2 runaway cells on 2 banks

[Ucatchmydrift]

Hi, i have two 24v lifepo4 banks in parallel (via busbars). They were top balanced when new to 3.65v. I have had them running for 6 months now. Coincidentally? the first cell on both banks seems to run away when all other cells are at about 3.33v (according to the jk bmses).
I have balancing atm set to start when one cell hits 3.35v+. The jk bmses have 2A of balancing, but this is semingly not enough. I have now lowered the inverter boost voltage to 27.1v, which seems the max voltage i can get away with. 27.2v causes the first cells on each bank to hit 3.65v.

In the 6 months i have used the batteries, i dont think i have ever had them charge fully, as im pretty economic with the power i make. So, maybe this is the reason? (i would hope the cells were good in the first place, as are grade a direct from eve via fogstar uk..)

Just wondering the best way to tackle this? (without haiving to top balance every 6 months!)
Drain some power using a resistor or a small load on each of the first of the cells when at a high state of charge?

Thanks ??








All the connections should be good, and are new, nickel plated bussbars etc as above..

 

[HRTKD]

Have you verified the cell voltages with a voltmeter with the probes places on the cell's terminals (not the bus bar)?

Half the voltage issues seen on the forum are due to poor connections. You have a nice, tidy system there and the connections look right. But are the contact surfaces clean and corrosion free?

 

[Ucatchmydrift]

Have you verified the cell voltages with a voltmeter with the probes places on the cell's terminals (not the bus bar)?

Half the voltage issues seen on the forum are due to poor connections. You have a nice, tidy system there and the connections look right. But are the contact surfaces clean and corrosion free?

Thanks... I havent checked the cell voltages recently with a meter no, id assume the bmses are accurate, but ill give them a test just to make sure..

Ive checked all connections on both banks the other day, but i didnt go as far as removing any cell busbars to check for corrosion as everything is so new. Pretty sure all connections in general are good, everything crimped with correct tooling (i build dc wiring harnesses for my job!- but everyone makes mistakes lol) at correct lengths etc

 

[HRTKD]

There was a thread a few months back where the forum member had those same cells and the cell terminals were bent. If yours are bent, it could cause a lack of surface contact. Check for gaps between the cell terminal and the bus bar.

Make sure the heat shrink on the negative cable lug is trimmed back far enough so that it isn't coming between the cable lug and the cell terminal.

 

[Ucatchmydrift]

There was a thread a few months back where the forum member had those same cells and the cell terminals were bent. If yours are bent, it could cause a lack of surface contact. Check for gaps between the cell terminal and the bus bar.

Make sure the heat shrink on the negative cable lug is trimmed back far enough so that it isn't coming between the cable lug and the cell terminal.

Thanks, ill check later...

 

[740GLE]

I don’t like the look of that black negative sense lead, that sharp bend is doing nothing good.

I have to imagine this negative ring terminal is the same on both batteries?

All the other sense leads look mighty proper and I have to say good job on the looming and zip ties.

 

[fafrd]

Hi, i have two 24v lifepo4 banks in parallel (via busbars). They were top balanced when new to 3.65v. I have had them running for 6 months now. Coincidentally? the first cell on both banks seems to run away when all other cells are at about 3.33v (according to the jk bmses).
I have balancing atm set to start when one cell hits 3.35v+. The jk bmses have 2A of balancing, but this is semingly not enough. I have now lowered the inverter boost voltage to 27.1v, which seems the max voltage i can get away with. 27.2v causes the first cells on each bank to hit 3.65v.

In the 6 months i have used the batteries, i dont think i have ever had them charge fully, as im pretty economic with the power i make. So, maybe this is the reason? (i would hope the cells were good in the first place, as are grade a direct from eve via fogstar uk..)

Just wondering the best way to tackle this? (without haiving to top balance every 6 months!)
Drain some power using a resistor or a small load on each of the first of the cells when at a high state of charge?

Thanks ??

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All the connections should be good, and are new, nickel plated bussbars etc as above..

I’d suggest you pick up an active balancer.

I have this one and have been happy with it: https://www.amazon.com/Cywhrvzsf-Eq...cphy=9032079&hvtargid=pla-1464402197729&psc=1

Once you are properly maintaining top balance, you can determine whether you have any cells losing capacity faster than the others by looking at which cells hit the lower knee first when the battery is discharged.

 

[Ucatchmydrift]

I don’t like the look of that black negative sense lead, that sharp bend is doing nothing good.

I have to imagine this negative ring terminal is the same on both batteries?

