# Low cell help



## evmetro (Apr 9, 2012)

I have a new pack of 45 100 ah cells, and one is giving me trouble. I top balanced in parallel all 45 a few months ago to 3.4 volts. Now I have the pack hooked up, the Orion picked out one cell that was reading 3.28 volts while all the others read 3.333. I brought that cell up with my power supply, but the next day it was back to 3.28. I did this again, and the next day it was at 3.28 again. No miles driven, swapped battery position and bus bars. The cell performs the same way when on the bench by itself. Thoughts?


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## Roy Von Rogers (Mar 21, 2009)

Not sure what type of electronics, but if it has shunts for each cell, swap that one with another and see if the problem follows.

Roy


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## evmetro (Apr 9, 2012)

Same symtoms with no electronics conneted, cell out of car and on bench.


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## mora (Nov 11, 2009)

My friend had a similar problem with his pack. Those were Winston/TS cells though. One single cell seemed to have lower capacity but still performed fine. No excessive sagging or warming. He pulled it out of his pack and I examined it a bit using Powerlab. It had 93Ah capacity when discharged down to 2.6V and charged to 4.0V. But during charge, at 3.6V, there was only 70Ah put back to the cell. I repeated this same test for 5 times and always got same results. I cleaned cell terminals after two cycles and tightened charging lugs to terminals. No difference.

I compared this cell to one of my similar Winston cells. Same excercise: discharge to 2.6V and charge to 4.0V. Cell had well over 90Ah capacity but at 3.6V there was ~90Ah put back to the cell.

I still have to measure its internal resistance using external load. Powerlab gives same results for both cells regarding internal resistance.

That faulty cell remained "undercharged" as BMS decided its full at 3.6V. Removing it from pack gave him about 10 miles more range.

Attached picture shows voltage from 2.6V to 4.0V for both cells. See the difference?


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## Frank (Dec 6, 2008)

I'd probably change it out for a new one, it should sit at the same voltage as the others.


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## onegreenev (May 18, 2012)

Take a known good cell and that odd ball one and do a capacity check. Take them both down to like 2.7 volts static then charge them up and see how many amp hours each takes. If the odd ball one gives you the same then there is no reason to remove it. If there is a large difference then it should be replaced. Do you have a spare to replace it with?


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## Zak650 (Sep 20, 2008)

I've done some bench testing of capacity balancing by adding small lifepo4 cells in parallel to cells of slightly less capacity. It seems to work so you might consider adding a headway or some other cell that is the right size wired in parallel to the problem cell to make up for it's the lack of capacity.


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## kennybobby (Aug 10, 2012)

*How many Coulombs...*

Mora and EVMetro: 

It seems to me when charging (or discharging) cells that measuring Voltage without Current and Time is nearly useless--what was the current reading while charging the cell? Holding a constant current while closely monitoring the actual time to the nearest minute will tell you exactly what was put into the cell, e.g. 5 amps for 12 minutes is exactly 1 A-hr. ( or 1 amp for 60 minutes, etc.)

Only by monitoring current and time can you expect to know the actual SOC of a cell. If you put 100 A-hr into a cell with that rating, then by definition it is at 100% SOC. And then you can compare whether the OCV after the resting period is in the expected voltage range. 

If for some reason the 'low' cell started the charging process from an unequal and lesser SOC than another, then it would likely read a lower OCV at the end and could still be a good cell.


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## BentMike (Oct 20, 2013)

This sort of a topic shift, but maybe a yes or no can cover it...

This is not a rhetorical question, I need more clarity on all things balancing.

Would not bottom balancing this pack, and paying little additional attention to the odd cell, work just fine? 

BentMike


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## Zak650 (Sep 20, 2008)

evmetro said:


> I have a new pack of 45 100 ah cells, and one is giving me trouble. I top balanced in parallel all 45 a few months ago to 3.4 volts. Now I have the pack hooked up, the Orion picked out one cell that was reading 3.28 volts while all the others read 3.333. I brought that cell up with my power supply, but the next day it was back to 3.28. I did this again, and the next day it was at 3.28 again. No miles driven, swapped battery position and bus bars. The cell performs the same way when on the bench by itself. Thoughts?


One item of note in this statement: 3.4 volts isn't really a good target voltage for these types of cells, it's too far away from the upper "knee" to be an end voltage target for top balancing. 3.4 volts at the end of charge doesn't equate with the cell being "full".


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## onegreenev (May 18, 2012)

Since your doing a top balance, try bringing up the voltage limit to like 3.55 volts and then let them rest and see what happens. It could be, like zak mentioned, that your end top voltage is just too low to actually bring the cells to a full state? I still say bottom balance but with an active balancing BMS you really can't do that. But you could bring up the high voltage to 3.55 which is still below full but high enough up the curve to give a better indication of good balance. 

Have you driven the car with it the way it is? You know, you could just take it for a short drive and then let the cells sit a bit then check the voltages. Is that one pretty much the same as the others? When talking a short drive, I am saying like a mile then let them rest. That is to burn off the top of the charge and then let them settle to near static. I'd almost bet that cell is just fine. 

Pete


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## Frank (Dec 6, 2008)

Good points about 3.4 being too low. I'm a fan of bottom balancing and no BMS but the Orion wants to top balance by default unless you disable it. If top balancing I'd use something higher as suggested than see where things end up.


