Problem with JK-BMS B2A24S20P, both showing identical problem with cell 9 on 2 separate batteries

[sunshine_eggo]

Right, I said it.
Right, @upnorthandpersonal said it was a bad idea so I dropped the idea like 25 posts ago.
Why do you keep bringing it up?

Because you did, and you haven't acknowledged there's a difference between this:

Unless I misunderstood him, mod said that's not a good idea. https://diysolarforum.com/threads/p...9-on-2-separate-batteries.100491/post-1366204

and what I proposed.

They appear to be completely different things.

And why do you keep repeating the word insane and applying it to various peoples' ideas here, each time with zero explanation?

Informal definition is shocking; outrageous, e.g., "Bro was making insane amounts of kWh!"

Maybe start by revisiting the definition of "insane." You're misusing the word.

The idea that you could swap the sense leads for cell #4 and #9 is "insane." You will be measuring the wrong things, and the BMS will shit its pants.

Thinking a 4mV voltage drop/bump under 50A of current is going to interfere significantly with balancing is "insane," ESPECIALLY if an active balancer is involved. The resistance of a charge/discharge circuit does not alter the mAh requirements needed for balance. It changes the Wh which simply alters efficiency, not total mAh needed to maintain balance.

What am I not explaining?

I get that I'm probably coming across as a dick. That is not my intent. I rarely mince words, and I hope you get that I don't take myself too seriously with responses like this:

 

[robbob2112]

Because you did, and you haven't acknowledged there's a difference between this:



and what I proposed.

They appear to be completely different things.



View attachment 282355

Informal definition is shocking; outrageous, e.g., "Bro was making insane amounts of kWh!"



The idea that you could swap the sense leads for cell #4 and #9 is "insane." You will be measuring the wrong things, and the BMS will shit its pants.

Thinking a 4mV voltage drop/bump under 50A of current is going to interfere significantly with balancing is "insane," ESPECIALLY if an active balancer is involved. The resistance of a charge/discharge circuit does not alter the mAh requirements needed for balance. It changes the Wh which simply alters efficiency, not total mAh needed to maintain balance.

What am I not explaining?

I get that I'm probably coming across as a dick. That is not my intent. I rarely mince words, and I hope you get that I don't take myself too seriously with responses like this:

View attachment 282356

You are right..and you are a dick.. but who let you out of your cage?!?!?

 

[kolek]

Guys, until I get a chance to test what I hope will solve this issue, I'm pretty much done here. But obviously feel free to carry on without me.

 

[OgreMustCrush]

Oh gotcha I understand now. So you're seeing this similar cell 9 balancing issue on the newer JK PB model?

All my data was from the JK PB inverter BMS, but I suspect you would see it on any BMS capable of active balancing when you have unequal resistance busbars. It's going to be less likely to occur on BMS with passive balancing as they pretty much by definition only balance at absorption since they are burning off charge through resistors on high cells rather than moving charge from high cells to low cells. Plus the fact that they tend to be more poorly balanced in the first place, one cell being 20-40mV lower stands out less when you have 100mV difference spread across the cells.

I will say that after raising my balancing voltage to 3.43, my absorption voltage to 3.5, and using a charge current profile on my inverter that aggressively ramps down current as SOC approaches 100% (making sure that cell voltages never climb above balance voltage while at high current) cell voltage on cells 9 (and 8) are much closer to the average than they used to be. Only have about 20mV of total delta at peak voltage on my worse pack now, and it's down below 10mV in like 5 min. My other pack is pretty much not having to balance at all.

 

[kolek]

Hey guys, on the JK, if you set balancing to start at say, 3.4V, does it stop balancing if even 1 cell is below that voltage, or the average needs to be below that voltage for it to stop?

 

[Hedges]

I hope it balances if even one cell is above 3.4V

"top balancing" ... just think what could happen if it didn't!

 

[kommando]

I hope it balances if even one cell is above 3.4V

The JK PB does this. I set to 3.449V if the battery is already well balanced, if its out of balance I drop it to 3.420V and then raise it once it behaves.

