
> 
> If you have ever pulled the pack down to 30% then that one cell may have been pulled way below "0%" into the damage zone. That is the reason why you should bottom balance rather than top balance your cells/pack.


That's not true at all! If you have a BMS, then the BMS will tell you when any given cell reaches the low voltage and you simply consider the pack discharged at that point. Simply by pulling it lower than 30% doesn't mean you will have damaged the cells, unless you are NOT using a BMS and simply use overall pack voltage as a state of charge measurement.

Personally, I think it is worse if you bottom balance. You have a higher chance of destroying all your cells at the same time, as opposed to killing just one or two.




kennybobby said:


> At nominal 1C discharge i have found that below about 3.0 to 3.1 volts there is only about 5 minutes of very poor quality saggy power left--the last 5 or 10 % is not very useful power for practical driving purposes.


Cool -- that's interesting information! I find it is highly dependent on the cell chemistry and the temperature. What brand/model cells are you using? What was the outside air temperature when dong this test? What size cell?

The thing is, 5-10% is quite a bit of energy when you are talking about a high amp hour cell. I have 200ah cells. 10% of my pack is about 3000w/hrs, which is easily about 10 miles of range (at 300whr/mile -- but my car can definitely do better than that). 10 miles is quite a bit, and is definitely useful power...so I think it depends on the size of your cells and pack.

-corbin


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## tomofreno (Mar 3, 2009)

Sounds like there was no problem with the cell last winter, but now it is dropping to 2.8V when the pack is still at 50% or more SoC. It seems it has either lost capacity, developed high ir, or some combination. You could bench test it with something like a Powerlab to determine which, but practically speaking it doesn't much matter. It needs to be replaced, or it will limit performance of the pack. If you continue driving well beyond 50% SoC it may reverse the cell since it is already down to 2.8V there. You could drive a little further (close to home) and see how it behaves as SoC lowers further since you have cell V readout with your elithion. But the cell definitely seems to have a problem. I would just replace it.


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## corbin (Apr 6, 2010)

tomofreno said:


> Sounds like there was no problem with the cell last winter, but now it is dropping to 2.8V when the pack is still at 50% or more SoC. It seems it has either lost capacity, developed high ir, or some combination. You could bench test it with something like a Powerlab to determine which, but practically speaking it doesn't much matter. It needs to be replaced, or it will limit performance of the pack. If you continue driving well beyond 50% SoC it may reverse the cell since it is already down to 2.8V there. You could drive a little further (close to home) and see how it behaves as SoC lowers further since you have cell V readout with your elithion. But the cell definitely seems to have a problem. I would just replace it.


Thanks tomo -- that's pretty much the conclusion I was coming to. Unfortunately, it might be hard to find the specific 200ah cell I have in the size I need. 

What would happen if the cell did reverse? Would it start to negatively affect the rest of the pack?

It almost seems to make the most sense to just kill the bad cell, and then replace it once it is no longer taking a charge. However, it it for some reason would affect the rest of the pack, then that might be a bad idea.

Another consideration: that cell is also the first to reach the top voltage when charging...(clearly due to its higher internal resistance). So, it is not only limiting my discharge, but also my charge. 

Today, when it is significantly warmer out (say 60F), the cell was behaving fine at 50% SOC.

-corbin


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## tomofreno (Mar 3, 2009)

I don't think it will harm the rest of the pack, but I've never reversed one so don't know if there is danger of fire...Pete (one green ev) might know. Myself, I wouldn't take the risk.


corbin said:


> Today, when it is significantly warmer out (say 60F), the cell was behaving fine at 50% SOC.-corbin


 That is interesting, and makes me think even more that you should get it out of there. I would bypass it for now, adjust your charger limit voltage, and run with an n-1 pack until you get a replacement. If you get a sudden cold spell it will greatly limit your range.

Edit: I suppose you could buddy-pair two 100Ah if you have the space for them.


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## corbin (Apr 6, 2010)

tomofreno said:


> I don't think it will harm the rest of the pack, but I've never reversed one so don't know if there is danger of fire...Pete (one green ev) might know. Myself, I wouldn't take the risk.
> That is interesting, and makes me think even more that you should get it out of there. I would bypass it for now, adjust your charger limit voltage, and run with an n-1 pack until you get a replacement. If you get a sudden cold spell it will greatly limit your range.


Thanks; I think this is good advice. For now, I might choose to ignore it, and experiment with the cell to see exactly what is happening with it. But I'll start looking for a replacement.

-corbin


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## otedawg (Apr 23, 2012)

Corbin,
I reversed a CALB 100AH, and it was perfectly fine - not even bulging. 