All the other sense leads look mighty proper and I have to say good job on the looming and zip ties.

Thank you.. Fair point about the black wire, i have straightened it out now ?.. The other bank has no kink in the wire, and the type of ring term is the same type on both banks. I could add another bus bar at the end and attach the neg wire using the same size ring terminal as the other sense leads, but i would be supprised if that was the cause of the problem?

Im running a home assistant add on called batmon that connects to the bmses, it was 'power cycling' (whatever that means? - bluetooth? or actually turning them on and off? - im not sure) them regurly as it was having problems connecting to both bmses (i still have it running, but connected to just one bms with power cycling turned off now). Its possible, if it was actually turning them on and off (very quickly presumably) many times a day, that could have caused it, but i wouldnt be supprised that the cells were unbalanced before i started playing with the HA batmon addon..

 

[Ucatchmydrift]

I’d suggest you pick up an active balancer.

I have this one and have been happy with it: https://www.amazon.com/Cywhrvzsf-Eq...cphy=9032079&hvtargid=pla-1464402197729&psc=1

Once you are properly maintaining top balance, you can determine whether you have any cells losing capacity faster than the others by looking at which cells hit the lower knee first when the battery is discharged.

Presumably, i could test that by draining off some of the power from the two cells when at a high state of charge with a resistor, then once balanced, check which cells drain first and by how much?

 

[MisterSandals]

In my experience most of the wired chit seems to happen to cell #1.
Have you tried moving that cell to a different position?

And, label your cells and take good notes. Your cell in position #1 is a good one to keep an eye on. Cell in position #6 looks like your strongest (highest capacity cell).

 

[Horsefly]

I’d suggest you pick up an active balancer.

The BMS he is using has active balancing built in. In fact, it was balancing to the tune of over 2A when he took the screengrab of the BMS.

 

[Ucatchmydrift]

In my experience most of the wired chit seems to happen to cell #1.
Have you tried moving that cell to a different position?

And, label your cells and take good notes. Your cell in position #1 is a good one to keep an eye on. Cell in position #6 looks like your strongest (highest capacity cell).

I havent tried moving the cells yet, they are compressed and was hoping not to have to do that, although im of the understanding that they only actually need to be compressed for the first few charge cycles to force out the detrimental 'bubbles' from within the battery chemistry.
I may try that though in the future

 

[goldserve]

In general, correlate the BMS readings with a multimeter and if it is really that one cell that is high, put a 5 ohm resistor across that cell only and see the voltage drop. You may have a while to go because the other cells are at 3.3x volt and there could be still a lot of charge before the voltage rises. Once you hit 3.4V, the other cells will start rising more quickly.

 

[Ucatchmydrift]

In general, correlate the BMS readings with a multimeter and if it is really that one cell that is high, put a 5 ohm resistor across that cell only and see the voltage drop. You may have a while to go because the other cells are at 3.3x volt and there could be still a lot of charge before the voltage rises. Once you hit 3.4V, the other cells will start rising more quickly.

Will do.. I ordered 2x 5 ohm 100w resistors earlier.. I thought it may take a while as like you say, at 3.3v, the rest could be 10-20% lower capacity?! Currently both banks are at about 80% soc, and the cell difference is 0.000v on the top bank and 0.001v on the bottom with the house drawing just 140w from the banks, which is how they normally sit. Even when drawing 2.5kw+ the cell difference is only ever normally 0.01-0.02v.

 

[fafrd]

Presumably, i could test that by draining off some of the power from the two cells when at a high state of charge with a resistor, then once balanced, check which cells drain first and by how much?

Yes, you can perform a manual top balance but it takes time, has to be done carefully to avoid tripping your BMS protection, and since it’s almost certainly something you’ll need to do on a repeated basis of at least every 12 months if not 6, I’m not sure it’s worth the effort.

You can purchase the 8S active balancer directly from Heltec for $28 through AliExpress (which is how I bought mine): https://m.aliexpress.us/item/225580...19ba4f&afSmartRedirect=y&gatewayAdapt=glo2usa

 

[Ucatchmydrift]

Yes, you can perform a manual top balance but it takes time, has to be done carefully to avoid tripping your BMS protection, and since it’s almost certainly something you’ll need to do on a repeated basis of at least every 12 months if not 6, I’m not sure it’s worth the effort.