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## mora (Nov 11, 2009)

*Re: How many Coulombs...*



kennybobby said:


> Mora and EVMetro:
> 
> It seems to me when charging (or discharging) cells that measuring Voltage without Current and Time is nearly useless--what was the current reading while charging the cell? Holding a constant current while closely monitoring the actual time to the nearest minute will tell you exactly what was put into the cell, e.g. 5 amps for 12 minutes is exactly 1 A-hr. ( or 1 amp for 60 minutes, etc.)
> 
> Only by monitoring current and time can you expect to know the actual SOC of a cell. If you put 100 A-hr into a cell with that rating, then by definition it is at 100% SOC. And then you can compare whether the OCV after the resting period is in the expected voltage range.


I rely on Powerlab reading current and voltage correctly. It calculates Ah in and Ah out more precisely than I could do with stopwatch and current meter.

Both cells were charged at 30A until the flat part of the curve. Constant voltage after that. CV part on bad cell was way longer than on good cell.

Bad cell has 76Ah put back into it after 2 hours and 16 minutes. Same for better cell. The only difference here is cell voltage. Bad cell reached 3.6V at that time but better cell read 3.46V at the same time. When better cell reached 3.6V it had 89Ah put into it. Bad cell had 89Ah put back into it after floating 8 minutes at 4.0V (CV mode).

If you didn't monitor voltage but only time at constant current you would have taken bad cell way beyond 4.0V by the time it reached its rated capacity.

I'm saying in this case if BMS cuts charging when weakest cell reaches 3.6V it doesn't mean that cell is "full". All the other cells are undercharged when BMS cuts charging. Both of these tested cells have their rated capacity left. If bad cell was charged to 3.6V and then discharged down to 2.6V it only had less than 80Ah to give.

I'd replace that cell anyway.


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## evmetro (Apr 9, 2012)

Thanks for all the replies. I will try the higher balance voltage first.


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## Red Neck (Feb 1, 2013)

Top balance at 3.9v with a power supply. The current will be basically zero when reached. 3.4, 3.5, 3.6 and 3.7v are all too low.

You will not really top balance.

And as someone mentioned. Get a small cell (20 or 40Ah) and wire it in 
parallel to that one cell or exchange that cell with a new one..


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## EVfun (Mar 14, 2010)

Red Neck said:


> Top balance at 3.9v with a power supply. The current will be basically zero when reached. 3.4, 3.5, 3.6 and 3.7v are all too low.


Don't balance a LiFePO4 cell by charging to 3.9 volts and holding it at that until the current is nearly zero. That is overcharging, and not good for the cell. Save those voltages for other Lithium chemistries.


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## evmetro (Apr 9, 2012)

I top balanced at 3.5, and they all came back down to 3.334 within 30 minutes. Cell 5, the one in question, came back to 3.28. I hooked up my power supply and brought cell 5 up to 3.5, and then took a road test. Below is a pic of my dash after returning from the road test. That odometer reflects the length of my test drive, and I put a gauge there for cell 5 so that I could keep an eye on it during the drive.










Here are my 45 cells after the road test. I was only able to get a 35 mile range, and cell 5 brought me in. The SOC and DOD pids seemed to reflect cell 5's condition in the last 2 miles, and cell 5 voltage began a more rapid drop. 










You can see cell 5 compared to the others here. This pack is 14.4 kwh, so I should have 11.5kwh to use before reaching 20% DOD. When I recharged, I was only able to put 8.3 KWH back in, which sounds like the other 44 cells still had a bit more to offer. I contacted the manufacturer, (Tornado) and presented all of this info and my dash screen shots, and they agreed that the cell is defective. They stood behind their product and offered to send me another cell under warranty. I do not have the replacement cell yet, but I will report back when I get it. This gives me some time to do the paintwork on the car.


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## EVfun (Mar 14, 2010)

I'm not sure you are getting a full charge. It takes a day for my cells to drop back to 3.36 volts if I have fully charged. 

I would try fully charging the pack again, as your current system does it. Then, try charging #5 individually, until the voltage gets to about 3.60 volts and the current is down to about 5 amps. (if the voltage is a little higher the ending current should be higher, if it is lower the ending current should be lower)

The idea is to make sure the current charge cycle is really fully charging, and see if it is just a cell out of balance. If individual charging makes it line up with the rest better and it stays that way it was a balance issue. If it makes it better but then it goes back to being low in just a month you have a cell with a self discharge issue, and those cells seem to be on a path to failure. If it takes very little extra charge and then is lowest again on the next drive it is smaller in capacity in a strange way (low charged voltage is odd.)


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## evmetro (Apr 9, 2012)

Just wanted to update this thread on the situation. I have received my replacement cell from Tornado. They sent it from China via air, and it was delivered to my hands right at my shop. I did not pay for the cell or the shipping. All the cells are reading the correct numbers now, and the new cell is balanced to the pack @ 3.5 VDC. This warranty service was very important for me to see, since I plan to purchase more packs in the near future.


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## onegreenev (May 18, 2012)

That is excellent news. Its always good to see companies standing by their products to this extent. 

Pete


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