I stupidly hooked up my charger one cell short, and now I've got a 51 cell pack instead of 52... Anybody know how I can bring a 0 internal resistance cell back to life? 

Thanks,
Otedawg


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## onegreenev (May 18, 2012)

corbin said:


> Thanks; I think this is good advice. For now, I might choose to ignore it, and experiment with the cell to see exactly what is happening with it. But I'll start looking for a replacement.
> 
> -corbin


Corbin,

At about 2.8 static voltage is pretty much empty. It is pretty much the same with all the LiFePO4 cells. 

http://www.diyelectriccar.com/forums/showthread.php/fast-charging-80814.html

As for your cell that is showing low well before the others, get it out. If it goes reversal it can take a few cells with it. It will get scorching hot if it goes reversal. 

I would pretty much reconsider bottom balancing your cells and ditching the BMS. Don't experiment. There has been enough of that already to know what the outcome will be without doing it yourself. I lost two cells because of running low two cells in my pack. What was interesting is that the car still ran but a bit more sluggish. I was really unaware of a HUGE issue happening. If you could have seen the the cells pop when I released the pack clamp you would have been amazed. 

You can run a cell below the static minimum as long as the cell bounces back to above the minimum. If you are at 2.8 volts static. Like if you sit for a couple minutes then you are pretty much empty and its time to charge up your cells. I have taken my cells well below that 2.8 volts under heavy load and that sag just popped right back up. But if your cells don't pop much past 3 volts then your very close to being empty. All the graphs I have seen pretty much show the same. At the knee on the discharge the voltage drops off like a rock dropped off a cliff into the lake. That fast. You can see in the graph shown, the discharge one that is linked above, the time from when the cell reached 3 volts to 2.5 volts is really a very short time. Running a car that would be more like seconds under heavy amperage and if you have a cell that is done before the others you WILL loose that cell. 

Pete


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## onegreenev (May 18, 2012)

otedawg said:


> Corbin,
> I reversed a CALB 100AH, and it was perfectly fine - not even bulging.
> 
> I stupidly hooked up my charger one cell short, and now I've got a 51 cell pack instead of 52... Anybody know how I can bring a 0 internal resistance cell back to life?
> ...


Then you did not reverse the cell. If you reverse one of these you WILL loose it.


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## corbin (Apr 6, 2010)

onegreenev said:


> Corbin,
> 
> At about 2.8 static voltage is pretty much empty. It is pretty much the same with all the LiFePO4 cells.
> 
> ...


Cool Pete,
that's great info. I'm still not a fan of bottom balancing -- it wouldn't help me in any way in this situation, and the BMS has been doing its job fully: it has told me when the cell is starting to die, and I can replace it.

I'll start the search for a new one, and carefully monitor the existing cells.

-corbin


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## corbin (Apr 6, 2010)

I should also add, the cell has always hopped back up to > 3.0 volts; it never seems to sit at 2.8 (or lower) for long (like, less than a minute or so for it to regain voltage).

corbin


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## mora (Nov 11, 2009)

I've reversed a single Winston cell. Drove the pack really empty and them some more. There was barely enought power to move the car in the end and after popping the hood and measuring cells one cell gave negative voltage reading. I put 50-100A through it for a minute or two. It was not even warm. That cell came back to life and isn't even the weakest one in my current pack. It might be the weakest after some time, but its been 6 months with the same pack and no problems.

Yes, BMS was screaming but I went ahead and drove some more. That sudden stop comes really quickly after cells sag to 2.8V at 1C. In my case there is about one mile left to drive at maximum.


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## corbin (Apr 6, 2010)

mora said:


> I've reversed a single Winston cell. Drove the pack really empty and them some more. There was barely enought power to move the car in the end and after popping the hood and measuring cells one cell gave negative voltage reading. I put 50-100A through it for a minute or two. It was not even warm. That cell came back to life and isn't even the weakest one in my current pack. It might be the weakest after some time, but its been 6 months with the same pack and no problems.
> 
> Yes, BMS was screaming but I went ahead and drove some more. That sudden stop comes really quickly after cells sag to 2.8V at 1C. In my case there is about one mile left to drive at maximum.


Mora -- that's interesting! How many cells do you have, and what is the rating (ah)?

corbin


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## otedawg (Apr 23, 2012)

onegreenev said:


> Then you did not reverse the cell. If you reverse one of these you WILL loose it.


Thanks - I'll keep that in mind.