 

[kolek]

@upnorthandpersonal today I was able to test your idea of puttting the clamp meter on the cell #9 BMS lead.

What I witnessed is every minute or so, it would pulse about 1.5A on that lead on and off for maybe 15 seconds, but the vast majority of the time, it wasn't doing anything. And sometimes I even saw the polarity of the flow switch to negative, like it was switching to drain mode for a second or 2. Don't know if this is normal behavior or what. This is on a cell that is around 0.22V lower than the rest of the cells during peak charging. The voltage gap was barely narrowed, if at all. I thought it was going to be continuous 2A adjusting.

But today is the first time the BMS has had a chance to start balancing these cells so maybe I need to just be patient and let it do it's thing.

I also had to change the BMS settings of Cell OVP from 3.6 to 3.62 and Cell OVPR from to 3.55 to 3.6 because otherwise the BMS was turning off charging because some cells were hitting 3.6 while cell #9 was still sitting much lower.

The Deye has Float set to 56V (3.5V) and Absorption set to 56.1V. Battery comms are not set up.

Thoughts?

 

[kommando]

Don't know if this is normal behavior or what.

Put the clamp meter on another balance wire and see if you see the negative pulse, it may be the sampling/measurement cycle and so common to all the active balancing wires.

 

[upnorthandpersonal]

Don't know if this is normal behavior or what

Normal.

maybe I need to just be patient and let it do it's thing.

Yep. They're large cells, and if there is a decent imbalance, it will take some time.

 

[OgreMustCrush]

@upnorthandpersonal today I was able to test your idea of puttting the clamp meter on the cell #9 BMS lead.

What I witnessed is every minute or so, it would pulse about 1.5A on that lead on and off for maybe 15 seconds, but the vast majority of the time, it wasn't doing anything. And sometimes I even saw the polarity of the flow switch to negative, like it was switching to drain mode for a second or 2. Don't know if this is normal behavior or what. This is on a cell that is around 0.22V lower than the rest of the cells during peak charging. The voltage gap was barely narrowed, if at all. I thought it was going to be continuous 2A adjusting.

But today is the first time the BMS has had a change to start balancing these cells so maybe I need to just be patient and let it do it's thing.

I also had to change the BMS settings of Cell OVP from 3.6 to 3.62 and Cell OVPR from to 3.55 to 3.6 because otherwise the BMS was turning off charging because some cells were hitting 3.6 while cell #9 was still sitting much lower.

The Deye has Float set to 56V (3.5V) and Absorption set to 56.1V. Battery comms are not set up.

Thoughts?

The balance current is never constant since the way the JK BMS implements it is using a high cell to charge a supercapacitor, then dumping that charge to a low cell. It's also more effective at pulling down high cells than bringing up low cells due to the process not being 100% efficient.

 

[apzs]

So, guys. Thank you for this thread. I feel not alone now.

I have a two DIY banks made identically. They were working well for 9 months first and 6 months second. Recently I've noticed the issues with ... cell #9 ! Today I rolled back my BMS firmware to 15.30, because it's old enough to believe that it was worth to try.

What I did in the meantime? I was thinking that my cell #9 is faulty. So I replaced all the cells in the bank (6 months working), and I decided to test capacity and the curve on all of them, every single one. I replaced them with newly delivered - and all tested - cells.

Now, first thing, all the old cells are almost _identical_ with capacity (delta 2Ah, after testing them with 40A), curves are almost identical - differences are marginal.

And the bank built with the newly delivered and tested cells is behaving the same, with cell #9 dropping the voltage way sooner than others. And what's more. My observation is that it is happening ~3,15-3,10 V. With lower voltage it is getting even worse.

But the summary is the same. Two BMS, from different delivery, from different sellers, and slightly different hardware revisions / two banks, two different cells delivery - but the same type, almost identical capacity and IR. Final result the same as the OP.

