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Discussion Starter #1
Hi,

I am SID. I am having issue with one cell of my LIFEPO4 battery set of WINSTON make model no: WB-LYP200AHA lifeypo4 (3.3V/200 AH)

Please find the snapshot of BMS for a 40% discharged battery set. Issue with cell no 1.

As you see other cells seems ok.

Is there a way I can revive or do something to get the capacity back for the bad cell ?

Thanks in advance.

Regards
SID
 

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Discussion Starter #2
Hi,

Please ignore other parameters on BMS since I have not connected anything else then battery voltage on BMS.

Regards
SID
 

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Hi
I have moved your thread to the right place

You need to equalise all of your cells

You can "top balance" or "bottom balance"

Basically you either fully charge all of them - right up above the normal full charge - and then connect them together in parallel to balance

Or you discharge them down to the bottom charge and do the same

Most BMS's expect you to Top Balance

Then see what happens - if that was the problem then you will have fixed it

BUT if the problem persists then you have a bad cell and need to replace it
 

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It looks like you have 2 "bad" cells, both 1 and 2 are way out of range with the rest, or there is an instrumentation issue affecting those 2 measurements. Maybe the BMS is drawing it's own control power from those bottom 2 cells and caused an imbalance.

If you can access the cells and have an adjustable bench power supply, then you could try just adding some charge to those cells to bring them in line with the others, and see if they behave. Otherwise might have to dismantle and test each cell for capacity and condition.

Good luck let us know what you find.
 

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Discussion Starter #5
@ Duncan,

Thanks for moving the thread to the right place.

You mean I remove all the cells from the Car and charge , discharge them all , then connect them all in parallel (all 16) to balance them all and then install them back , right ?

The snapshot I have attached is the external BMS but that just shows the battery voltage , I have not connected any charge or load anything. The BMS is already there in the car and attached with the controller.

Is there a way to do it without removing the batteries from the car ?

I have attached wires to each cell all the way to BMS which can take around 10 Amps and fuse protection.

Regards
SID
 

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Discussion Starter #6
@ Kennybobby,

Yes I was also suspecting the same about cell no. 1 and two both.

The instruments issue is not related , please ignore it. I have not connected any charge , load circuit with the same. Those are random numbers. I have just tapped each battery to monitor each voltage of the battery.

The car already has everything including BMS but was not allowing me to monitor each cell and was quitting at 40% of the battery , so I installed this BMS , actually it's just a cell monitor.

Yes , I have access of each cell. I will add some charge and bring them inline with the voltage of others and see.

I hope and wish this will resolve the issue , or else removing all cells under the front seat is a big job.

Luckily when I removed front seats sometime back , I had tapped wires from each cell.

These cells are 200 AH , and the wires I have attached can take 10 AMPS max. Perhaps discharging those two cells will be an issue since I cant draw heavy current.

Will charge at 5 AMPS first and see if I can increase and level both the cells.

Will keep you posted.

Thanks n regards
SID
 

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Hi
With "Top Balance" - you fully charge them all

You don't need to take them out of the car as long as you can safely get to their terminals

You can do this with thin wires and a small power supply - you just need patience - lots of it!

The final balancing when all cells are up there is only very low current -

No matter what you do you will NEED to balance your cells - some BMS's do have a balancing feature but that is only a "fine tuning" balance you need to do the main balance yourself
 

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Discussion Starter #8
@ Duncan,

Thanks a lot for your inputs.

SO I am following below steps , please correct me if I am wrong.

Once my Cars says no to move at the voltage levels snapshot attached here. See cell no. 1 & 2. (They are 2.944 and 3.198). (My EV stops at the snapshot level after moving around 45 Kms)

I also get cell no. 1 and 2 to the same level as others , right ? One stupid question , up to what decimal I balance ? two ?

Once I charge the cell no. 1 and 2 to match the rest of the others , I do normal charge using the given car charger . Right ?

I guess I will have to do this several times ?

I will be using a standard variable smps with adjustable voltage and current levels (max up to 5 amps) and set the voltage as other cells. The cell will stop absorbing the current once it reached to the set voltage.

I have an ammeter in SMPS which shows the currently consumed current.

Regards
SID
 

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Here's what i would do: Hook up to cell #1 and set voltage to charge at 3.2, then after the current slows down under 1 Amp, add in series cell #2 and set the smps to 6.6 until current slows down again.

You could also do this for some of the other "low" pairs, e.g. 5&6 are about 30 mV lower than the rest. It looks like a good target since most of your cells are about 3.29 +/- 0.01

This will bring them up into a range much closer to the others and you can then charge up the entire pack with your EVSE.

Does your BMS control the charging also--can it shut off the charger if a cell voltage goes above some upper limit?
 