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## tomofreno (Mar 3, 2009)

corbin said:


> ...I'll start the search for a new one, and carefully monitor the existing cells.-corbin


 Don't you have a long drive to work? Seems it could be a problem if it starts diving in voltage halfway there. I bypassed one of my 36 cells shortly after converting my car, and couldn't tell much difference in range or performance driving with 35. I think you have 48, so that's a only a bit over 2% loss in range and voltage if you remove one. Just have to make up a short cable...maybe you just want a little excitement.


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## kennybobby (Aug 10, 2012)

Today i was able to discharge 12 Bestgo 100Ahr cells for bottom balancing. They are brand new and came fully charged, OCV was 3.355. Took them 6 at a time down to 2.5 volts cut-off at 60 amps (so 1 minute = 1 Ahr). The time to cut-off ranged from about 94 to 97 minutes, and the cells came back up to about 3.0 volts within 10-20 minutes. 

Then they were loaded at 10 amps (so 6 minutes = 1Ahr) till they hit the 2.5 cut-off again. Now the time to cut-off was only 13 to 21 minutes (pulling an additional 2 to 3.5 Ahr), so the cells have given up their "100Ahr" capacity and are empty, 0% SOC. The float back up was very slow after this second round but i'm expecting that they will be at about 2.8 by tomorrow morning.

Once all 44 cells have been through this procedure then we will wire them up in series and charge the entire pack for a nominal 144 volts. Will post some graphs later.


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## mora (Nov 11, 2009)

corbin said:


> Mora -- that's interesting! How many cells do you have, and what is the rating (ah)?


45 cells in series, 90Ah cells.


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## corbin (Apr 6, 2010)

tomofreno said:


> Don't you have a long drive to work? Seems it could be a problem if it starts diving in voltage halfway there. I bypassed one of my 36 cells shortly after converting my car, and couldn't tell much difference in range or performance driving with 35. I think you have 48, so that's a only a bit over 2% loss in range and voltage if you remove one. Just have to make up a short cable...maybe you just want a little excitement.


I have a minimum 20 mile commute each way, but the kicker is a large hill on the way home. I usually charge just at work...so I usually drive 40 miles a day (sometimes 50 or so with errands and a trip to the gym).

I do have 48 cells...and yeah, dropping one would not be the end of the world. I'll do that once it gets worse.

--corbin


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## corbin (Apr 6, 2010)

kennybobby said:


> Today i was able to discharge 12 Bestgo 100Ahr cells for bottom balancing. They are brand new and came fully charged, OCV was 3.355. Took them 6 at a time down to 2.5 volts cut-off at 60 amps (so 1 minute = 1 Ahr). The time to cut-off ranged from about 94 to 97 minutes, and the cells came back up to about 3.0 volts within 10-20 minutes.
> 
> Then they were loaded at 10 amps (so 6 minutes = 1Ahr) till they hit the 2.5 cut-off again. Now the time to cut-off was only 13 to 21 minutes (pulling an additional 2 to 3.5 Ahr), so the cells have given up their "100Ahr" capacity and are empty, 0% SOC. The float back up was very slow after this second round but i'm expecting that they will be at about 2.8 by tomorrow morning.
> 
> Once all 44 cells have been through this procedure then we will wire them up in series and charge the entire pack for a nominal 144 volts. Will post some graphs later.



Kenny -- that is great information, and the type of stuff I was interested to hear. Did they come back up to 2.8 volts and sit around that, or 3.0? 

3ahr in a 48 cell pack would give: 3ahr*3.0v*48= 432watt/hr -- definitely enough to drive a mile in a typical car (mine gets 280-300watt/hr mile)...but, it would obviously be at a higher discharge rate, so one probably couldn't pull that many watt/hrs out at a higher rate.

corbin

corbin


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## Elithion (Oct 6, 2009)

corbin said:


> once the cells hit 2.8 volts, even under normal discharge (say 1C)
> 
> 
> Or, once the cell is always sitting at 2.8 volts?


The latter.

The former is the terminal voltage. The latter is the Open Circuit Voltage (OCV). 

OCV is a good indicator of SOC. Terminal voltage isn't.


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## corbin (Apr 6, 2010)

Elithion said:


> The latter.
> 
> The former is the terminal voltage. The latter is the Open Circuit Voltage (OCV).
> 
> OCV is a good indicator of SOC. Terminal voltage isn't.


Thanks Davide!

Also, I'm working on an Elithion BMS display. I'll send you details once I get it working to my satisfaction. 

-corbin


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## GizmoEV (Nov 28, 2009)

*Re: How are you Balancing, at the top or bottom?*



corbin said:


> Personally, I think it is worse if you bottom balance. You have a higher chance of destroying all your cells at the same time, as opposed to killing just one or two.


