Jk bms balancing issues

[GuyS]

Hi, I've recently installed 16s 320ah diy battery with jk bms with 2A active balencer into my grid tied solar system.
At first everything seemed to be working correctly and the cells were all balanced and within 0.01v over several cycles.
I checked the cells a few days ago and noticed the uvp had kicked in and one cell was 0.3v lower than the others. I fully charged and top balanced and after my first cycle I checked my cells and again uvp had come on but this time 0.48v difference, which is very concerning!

I'm unsure what else to do other than remove that cell from the battery as it may be damaged? Or is my bms not correctly balancing? I have changed the balancing from 3.45v to 2v in an attempt to balance (even at low v) to ensure I don't cause more damage with the one low cell But it doesn't seem to be able to keep up and still falls out of balance.
What are peoples thoughts?

 

[Bob B]

If the cells fully balance at 100% charge .... balancing is not the problem.

Lowering the balance voltage to 2 volts is a good way to get them out of balance.

Move the cell to a different position while cleaning and checking all bus bar connections carefully ... If it is still low in the new position, you likely have a bad cell.

 

[ScrotusGobbleBottom]

I dont claim to be an expert but I dont believe there is a whole lot of energy between 3.05 and 2.5. This may be only a couple amps.

If these batteries are +/- .005 at 3.45 then they are mostly balanced. My cells have a .035 separation at 3.65 and I am not concerned about it because again, there isnt much energy between 3.45 and 3.65 in most cases.

I hope some experts chime in and correct or back me up.

I agree that lowering the balance voltage to 2 volts is a good way to get them out of balance. That is a terrible idea given how flat 3.2 -3.4 is on the voltage curve.

Moving the cell and double checking balance wire position and tightness would be a great idea as Bob B said also.

 

[GuyS]

I have changed the balance V back to 3.45v and started charging the battery. The power seems to go all into that cell and is quickly re-aligning to the other cells. I will swap the cell as advised and see what happens

 

[GuyS]

I dont claim to be an expert but I dont believe there is a whole lot of energy between 3.05 and 2.5. This may be only a couple amps.

If these batteries are +/- .005 at 3.45 then they are mostly balanced. My cells have a .035 separation at 3.65 and I am not concerned about it because again, there isnt much energy between 3.45 and 3.65 in most cases.

I hope some experts chime in and correct or back me up.

I agree that lowering the balance voltage to 2 volts is a good way to get them out of balance. That is a terrible idea given how flat 3.2 -3.4 is on the voltage curve.

Moving the cell and double checking balance wire position and tightness would be a great idea as Bob B said also.

Thanks, I'll perform another top balance too and check. I don't think my cells have charged past an average 3.42v since having them (according to jk bms) but my victron shunt reads about 0.02-0.03v higher so assume that's correct, my inverter reads lower so its a bit of a guessing game. I think I'll get a multimeter on it when fully charged tomorrow and check that too.

 

[ScrotusGobbleBottom]

I don't think my cells have charged past an average 3.42v since having them (according to jk bms) but my victron shunt reads about 0.02-0.03v higher so assume that's correct, my inverter reads lower so its a bit of a guessing game. I think I'll get a multimeter on it when fully charged tomorrow and check that too.

At 3.42 they should be VERY close in voltage. This doesnt mean they are balanced tho.

When I balanced my 280k's I charged them to 3.47 and held them there until .005 difference. Then charged them to 3.55 and held them there until .005 difference. Then I called it done... of course you need to start your JK balancer at 3.45 or +/- .02 or the other voltages and NOT even use your solar system during this process.

I guess you could use it the energy and recharge but it would stretch a 3 day process into a month.

 

[Skypower]

Thanks, I'll perform another top balance too and check. I don't think my cells have charged past an average 3.42v since having them (according to jk bms) but my victron shunt reads about 0.02-0.03v higher so assume that's correct, my inverter reads lower so its a bit of a guessing game. I think I'll get a multimeter on it when fully charged tomorrow and check that too.

Most likely you have a loose terminal fastener or a bad buss connection primarily between 15 & 16 and others. If that’s the case there definitely would be some heat development after a while in that area with a load. Unlikely a bad cell or bms. After confirming a good connection, you could use a bench top power supply to boost the low cell up to the others and then charge to normal bulk voltage at a lower current to give the bms the time it needs to repair the imbalance. Three things that can get a battery out of balance. Balancing below 3.4 (probably not this case). A bad connection. Repeatedly (cycles) not allowing enough time in bulk for balancing to occur.

