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JK BMS Cell Count Problem on B2A24S20P

upnorthandpersonal said:
There are two blown resistors (one of them shown in the picture) that correspond to the non-detected cells. These are 0.01R resistors on the input of the balance leads, before the mosfets. I think these are used here as a fuse, so it did it's job
Click to expand...

Yes, its' failure mode is to short circuit the gate to the drain, so it's prudent to have some sort of 'fuse' here. Most car audio amps have the exact same protection.

Interested to hear your outcome with this, most likely a failed mosfet. I have unit on the bench to repair as well.
 
sdj said:
Yes, its' failure mode is to short circuit the gate to the drain, so it's prudent to have some sort of 'fuse' here. Most car audio amps have the exact same protection.

Interested to hear your outcome with this, most likely a failed mosfet. I have unit on the bench to repair as well.
Click to expand...

Mosfets are the first thing I tested and they're all good.
 
Sorry, I have not had time to test. The resistors are replaced, but I need to hook it up to one of my packs to test. I should get to that after next week when things should be less hectic here.
 
dakamalu said:
How are things going?
Click to expand...

So, changing the resistors fixed it. I've not run extensive tests because I had the PCB out of the enclosure (less cooling) but it powers the load and balances cells. All 16 cells are accounted for in the app.
I did notice in the app that balancing start voltage was set to 3.0V which is very low. Not that this should really be an issue, but maybe it was just balancing all the time and this caused those resistors to give in.
 
Thanks so much to upnorthandpersonal for his help. I could hardly express how thankful I am to him.

But as many have pointed out, there must be something causing the problems in my 16S configuration with this JK BMS. Cell number 9 was always involved. And believe it or not, a week or so before I got the repaired BMS back from Finland, the one I was using failed. Three of the balancing lead resistors went bad and once again, cell number 9 was involved. I have also noticed that cell 9 is the one that seems to always be taking current when balancing is on.

Can I replace that single cell?
 
dakamalu said:
Can I replace that single cell?
Click to expand...

Yes - you can. Did you set the balancing start voltage to something higher than 3V (i.e., 3.45V or something)? It's not normal for those resistors to fail like that (they act as fuses, but still). Anything special about cell 9? What is its rest voltage? Do you have a multimeter and a power supply?
 
upnorthandpersonal said:
Yes - you can. Did you set the balancing start voltage to something higher than 3V (i.e., 3.45V or something)? It's not normal for those resistors to fail like that (they act as fuses, but still). Anything special about cell 9? What is its rest voltage? Do you have a multimeter and a power supply?
Click to expand...
I set it to 3.45V. I had it there originally, but things were failing so I tried a lower balance. It didn't help, so we are back to 3.45V. We'll see. I may have to improve my soldering skills. I have many multimeters. Power supply went bad, but at the moment I am charging the battery using the BMS you returned and it looks good for now.
 
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I have another thought. The 16 cells are connected in two groups of 8 each. Within each bank of 8, they are connected by bus bars, but the two banks are connected by a cable. Would that make a difference because cell 9 is the first one in the second bank and cell 8 has also gone bad twice. Cell 10 has also gone bad. No cells on the far ends have had problems. Not sure why that would affect the balance current, but who knows?
 
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That should not make a difference but the fact that it has to keep balancing sounds like it is burning up the balance resistor.
You would like to think the BMS would protect itself from situations like that but well...
 
Quattrohead said:
That should not make a difference but the fact that it has to keep balancing sounds like it is burning up the balance resistor.
You would like to think the BMS would protect itself from situations like that but well...
Click to expand...
That's what I thought. It is burning up the balance resistors even if I lower the balance current. Cell 9 is always involved. The other cells are close by. I have been through 3 JK BMS units and they all have the same issue so perhaps the arrangement is a problem or cell 9 itself is defective.

Hypothetically speaking, if cell 9 has internal problems that cause it to draw too much current before the BMS can respond it could cause problems. I have no way of testing that but it is an idea. If for example cell 8 was providing balance current to cell 9 when cell 9 had its issue, then both cells balance resistors would be fried if it happened too quickly for the BMS to respond.

The problem with that idea is that it should happen as soon as I start it up unless there are some dynamics within the cell that shift over time.
 
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fatdaddy said:
it might help to install some. if there is some sort of cell surge on the balance lead it would help protect the BMS if thats the issue? also might help with diagnostics if you are blowing fuses on that suspect cell 9?

Click to expand...
I may have to consider fuses if I cannot figure out why the balance resistors are getting fried.
 
dakamalu said:
I may have to consider fuses if I cannot figure out why the balance resistors are getting fried.
Click to expand...
Be careful about fuses. They can be a source of a new problem of intermittent interruption.
The the mini in-line fuse holder(pictured) works well…. for a while then un explained interruptions to my Solar Assistant from my new buck converter.
What happened is some sort of corrosion setup between the nickel plate for the fuse end and the brass contacts. I cleaned the junctions and used dielectric grease and they have been fine for over a year. A better way and much more secure is to solder and heat shrink the fuse.
IMG_1096.jpegIMG_1097.jpegIMG_1016.jpegIMG_1019.jpegIMG_1020.jpeg
 
Greetings all. Long-time listener, infrequent caller.
I have just received and installed a B2A24S20P for my 16s LiFePO4 setup. I also have a missing cell (and the alert that says cell count doesnt match setting). I redid the crimps on the wire on that cell and it did not resolve. The curious thing though is that as per the wiring instructions B+ gets its power from cell 16pos and my total pack voltage shows up as 50.3v which is the voltage of 15 cells, not 16.

Sent an email to support so... ?

Cell16_voltage.jpgCell15_voltage.jpgScreenshot_BMS.jpg

12x250 Watt panels
16 cell/ 48v
B2A24S20P BMS
Growatt 48V SPF 3000TL LVM
NYC and 25% off-grid :)
 
upnorthandpersonal said:
Total voltage is calculated as the sum of all cells, not the pack voltage directly.
Click to expand...
Thanks. I had a suspicion that might be the case because it might have also explained the slight discrepancy between the actual voltage shown on the multimeter at cell 15 and the BMS shown total voltage of the 15 visible cells.

I guess now I am back to just troubleshooting why cell 16 is not showing up. Note: I am currently not connected to any inverter/charger. I just want to make sure the BMS set up was solid before introducing any large currents.
 
Thanks @upnorthandpersonal & @Quattrohead. I will pull the connector and trouble shoot at that end, After 10+ years of marriage I tend to always just assume I am the problem, it would be great if this time the problem is actually on the other end,

Stay tuned.
 
pangrande said:
Thanks @upnorthandpersonal & @Quattrohead. I will pull the connector and trouble shoot at that end, After 10+ years of marriage I tend to always just assume I am the problem, it would be great if this time the problem is actually on the other end,

Stay tuned.
Click to expand...
Checked pins and connectivity through the connector. All tested fine. Still no response from support or the vendor I bought it from (18650batterystore.com) about the "missing" cell. I just gave up and pulled out the JK BMS and went to an older ANT Smart BMS. My plan is to leave both BMS's connected and turn off active balancing while I continue to troubleshoot.
 
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