Actually, if they all hit 0%SOC at the same time you will not be able to drive your car any further and no cell will get reversed. This has been demonstrated on more than one occasion. It is on the top end you want to make sure your charger doesn't over charge the lowest capacity cell.

EDIT: I was also going to add that the high IR cell may not be getting charged fully if the BMS is either stopping charge or bleeding off current from the cell. Since you are top balancing your pack charge it up and then charge just that one cell slowly until it is full. It might act quite differently on your drive.


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## corbin (Apr 6, 2010)

*Re: How are you Balancing, at the top or bottom?*



GizmoEV said:


> Actually, if they all hit 0%SOC at the same time you will not be able to drive your car any further and no cell will get reversed. This has been demonstrated on more than one occasion. It is on the top end you want to make sure your charger doesn't over charge the lowest capacity cell.


Hi Gizmo,
How? 
And what controller are you referring to?

Unfortunately, the older Netgain controller I'm using doesn't have a "minimum voltage" where it goes into "limp home" mode (or at least, I'm not aware of this).

If I hit 2.8v on each cell, I'm at: 2.8v*48cells=134v -- which is still enough voltage to make the car move. Unless every cell is incapable of providing any current, or provides so little current that you really notice. Maybe that is the case that happens when bottom balanced? I'm not sure (and don't want to test that theory, because if you draw more current you probably kill cells).

corbin


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## GizmoEV (Nov 28, 2009)

*Re: How are you Balancing, at the top or bottom?*



corbin said:


> If I hit 2.8v on each cell, I'm at: 2.8v*48cells=134v -- which is still enough voltage to make the car move. Unless every cell is incapable of providing any current, or provides so little current that you really notice. Maybe that is the case that happens when bottom balanced? I'm not sure (and don't want to test that theory, because if you draw more current you probably kill cells).


I'm surprised that your controller does not have a low voltage cutoff. Even then, you will never be able to get your pack to 0V open circuit with your controller by driving. When the cells get to the 2.0V range and a tiny load is applied the terminal voltage drops to nearly 0V so you won't be able to drive. I know Jack Rickard said that something went wrong with the heating system in his Escalade and it took his whole pack down to around 1.6V per cell. Since his pack was bottom balanced it came back just fine. Every cell went low at the same time. A top balanced pack would have reversed at least one cell. Furthermore, a BMS would not have been able to stop what ever went wrong with his heating system.

BTW, If you get a cell that low be sure to charge it slowly until it gets above 2-2.5V or so. I'm not sure what happens if you charge really fast with a cell that low but I've seen hints that it is good to be gentle on them at first.


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## corbin (Apr 6, 2010)

*Re: How are you Balancing, at the top or bottom?*



GizmoEV said:


> I'm surprised that your controller does not have a low voltage cutoff. Even then, you will never be able to get your pack to 0V open circuit with your controller by driving. When the cells get to the 2.0V range and a tiny load is applied the terminal voltage drops to nearly 0V so you won't be able to drive. I know Jack Rickard said that something went wrong with the heating system in his Escalade and it took his whole pack down to around 1.6V per cell. Since his pack was bottom balanced it came back just fine. Every cell went low at the same time. A top balanced pack would have reversed at least one cell. Furthermore, a BMS would not have been able to stop what ever went wrong with his heating system.
> 
> BTW, If you get a cell that low be sure to charge it slowly until it gets above 2-2.5V or so. I'm not sure what happens if you charge really fast with a cell that low but I've seen hints that it is good to be gentle on them at first.


Thanks David -- that's interesting info to learn. Of course...the 2.0v range seems like it is killing cells at that point, but I could be wrong!

--corbin


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## tomofreno (Mar 3, 2009)

*Re: How are you Balancing, at the top or bottom?*



GizmoEV said:


> ...A top balanced pack would have reversed at least one cell...


 If it did not have a bms. If it did, he would have gotten an LVC alarm when any cell hit the low threshold voltage. On my car that sounds a buzzer by the steering wheel, cuts the throttle by half, and turns on the "check engine" light. Also, why didn't his Ah gauge show his pack was low on charge? Didn't he wire the heater through the shunt? Or did he just not look?


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## GizmoEV (Nov 28, 2009)

*Re: How are you Balancing, at the top or bottom?*



corbin said:


> Thanks David -- that's interesting info to learn. Of course...the 2.0v range seems like it is killing cells at that point, but I could be wrong!