 

[apzs]

PS I forgot to mention, that during last 6 months I've upgraded firmware couple of times. But because of winter season, I didn't noticed any issues - my bank wasn't discharged and charged to often. Now from couple of weeks there was enough sun to system works like an ESS, and my problems started.

So I've downgraded firmware from 15.38 to 15.37 yesterday, but no luck - after initial connections and single cells test I mentioned in the previous post. So I decided to go down to 15.30 where I remember I had no issues last summer/autumn.

 

[rogerheflin]

So, guys. Thank you for this thread. I feel not alone now.

I have a two DIY banks made identically. They were working well for 9 months first and 6 months second. Recently I've noticed the issues with ... cell #9 ! Today I rolled back my BMS firmware to 15.30, because it's old enough to believe that it was worth to try.

What I did in the meantime? I was thinking that my cell #9 is faulty. So I replaced all the cells in the bank (6 months working), and I decided to test capacity and the curve on all of them, every single one. I replaced them with newly delivered - and all tested - cells.

Now, first thing, all the old cells are almost _identical_ with capacity (delta 2Ah, after testing them with 40A), curves are almost identical - differences are marginal.

And the bank built with the newly delivered and tested cells is behaving the same, with cell #9 dropping the voltage way sooner than others. And what's more. My observation is that it is happening ~3,15-3,10 V. With lower voltage it is getting even worse.

But the summary is the same. Two BMS, from different delivery, from different sellers, and slightly different hardware revisions / two banks, two different cells delivery - but the same type, almost identical capacity and IR. Final result the same as the OP.

Check what voltage balancing starts at in the BMS. If below 3.42v then balancing will pull charge out of cell 9 if cell 9 is connected to cell 8+ with a longer cable with more resistance than all of the rest of the cells. So when heavily charging cell 9's voltage will show high(cell + cable resistance*current) and gets charge sucked out, after fully charging cell 9 is lower voltage(because of the charge being sucked out) and way behind and the charge is being put back. Mine took about 90minutes each night on both batteries to put the charge back that it sucked out earlier.

 

[apzs]

In my case we're talking about 29cm of cable which has resistance 0,554 Ohm/km (based on the specs).

So correct me if I'm wrong, but this is 0,16mOhm for the cable + two tinned copper plugs (unfortunately no resistance in specs), but those are the best on my market.

And we're talking about something that was working fine for months.

 

[rogerheflin]

In my case we're talking about 29cm of cable which has resistance 0,554 Ohm/km (based on the specs). And we're talking about something that was working fine for months.

Yes. My cable is shorter than that and I have a 2/0 cable. And a few days ago under 60a of charging current I measured a 7mv difference on just the cable (cell terminal 8+ -> cell 9-). And with higher currents the difference would go up. So yours should be .160micro-ohms, at 60a that is 9.6mv. The balancer starts at 10mv, so depending on how accurate the voltage readings really are just a bit more current would cause the balancer to activate.

And worked fine for months could be that an upgrade changed a value of a setting, or simply that you had not noticed it was doing something weird. Mine worked fine, but when looking at what was happening with the balancing something was wrong. Especially after I sorted out my second battery had exactly the same symptoms.

Make sure your balancing is set at least 3.40-3.42. Below that during charging it will remove charge from cell 9, and when the charging current goes down it will add charge back to cell 9. Note google play photos for the app shows balancer start voltage of 3.0 so it must be default on at least some versions.

 

[apzs]

I just did a practical experiment. Voltage drop on this cable is 0,003V on 38A currently. I have no need to go up above 45A - usually 35-40A.

EDIT: I see in the cable datasheet that for 50mm2 it has 0,386 Ohm/km and 70mm2 - 0,272 Ohm/km. I'll try to order them + connectors and play with them as a connector between the levels.

 

[kolek]

So I replaced all the cells in the bank

Interesting... so what'd you do with the first set of cells? Asking for a friend. He likes to build doll houses for orphans and disadvantaged minority children out of old worthless cells like that.

