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

[kolek]

No absorbption timer, still looking for how to make that settings change. These are not my setting btw, just what the menu looks like.

 

[kommando]

Was that a B2A24S20P?

No, JK PB BMS 200A 2A balancer.

 

[upnorthandpersonal]

Should I try using the clamp meter to see if there's any flow on the balance lead to cell 9 when the battery is above 3.4V and it's the lowest voltage cell? That seems like a good idea.

Yes!

 

[Daddy Tanuki]

when i say absorption I am simply referring to common nomenclature used in lead acid as long as your float voltage is high enough to keep it balancing you should be OK. I prefer to use the absorb settings on my morning star to fill up at the end at a lower voltage than bulk, just high enough to keep balancing going before swapping to float at 3.4/cell

 

[kolek]

when i say absorption I am simply referring to common nomenclature used in lead acid as long as your float voltage is high enough to keep it balancing you should be OK. I prefer to use the absorb settings on my morning star to fill up at the end at a lower voltage than bulk, just high enough to keep balancing going before swapping to float at 3.4/cell

Ok, I'm starting to understand this better. Since I start balancing at 3.4, I need to raise my float to be higher than 54.4V, and it's currently 54. Oh. Duh, that never occurred to me. Ok light bulb went on in my head.

 

[Daddy Tanuki]

Ok trying to translate that into something I can understand. Maybe raising the float will prolong the time the battery has to balance? Sorry, I'm not really much of a battery charging guru. What absorption & float values would you recommend in this case to try and balance cell 9? Currently I have Float=54 and Absorption V=54.3, I lowered absorption from 55, but maybe I need to be raising it rather than lowering it. Just trying to be cautious because the higher I charge, the larger the gap gets between cell 9 and everything else.

Bro Absorb is too low, 54.3 = 3.39 volts per cells if you are not pushing 3.4 it will never balance. so bare minimum would be 54.4

but your photos show your pack voltage at 55.19 which is 3.44 if they were all the same. I would open the breaker for any charging source, and put a single cell charger on that one cell. bring it up closer to the rest or even slightly above and then run a charge cycle to see what happens

 

[kolek]

Bro Absorb is too low, 54.3 = 3.39 volts per cells if you are not pushing 3.4 it will never balance. so bare minimum would be 54.4

You caught my response before I finished editing it. I just managed to figure that out. Never made that connection until today. Duh.

 

[Daddy Tanuki]

You caught my response before I finished editing it. I just managed to figure that out. Never made that connection until today. Duh.

still charge up that cells and then when its charged observe it with a clamp meter on the balance lead for it to see if it is pulling excess while trying to balance, this will determine if it is the BMS screwing up or simply a as of yet undiscovered wiring issue. trust me I have pulled my packs wiring off and re cleaned them more than once for similar issues for me it was three cells one time were not charging up as well as the rest of them and it was not verifiable via voltmeter. it was the BMS leads for me.

 

[sunshine_eggo]

LFP requires varying degrees of absorption depending on charge voltage and initial C rates.

Low C rate - short absorption
High C rate - not as short absorption
High absorption voltage - short absorption
Low absorption voltage - not as short absorption

At high C rate, you hit absorption sooner than you otherwise would at a lower current, so some absorption is needed.

At low C rate, you hit absorption later than you otherwise would at a higher current, so little absorption is needed.

Examples:

3.65V, 0.5C, 0.05C tail current (common definition for fully charged cell): ~15-30 minutes absorption
3.45V, 0.5C, 0.005 tail current (near zero ): ~2 hours absorption
3.45V, 0.1C, 0.005 tail current (near zero): < 15 minutes absorption
3.40V, 0.2C, 0,02 tail current: ~5 hours absorption

 

[kolek]

still charge up that cells and then when its charged observe it with a clamp meter on the balance lead for it to see if it is pulling excess while trying to balance, this will determine if it is the BMS screwing up or simply a as of yet undiscovered wiring issue.

Yep, the light bulb finally went on, thanks to you. I'm going to do exactly that and will report back with my findings.

 

[Daddy Tanuki]

Yep, the light bulb finally went on, thanks to you. I'm going to do exactly that and will report back with my findings.

I should have hit you up last month, I was in your stomping grounds as I went to and from sasebo and iwakuni bases.

 

[OgreMustCrush]

I just want to note that on my packs I see cell 9 read as slightly higher than other cells due to the higher resistance (longer) busbar at the "bend" right up until the pack reaches the voltage knee, at which point things flip and it is almost always the lowest cell until everything balances out. It is very possible that at higher charge currents, that busbar resistance could dominate and actually result in cell 9 getting balanced the wrong way (having energy pulled from it instead of sent to it) for some time if the balance threshold voltage is set too low. Then once the pack reaches constant voltage and the current drops off the voltage on that cell would drop even more that it would otherwise.

You can see the effect of that busbar even at relatively mild charge currents, this is a ~600Ah system, so the charge current in this plot was never higher that 0.1C (was significantly higher earlier, inverter was ramping down.)

 

[kolek]

@sunshine_eggo @Daddy Tanuki
Today trying these settings:

Absorption 56.1
Float 56 (3.5V/cell) to prolong the time available for the BMS to do it's work

Seems reasonable?

 

[kolek]

You can see the effect of that busbar even at relatively mild charge currents,

Really appreciate your post. So are you saying the cell 9 busbar has more resistance because it's a couple centimeters longer than the other busbars, and that's behind these cell 9 issues? It's kinda hard to understand that an extra 2 centimeters of 300A solid copper busbar could make an issue.

