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An issue in balancing LifePO4 battery cells' top and bottom

Ok, are the plates copper and aluminum? Are the bolts short enough to achieve full clamping on the plates?
 
Having two different types of buss bar material can’t be good different resistance between the metals will most likely cause problems
 
Is cell #12 the lowest at a low state charge and the highest at at a high state of charge? If so you could have a bad terminal connection or it’s a bad/weak cell. If things are random, then it’s more likely an unbalanced condition exists. If the cells are in reasonable shape, 1amp balance current applied above 3.45 vpc with enough time at or above that voltage should do the job.
 
Ok, each terminal needs cleaning and antiox paste, also each of the aluminum plates need to be cleaned in an alcohol bath with flat sandpaper, and assembled INSTANTLY while still wet... aluminum is a pain to assemble without having oxide layers.
The copper plates also need cleaning prior to assembly, but antiox paste needs to be used with the terminals.
Off grid garage did a video on this mess a while back.
 
Ok, each terminal needs cleaning and antiox paste, also each of the aluminum plates need to be cleaned in an alcohol bath with flat sandpaper, and assembled INSTANTLY while still wet... aluminum is a pain to assemble without having oxide layers.
The copper plates also need cleaning prior to assembly, but antiox paste needs to be used with the terminals.
Off grid garage did a video on this mess a while back.

I’ve seen that video too. Aluminum does form a layer very quickly(minutes) with some alloys faster than others. The terminal tops, especially older ones have plenty of oxides. And need to be removed carefully. I made a lapping tool that only takes a few revolutions. The abrasive 300-400 grit has a thin double sided tape to keep it in place. The abrasive needs to be “unloaded” after each terminal with a clean paper towel or air. The terminals need to be cleaned before lapping will alcohol dampened paper towel. After lapping any dust can be wiped away with another alcohol dampened towel. Don’t clean all the terminals at once just a few to limit exposure time. When you apply an antioxidant, do so sparingly as to not get it on the threads because it could greatly increase the tension of the fastener due to thread lubrication. I now use marine “green grease” as a antioxidant. No metal(zinc) or carbon containing antioxidant. Because of what I’ve read; https://www.practical-sailor.com/boat-maintenance/conductive-greases-vs-corrosion
Lapping tool:1CE24E2D-AE4B-4770-9703-740F9CC52436.jpeg5E583165-A240-43B7-9A25-7039C09A79C7.png
 
It seems odd to me that the cell voltage pic shows cells 1-8 as ~3.2V and cells 9-16 as ~2.8V.

Is there any info on what was done with these cells (as groups of 8) or how/why the cells were placed in the order they are in now?
I think these cells are taken from 2 packs ( 2 modules - 8 cells from each. These cells are not brand new). I also suspect this cause the problem. Two days back cells V went above 4.2V. It also happened in first 8 cells. (I'm not sure why JK BMS failed to shoutdown). If so, How can resolve this problem?

1667673212853.png
 
Is cell #12 the lowest at a low state charge and the highest at at a high state of charge? If so you could have a bad terminal connection or it’s a bad/weak cell. If things are random, then it’s more likely an unbalanced condition exists. If the cells are in reasonable shape, 1amp balance current applied above 3.45 vpc with enough time at or above that voltage should do the job.
These cells are taken from 2 different modules I guess (8 cells from 1 module). This is random in the first 8 cells and the second.
 
Hi.

We are building a 3.2v 16 cells Lifepo4 pack, with JK BMS.
We did bottom balancing and top balancing of the pack.

But now some cells get over discharged and the cell volt difference is 0.5mv. (Also, observed that the cells gets over charged to 4v on the previous day).

In this case, 5 cells were at 2.7v and the remaining were at 3.24v.
View attachment 119153

Here is the setting of JK BMS (screenshot took bit later)
View attachment 119156

How do we set the parameters correctly to balance the pack at “top” and “bottom” both?

Your advice is highly appreciated.