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Discussion Starter #10
@ Kennybobby,

Thanks a lot. I will do that.

Answer your question below(Does your BMS control the charging also--can it shut off the charger if a cell voltage goes above some upper limit? )

I use the charger supplied by the EV manufacturer , it has built in BMS. Hopefully it does control the upper charging voltage. The one in the picture is which I have installed just to monitor the cell voltages. Not the entire BMS.

The BMS does stop the car immediately once the cell 1 voltage goes below 3 Volts , so I am assuming it must not be allowing the cell to go above upper limit.

By the way what should be the upper limit of the cell ?

A fully charged battery in my monitor BMS shows 3.35V

Regards
SID
 

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Discussion Starter #11
Hi Everyone,

Finally I concluded that the cell is Bad. ( I did charge the cell externally full and did a drive test. The same cell was dipping in voltage again)

I have a new cell now with me.

Can anyone let me know the proper process to replace the cell ? The total bank is of 16 cells and I want to replace first cell which is giving trouble.

The main thing which I want to know is should I balance the cell to the other cells before replacing or after ?

Thanks.

Regards
SID
 

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Hi SID,

What I would do is put the cell into the pack and charge the entire pack,
At the same time I would be present for the whole process with a multimeter (volts) on the new cell and another multimeter on one of the better cells.
You will see what is happening as the voltages start rising.
If the new cell reaches around 3.5Volts (top voltage for LiFePO4 cells is 3.65V) before the others, terminate the charge and then drain the new cell with a car headlight (or something similar) to discharge it a bit, say for 10 minutes. After this, disconnect the car headlight, and put the whole pack back on charge, monitoring as before. Repeat this until all cells reach 3.5V together, or close to it. Then try the above for the top voltage of 3.65V. Once they are all close, a few more discharge/charge cycles will sort them out.

If the new cell doesn't reach the top voltage of 3.65V before the originals, it will have to be charged up individually to 3.65Volts then placed back into the pack.

Don't leave the process at any stage - monitoring is very important if you want to keep your cells healthy.

Eric
http://www.evric.kestar.com.au
 

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for LiFePO4 cells, most chargers are set to shoot for an average volts-per-cell of 3.65 at 'end-of-charge', which settles down to about 3.35 after the charger disconnects.

if your goal is to top balance, you can use your pack charger to get 'close' to end of charge, where the voltage begins rising the last few volts, and then:
- disconnect the pack charger
- use a variable power supply to bring EACH cell up to 3.65v, and leave it on until the charge current drops to below 1 amp indicating the cell is 'full'.

you can set the voltage and max current on a variable power supply to prevent overcurrent charging thru your monitor wires, or clamp direct to battery terminals if you have access.
 

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Discussion Starter #15
Thanks a lot Dan.

I will do that.

However the battery pack is getting settled now.

After replacing the cell I am getting around 70 Kms range after around 7-8 cycles. I am hoping that I will reach to peak which is 100 Kms.

In the meanwhile I will do as you have suggested.

Regards
SID
 

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With C = 200 Ah cells, the Charge Procedure would consider C/20= 10 as the cutoff point for charging, that is to stop charging when the current drops to 10 Amps during the CV portion. This is based upon their testing of cells to charge them quickly and (hopefully) not do damage.

The full voltage is about 3.33x. To hold it at 3.65 until it the current drops to 1A will be overcharging the cell and violates the Procedure.

Unless you need every last Coulomb of charge that might fit in the cell, it would be safer to charge to a lower voltage. For example you could put a charger at 3.329 VPC on the pack and let it go till the current stops--this would mostly fill the pack and not risk damage from overcharging.
 

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Discussion Starter #17
Dear Kennybobby,

Thanks for your insight technical info.

I will definitely follow the same.

Thanks a lot .

Regards
SID
 

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With C = 200 Ah cells, the Charge Procedure would consider C/20= 10 as the cutoff point for charging, that is to stop charging when the current drops to 10 Amps during the CV portion. This is based upon their testing of cells to charge them quickly and (hopefully) not do damage.

The full voltage is about 3.33x. To hold it at 3.65 until it the current drops to 1A will be overcharging the cell and violates the Procedure.

Unless you need every last Coulomb of charge that might fit in the cell, it would be safer to charge to a lower voltage. For example you could put a charger at 3.329 VPC on the pack and let it go till the current stops--this would mostly fill the pack and not risk damage from overcharging.

I am not suggesting to hold the end voltage until c/100 with every charge, just for the INITIAL manual charge, when you are starting with a pack of cells that might be at different states of charge if you are intending to top balance. This helps make sure that all cells are 'full' to the same level at first, so that whatever BMS you use, if you use one, won't have to deal with any cells hitting the final voltage still under CA current which is probably more than the BMS shunts can handle.
 
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