But it's amazing that your chart is showing exactly the same issue as I'm facing, that cell 9 goes out of balance as the current drops off.

Is your cell 9 eventually coming into balance during the float with the other cells at higher voltage? From that chart, it looks like the BMS is not managing to balance it. What is the actual voltage gap you're seeing?

 

[Daddy Tanuki]

@sunshine_eggo @Daddy Tanuki
Today trying these settings:

Absorption 56.1
Float 56 (3.5V/cell) to prolong the time available for the BMS to do it's work

Seems reasonable?

very.. track it for a week or so bare minimum.

 

[rogerheflin]

Really appreciate your post. So are you saying the cell 9 busbar has more resistance because it's a couple centimeters longer than the other busbars, and that's behind these cell 9 issues? It's kinda hard to understand that an extra 2 centimeters of 300A solid copper busbar could make an issue.

But it's amazing that your chart is showing exactly the same issue as I'm facing, that cell 9 goes out of balance as the current drops off.

Is your cell 9 eventually coming into balance during the float with the other cells at higher voltage? From that chart, it looks like the BMS is not managing to balance it. What is the actual voltage gap you're seeing?

At 50+A of current, and measuring down to .01v differences it takes very little resistance changes. 6" of 2/0 has .000078 ohms of resistance. At 50A that is 4mv of extra voltage drop (use 2 x 2/0 and you get 2mv difference, and shorting it to under 3" each and you still have 1mv--so just about ANY extra wire length will be enough to be at risk of causing the balancer to mess it up). if you were to move the balance wire on cell 8+ to cell 9- then cell 8 would be the out of balance cell. I changed mine to not balance below 3.42 as I was hitting 3.4 during some of the 50-70A charging and it was getting out of balance and having to be brought back in the final phases of charging.

My min cell to max cell was almost the same until the battery was charged and then the charged cells went to 3.6 and left cell 9 at 3.37 or so for hours of rebalancing.

 

[kolek]

My min cell to max cell was almost the same until the battery was charged and then the charged cells went to 3.6 and left cell 9 at 3.37 or so for hours of rebalancing.

Wow that is nuts. So after the hours of rebalancing, did the issue resolve? Or you're still seeing it happen every day?

 

[rogerheflin]

Wow that is nuts. So after the hours of rebalancing, did the issue resolve? Or you're still seeing it happen every day?

the balancing voltage was set to start too low so it was balancing while the battery was charging and the extra cable on cell 9 caused cell 9 to show high. It repeated every day until I raised the balancing voltage to 3.42 (so it does not ever balance while charging) and now it is fine.

 

[kolek]

the balancing voltage was set to start too low so it was balancing while the battery was charging and the extra cable on cell 9 caused cell 9 to show high. It repeated every day until I raised the balancing voltage to 3.42 (so it does not ever balance while charging) and now it is fine.

Ok great info.
Do you still have balancing voltage set to 3.42?
What do you have your float and absorption values set to now?

 

[rogerheflin]

Ok great info.
Do you still have balancing voltage set to 3.42?
What do you have your float and absorption values set to now?

I have left my balancing set to 3.42. I have verified that at the end of the charging cycles that my cells hit 3.44-3.45 so high enough to balance, and have verified that the cells are all about the same now. I lowered my limits in the jk bms based on another thread with recommended settings so the cells stop around 3.45 (vs 3.65). On the eg4 with closed loop the float and absorption settings cannot be changed.

Month is the pattern for the month when it fully charges. Day bad is the plot from a high delta day before I raised the balancing voltage.

 

[kolek]

I lowered my limits in the jk bms based on another thread with recommended settings so the cells stop around 3.45 (vs 3.65).

Thanks again for this info, very helpful. So your JK-BMS is handling the daily high voltage disconnects (deciding when to stop the charging) rather than your inverter?

 

[kolek]

If this issue is common to all JK-BMS, or all BMS in general, due to the longer busbar on cell 9 or whatever, why is nobody talking about it? Is this only an issue with JK?

 

[Daddy Tanuki]

If this issue is common to all JK-BMS, or all BMS in general, due to the longer busbar on cell 9 or whatever, why is nobody talking about it? Is this only an issue with JK?

easy solution, I bought 2 gauge cable and crimp fittings and made my own cables for my builds. this allowed me to measure the cable and make all of the links the same length, even the ones that go from #8 to #9.

 

[rogerheflin]

Thanks again for this info, very helpful. So your JK-BMS is handling the daily high voltage disconnects (deciding when to stop the charging) rather than your inverter?

Yes, the JK/BMS is taking care of it. I run mine in closed loop with the BMS talking to the inverter.

If this issue is common to all JK-BMS, or all BMS in general, due to the longer busbar on cell 9 or whatever, why is nobody talking about it? Is this only an issue with JK?

If there is a longer cable between the 8+ and 9- wire/battery sensor then it would effect any BMS because it would change the voltage seen under load.

 

[kolek]

easy solution, I bought 2 gauge cable and crimp fittings and made my own cables for my builds. this allowed me to measure the cable and make all of the links the same length, even the ones that go from #8 to #9.

So you actually knew that having a slightly longer 300A busbar was going to cause this your #9 cell to be harder to balance? C'mon you gotta be joking. The vast majority of battery builds on this site are not taking that into account.