 

[740GLE]

Do you typically run the battery down to that low a SOC?

The BMS is reporting 0% due to that low cell.

 

[GuyS]

So after 3 days of top balancing and moving cells. Used today for the first time and did the same thing. 0.005 difference at 3.63v all good at 50% when I checked then came back a few hours later and had 0.5v difference.... only to cell 16, same as before. I did swap cells around so it's not a cell problem. I can only assume the bms is faulty. Not to mention the difference hadn't triggered the bms to stop which is also worrying, it only stopped because it hit the low voltage cut off for that one cell. Any more ideas or would you recommend new bms?

 

[GuyS]

Do you typically run the battery down to that low a SOC?

The BMS is reporting 0% due to that low cell.

No, only happened because that one cell

 

[GuyS]

Most likely you have a loose terminal fastener or a bad buss connection primarily between 15 & 16 and others. If that’s the case there definitely would be some heat development after a while in that area with a load. Unlikely a bad cell or bms. After confirming a good connection, you could use a bench top power supply to boost the low cell up to the others and then charge to normal bulk voltage at a lower current to give the bms the time it needs to repair the imbalance. Three things that can get a battery out of balance. Balancing below 3.4 (probably not this case). A bad connection. Repeatedly (cycles) not allowing enough time in bulk for balancing to occur.

Connection was and is good. Have taken apart to top balance to reassemble and same issue. This is the first cycle after top balance and hit 0.5v difference after being 0.005 for the rest of the range

 

[Skypower]

What does the wire resistance say? Screen shot?

 

[GuyS]

What does the wire resistance say? Screen shot?

 

[Skypower]

I’m not seeing anything wrong in the screenshot. Let me get this straight. You top balanced and everything was good. You swapped around the cells (position16) put back in service and on the very first cycle, position 16 was weird again.
Here’s the thing, normally once a pack is balanced, it should be able to go several cycles before a balance circuit is necessary. So, what is different about that location? Temperature? Positive output cable? Buss plate? The good news, the cells are probably good.

One other thing to check. What is the actual voltage of the cell, does the bms read the correct voltages in that position? Have you removed and replaced the balance connector from the bms? Intermittent loose connection?
Really want the bms and cells to be ok and something simple to blame.

 

[Skypower]

When we mean clean terminal, it doesn’t mean just wiped. It means no oxides. This is removed with a few twists of 3M cloth to both contact surfaces.

 

[GuyS]

When we mean clean terminal, it doesn’t mean just wiped. It means no oxides. This is removed with a few twists of 3M cloth to both contact surfaces.

All the wires and connectors are new and batteries are inside so no oxide at all.

 

[GuyS]

I’m not seeing anything wrong in the screenshot. Let me get this straight. You top balanced and everything was good. You swapped around the cells (position16) put back in service and on the very first cycle, position 16 was weird again.
Here’s the thing, normally once a pack is balanced, it should be able to go several cycles before a balance circuit is necessary. So, what is different about that location? Temperature? Positive output cable? Buss plate? The good news, the cells are probably good.

One other thing to check. What is the actual voltage of the cell, does the bms read the correct voltages in that position? Have you removed and replaced the balance connector from the bms? Intermittent loose connection?
Really want the bms and cells to be ok and something simple to blame.

Yes correct. Location is negligible as they are all together inside (so temp is always room temp) yes positive cable output is from cell 16 and the bms requires 2 wires to attach to 16, plus the victron shunt is attached to 16 so total 4 wires.
The bms was reading correct when I checked a month or so ago but its worth checking it's still correct. Will do this tonight.

 

[kommando]

The UK has a relatively damp atmosphere, even inside metals oxidise. So to be 100% sure of the connection use the 3M on both contact surfaces even if the battery has never been outside.

 

[DIYrich]

How many amp hours or kwh did you get out of the battery when you saw the 0.5v difference?

 

[Skypower]

All the wires and connectors are new and batteries are inside so no oxide at all.

Aluminum oxidizes in minutes and you really can’t see it …until you see it side by side after it’s been brightened. Most of the time people get away without any prep. But it’s not a bad idea to eliminate all the variables.