I'm sure it isn't good for them! I remember Bill Dube stating that an A123 cell isn't damaged by 0V but I don't remember which cell he was referring to. I still don't like the idea of running them that low. I stay away from both ends of the spectrum with my pack.



tomofreno said:


> If it did not have a bms. If it did, he would have gotten an LVC alarm when any cell hit the low threshold voltage. On my car that sounds a buzzer by the steering wheel, cuts the throttle by half, and turns on the "check engine" light. Also, why didn't his Ah gauge show his pack was low on charge? Didn't he wire the heater through the shunt? Or did he just not look?


He was asleep in bed. He wasn't around to hear or see anything. He discovered it when he went out in the morning. Apparently the heater stuck on rather than cycling to keep the pack warm. It was at least his third demonstration, obviously unintentional, of the benefit of bottom balancing a LiFePO4 pack.


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## kennybobby (Aug 10, 2012)

Howdy Corbin,

The cells from yesterday's testing were at 2.8 OCV this morning. 

There was one set of 6 from several days ago that only had the initial 60 amp draw down to 2.5 cut-off, and they were up to 3.1 OCV. i hooked that set of 6 up for a 30 amp draw which only took 6 minutes (pulling out an additional 3 Ahr).


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## tomofreno (Mar 3, 2009)

*Re: How are you Balancing, at the top or bottom?*



GizmoEV said:


> He was asleep in bed. He wasn't around to hear or see anything. He discovered it when he went out in the morning. Apparently the heater stuck on rather than cycling to keep the pack warm. It was at least his third demonstration, obviously unintentional, of the benefit of bottom balancing a LiFePO4 pack.


Ahh, that would do it. Could also overheat the cells - why it is nice to have an overtemp switch on a cell in each box with the AC input to the heater wired through them.


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## Rastusmalinus (Sep 26, 2011)

I recently purchased 3 new replacement cells for my pack. After I checked the voltage, installed, and charged them with the pack, the voltage wouldn't reach the level of the other cells. Also, they sagged worse under load and discharged faster than the rest of the pack. After three cycles with the same results, I wrote the company a nasty letter basically accusing them of sending me used cells. The guy called me and said to charge them individually. I put together a single cell charger and did as he said. Those cells now charge and discharge just like the other cells.

This is a very long story to say that maybe your cell is out of balance with the others and should be charged individually up to 3.6V or so.


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## GizmoEV (Nov 28, 2009)

*Re: How are you Balancing, at the top or bottom?*



tomofreno said:


> Ahh, that would do it. Could also overheat the cells - why it is nice to have an overtemp switch on a cell in each box with the AC input to the heater wired through them.


Or in his case, a low voltage cutoff to the heater system which is running off of pack voltage. This could be easily done with one of the nifty volt meters he has which can control a relay or contactor.


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## corbin (Apr 6, 2010)

Rastusmalinus said:


> I recently purchased 3 new replacement cells for my pack. After I checked the voltage, installed, and charged them with the pack, the voltage wouldn't reach the level of the other cells. Also, they sagged worse under load and discharged faster than the rest of the pack. After three cycles with the same results, I wrote the company a nasty letter basically accusing them of sending me used cells. The guy called me and said to charge them individually. I put together a single cell charger and did as he said. Those cells now charge and discharge just like the other cells.
> 
> This is a very long story to say that maybe your cell is out of balance with the others and should be charged individually up to 3.6V or so.


That's an interesting thing to look for; However, the BMS is telling me the low cell reaches the high voltage first...so I think it is getting fully charged. It sounds like yours wasn't getting to the high voltage ever due to the charger shutting off early.

corbin


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## stickytechnology (Sep 19, 2010)

corbin said:


> That's an interesting thing to look for; However, the BMS is telling me the low cell reaches the high voltage first...so I think it is getting fully charged. It sounds like yours wasn't getting to the high voltage ever due to the charger shutting off early.
> 
> corbin


I wonder if you might have a terminal contact issue. Increased internal resistance could cause both symptoms (highest cell under charge, and lowest cell under discharge), and terminal resistance is part of that.


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## corbin (Apr 6, 2010)

stickytechnology said:


> I wonder if you might have a terminal contact issue. Increased internal resistance could cause both symptoms (highest cell under charge, and lowest cell under discharge), and terminal resistance is part of that.


Good idea! I took a look at the terminal, and the bolt was tight. I didn't take off the battery bar...but I might have to do that soon.


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## corbin (Apr 6, 2010)

Here's a video of my bad cell under load:

http://youtu.be/T_ldO62OoWs

It is also showing off my new display (see corbinstreehouse.com or the other thread I started on it). 

At 77% SOC, under hard acceleration (maybe 300-400 amps, which is at most 2c for 200Ah cells), cell #38 is sagging to 2.57 volts. I'll post a screen shot from the video showing this soon in the other thread.

corbin


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