You can purchase the 8S active balancer directly from Heltec for $28 through AliExpress (which is how I bought mine): https://m.aliexpress.us/item/225580...19ba4f&afSmartRedirect=y&gatewayAdapt=glo2usa

The jk bmses do have a 2amp balancer on board, which i did have set to balance above 3.4v, (now set to 3.35v). Problem is, that im not sure that in the 6 months ive been using the banks, they have ever been charged to 3.4v or above, as its been winter, and when charging from the grid at night on cheap rate electricity, i would just put in a couple of hours charge or whatever i though would bring them to about 80% soc. - and im wondering if thats the reason they have gotten out of balance?.. Not sure having another 3amps of balancing would have made any difference..

 

[Ucatchmydrift]

And in the summer now, on sunny days i would switch on my sons crypto mining rig to use any extra power, resulting in the banks getting to around 80-90% soc each day. Only just tried reaching 100% soc the other day to test them ?

 

[fafrd]

Will do.. I ordered 2x 5 ohm 100w resistors earlier.. I thought it may take a while as like you say, at 3.3v, the rest could be 10-20% lower capacity?! Currently both banks are at about 80% soc, and the cell difference is 0.000v on the top bank and 0.001v on the bottom with the house drawing just 140w from the banks, which is how they normally sit. Even when drawing 2.5kw+ the cell difference is only ever normally 0.01-0.02v.

You can only effectively balance at the knee. Ignore any cell voltage below 3.5V (or certainly 3.4V).

Bleed off the two runner cells to 3.4V then if you can, charge at a charge current slightly higher than the bleed current at 3.4V and charge until a 3rd cell hits 3.4 while continuing to bleed the two runners.

Now you can stop bleeding and charge the battery until the 3 high cells get up to 3.6 or at least 3.5.

Once you’ve got the highest 3 cells roughly in balance, you can continue the process to get those 3 balanced with the next highest cell, etc…

Once you’ve got the highest 3 or 4 cells in balance, probably time to discharge the battery until a first cell hits the lower knee at 3.0 or at least 3.1V.

If that cell is one of the cells you balanced at the top, that cell is limiting the working capacity of the entire battery and continuing to improve the quality of the top balance will only allow you to increase boost voltage closer to 28.8V but will not gain you any additional capacity.

If another cell hits the lower knee first, boost it’s voltage using a 3.6V power supply until it matches the voltage of the next lowest cell then stop boosting and discharge again until those cells hit the lower knee again).

Now boost both of those cells until they match the voltage of the 3rd-lowest cell.

Repeat the discharge and if none of those 3 bottom-balanced cells hitting the lower knee were one of your 3 top-balanced cells, go ahead and boost all 3 to match the 4th-lowest cell.

Continue to bottom balance to the 5th lowest or even 6th-lowest cell until you have bottom-balanced all lowest cells to the lowest of your 3 top-balanced cells.

You can now charge the full battery back to the top where the first cell to hit the upper knee that was part of the bottom-balanced set is the reference cell determines overall battery capacity and should no longer be touched (neither boosted nor drained).

Boost all other cells to match the voltage of that reference cell.

Getting all the cells balanced to 3.5V should be relatively easy. Getting balanced to 3.6 or even 3.65V just requires a lot of time and attention.

Wherever you end up, that determines your boost voltage and monitoring how quickly the battery strays from that well-balanced starting point will tell you how often you need to go through this painful exercise.

A 5A active balancer that you just attach on whatever schedule you think makes sense is much, much easier…

 

[fafrd]

The jk bmses do have a 2amp balancer on board, which i did have set to balance above 3.4v, (now set to 3.35v). Problem is, that im not sure that in the 6 months ive been using the banks, they have ever been charged to 3.4v or above, as its been winter, and when charging from the grid at night on cheap rate electricity, i would just put in a couple of hours charge or whatever i though would bring them to about 80% soc. - and im wondering if thats the reason they have gotten out of balance?.. Not sure having another 3amps of balancing would have made any difference..

I elected to go the route of independent active balancer rather than integrated active balancer for exactly this reason. I don’t want to be forced to fully charge my battery to keep it balanced.

I actually maintain a bottom-balanced battery because I discharge to the lower knee each and every day, rain or shine, winter or summer, while even on the highest-production summer days I don’t get much over 80% SOC…

So I’m using my Heltec Active Balancer to maintain bottom balance at the lower knee (whenever I see it straying too fat off which is about every ~6-months or so).

 

[fafrd]

And in the summer now, on sunny days i would switch on my sons crypto mining rig to use any extra power, resulting in the banks getting to around 80-90% soc each day. Only just tried reaching 100% soc the other day to test them ?

Have you calculated what crypto-mining is paying you on average $/kW for excess solar power?