 

[apzs]

Interesting... so what'd you do with the first set of cells? Asking for a friend. He likes to build doll houses for orphans and disadvantaged minority children out of old worthless cells like that.

Third bank was planned, so I had extra 16 ordered. When I've noticed issues, I decided to do a switch as they were just waiting ;-)

 

[apzs]

I just did a practical experiment. Voltage drop on this cable is 0,003V on 38A currently.

I had a chance to do another test now. As the bank is fully loaded and sun is powering my home, I've checked voltage drop with 0A on the cable #8-#9. It's 0,000V (which was expected).

But you're right @rogerheflin about 3.0V balancing by default on JKBMS mobile app. It is still there in the recent version on iOS, but I have it bumped to 3.42V.

 

[PVGeezer]

I can chime in here by saying that this issue may not to be limited to the JK BMS. I'm currently building 6 16S MB31 boxes with the Seplos V3 BMS and Enerkey 4A balancers.

At the moment I'm top balancing box 2 of 6. Power supply set at 57V and 5A. Both boxes 1 and 2 are/were exhibiting lower voltage on cells 8 and 9. I just noticed this potential pattern yesterday after seeing it on pack 2. I wouldnt actually call it a pattern till I see it on a third pack.

Pack 1 max cell voltage difference was 150 mA and 8 and 9 came within 10 mA in 24 hours so I called it good.

Pack 2 cell voltage difference was 190 mA yesterday. The pack was just reaching full charge. We'll see what it is today.

The Enerkey balance leads are all the same length and the pack charge current is down around 1 or 2 amps so I have my doubts about variation in busbar or lead resistance being the issue. I haven't looked closely at it yet though.

An alternate idea, just conjecture on my part, is that it's temperature related. It's fairly cold in my airplane hangar where I'm building these so battery temps are down around 5 or 6 C. I have heat pads in the boxes but they aren't set to run till the cell temps get down to 0 C. The electronics internal temps are running in the 25C range. Cells 8 and 9 are obviously farthest from these heat sources so might be running a bit cooler than the rest of the cells. Just a thought.

 

[apzs]

In my case the room temperature is between 21-23C.

What I did yesterday was to measure temperature with some cheap IR temperature sensor, and on both banks, cell number 9 had lower temperature than others - even cell number 8. All of them during the charge had 28.xC, but cell number 9 had 27.xC. Difference was low, around 0,5C, but still, something I noticed.

 

[PVGeezer]

I'll take my IR thermometer over and check the cell temps today as well. I don't think they can be much more than a degree or so different. The 4 Seplos NTC sensors are all within a degree or so. Unfortunately the sensor wires weren't long enough to reach back to cells 8 and 9 so I have the rear most ones on cells 6 and 11.

 

[PVGeezer]

This morning all cells in box 2 are also in good balance, within 4 mV. 8 and 9 are at 3.546, the others at 3.550. There is about a 1 C temp difference between the front and rear cells. Starting to believe it is the slight temp difference, but it seems surprising that a 1C delta is all it would take.

Starting top balance on pack 3 now, we'll see if it does the same thing.

 

[rogerheflin]

I can chime in here by saying that this issue may not to be limited to the JK BMS. I'm currently building 6 16S MB31 boxes with the Seplos V3 BMS and Enerkey 4A balancers.

I would expect this to be potential battery layout issue with ANY bms. It might be why some of the BMSes have the cell difference set higher than 0.010v to start balancing. I think someone said they had seen some commercial batteries have balancing differences of .03v and that would mean that the balancer is almost useless, and/or only works in the most extreme cases of out of balance.

And if cell 8+ -> cell 9 - includes more wire plus a fuse then the voltage difference would be greater.

I have seen enough companies "fix" symptoms without having the slightest idea of what is going on (upping the voltage difference to 30mv to fix a cell doing the above), and or lowing the start balance voltage to 3.0v for some unknown reason. Neither of which do anything except cause some other problem but they might hide some observed behavior that is not a real problem and/or is normal.