Thanks,
Flash
About 12 hours back, I changed the start balance Volt to 3.3V and charged the pack through the Inverter. Then I saw that 1 cell (Cell #8) goes beyond 3.65V. JK BMS balance gave a red alert and reduced the Volt.
But since it happened several times for the same cell, I turned off the charging and keep the cells to balance the load by BMS.

After around 8h of balancing this is how it looks like. Only the Balalnce is ON.
I'm planing keep this untill all the cells get balance around 3.3V (Which is the balancing start Volt in the BMS) and then charge (10A through Inverter - grid power) till 3.65V (Not sure how many cells will reach to that level). Keep pack several hours ( or few days) untill get balance at around 3.65V.

What do yiu think about this apporach? Will this work? Any recomondations?

What do you
1667674096482.png
 
Two days back cells V went above 4.2V. It also happened in first 8 cells. (I'm not sure why JK BMS failed to shoutdown).

That shouldn't have happened, because in your bms settings, your cell OVP was set to 3.60v. Which should have disconnected to the bms.

Maybe we have *two* problems - a bms malfunctioning or wired incorrectly, and a badly unbalanced bank because it has been instructed to start balancing too low in an attempt at a faux-bottom balance. (doing a bottom balance means *manually* discharging the cells yourself - the bms won't do a bottom balance for you).

The only way to resolve this problem is to do a manual top-balance as linked to earlier I'm afraid. And possibly a bad bms (or cell wiring) to boot, since it allowed cells to go over the OVP protection limit.
 
Looks like it's working as designed.
It will take time to get it straightened back out.
If you slow the charging, it won't shut down as much.
But this is not normal, right? Only one cell at 3.65V and all the other cells are way below that volt. That's why I'm trying to balance at 3.30V before charging again targeting 3.65V for all cells. What settings should I need to change to get there?
 
But this is not normal, right? Only one cell at 3.65V and all the other cells are way below that volt. That's why I'm trying to balance at 3.30V before charging again targeting 3.65V for all cells. What settings should I need to change to get there?
Lower the amperage so that the balancing can keep up.
No, not normal. But you have balanced too low and gotten everything out of wack.
Just give the BMS time to correct it.
 
You need voltage delta in order to balance. You generally won't see this until the upper region of the charge curve. You are now in this region and the balancer is trying to do its job. Give it time and as you cycle the battery it should come into balance.
 
Two days back cells V went above 4.2V. It also happened in first 8 cells. (I'm not sure why JK BMS failed to shoutdown). If so, How can resolve this problem?
This does not make sense. Your cell OVP is 3.6V.

You really need to post a pic of how this is wired. This should never happen as you have the BMS set.
 
Somebody more familiar with the JK bms than I am, may also see if that toggle for "charge" needs to be activated, because not being an owner, it almost looks like balancing overall has been turned off.

Charge ON. Discharge OFF. Balance ON. 3.45v for balance start.

[UPDATE - Charge ON. Discharge ON. Balance ON. Thanks to RCinFLA in the next msg for that.]

I see these are getting toggled depending on which pic of the bms settings you look at. :)

The good thing is that at least the JK bms has 1A or so of balancing, but appears to need to be actually enabled during "charge".

Maybe an owner can help - I'm guessing here not being an owner.
 
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I would recommend leaving charge and discharge 'ON'.

Whenever you disable either, there is an override whenever passed current path exceeds a few amps to prevent overheating the body diode carrying the pass current. Override re-enables both series back-to-back MOSFET's like both paths are 'ON'. The BMS monitors current magnitude and direction to re-apply disable setting when passed current drops again to level that would not cause too much body diode heating. You can notice when the override happens as the voltage drop across BMS drops from about 0.5-0.8v to very low voltage drop.

For lower charging current, having discharge disabled can screw up chargers absorb and float voltages due to the extra diode drop between charger and battery. For lower inverter load current, having charge disabled can screw up inverter low voltage shutdown due to extra diode drop between battery and inverter.

The MOSFET enabling/disabling control is very slow switching so if the current varies and drops below the override current there is momentary current in the disabled path.
BMS charge-discharge Disable function.png
 
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