Cell voltage imbalance only when fully charged

[jeffc2021]

I have a 24v 230ah LiFePo4 battery pack made from 8s 3.2v 230ah cells from Docan Technology through Jenny Wu with a JBD 150a BMS. When they arrived a while ago I checked that the voltages of each cell are near identical, and from some suggestions, top balancing isn't necessary if they are new and the same voltages.

However I have a problem when the battery pack is fully charged or near 230ah, two of the cells have much higher voltages than the rest according to the BMS. Below is what I see on the JBD BMS Bluetooth app:

Normal, not fully charged(also very similar voltage difference when charging):


Cells 2 and 4 higher voltage when fully charged:



Does this mean I have top balance all the cells? I had thought that if the cells are tested from the manufacturer, are grade A, and have the same voltages when they receive and test them, then they should be balanced?

 

[MisterSandals]

and from some suggestions, top balancing isn't necessary if they are new and the same voltages.

It is fairly meaningless that the cells are the same voltage when at a low state of charge. The reason to top balance is to avoid having out of balance issues at higher states of charge. Others have differing opinions of course.

Is your second pic the state that you consider fully charged?
Can you describe how you charged them? What are your charging settings and charge rate?

If your charge and discharge range for your normal use pattern is not disrupted by the BMS cutting out, you're in decent shape and the BMS balancing may eventually balance the cells. Set the start balancing parameter to 3.4V and give it a few charge cycles.

 

[mikefitz]

With that inbalance at only 27 battery volts I suggest using a DC resistive load across the high cell and manually discharging energy whilst observing cell volts via the app. An automotive 50 watt automotive bulb is a useful load. Apply across the high cell for 10s of seconds at a time.

Mike

 

[MisterSandals]

With that inbalance at only 27 battery volts I suggest using a DC resistive load across the high cell and manually discharging energy whilst observing cell volts via the app. An automotive 50 watt automotive bulb is a useful load. Apply across the high cell for 10s of seconds at a time.

This is a clever idea.
I would think that doing several iterations of this (manual cell discharging) and whole battery charging would get your battery to a better top balanced state.

 

[jeffc2021]

Is your second pic the state that you consider fully charged?
Can you describe how you charged them? What are your charging settings and charge rate?

Yes because the BMS indicated that it's 239ah remaining and automatically cuts off charging. I've been charging the whole pack with a 300w solar panel via a MPPT SCC that's 24v at 40a max. If I recall correctly the scc charged the pack at a couple volts higher than 24 and at 1 - 9 amps during the day.

 

[jeffc2021]

If your charge and discharge range for your normal use pattern is not disrupted by the BMS cutting out, you're in decent shape and the BMS balancing may eventually balance the cells. Set the start balancing parameter to 3.4V and give it a few charge cycles.

Yeah I don't plan to use more than 50% of the capacity regularly, and I've lowered the voltage for the balancing parameter from the default.

Are you saying that the BMS should eventually balance the cells even if the cell voltages are imbalanced near full charge? Because when the pack is not full, the cells are close to equal voltages and thr BMS shows that the voltage difference is only 0.015v or less, so I'm assuming BMS won't turn on balancing. Will the BMS still balance in this situation?

 

[jeffc2021]

With that inbalance at only 27 battery volts I suggest using a DC resistive load across the high cell and manually discharging energy whilst observing cell volts via the app. An automotive 50 watt automotive bulb is a useful load. Apply across the high cell for 10s of seconds at a time.

Mike

That seems like a good idea since I don't have a 3.2v charger. Do I just connect this bulb or load directly across the terminals of the high cell while all the cells are still connected in series to the BMS?

 

[MisterSandals]

That seems like a good idea since I don't have a 3.2v charger. Do I just connect this bulb or load directly across the terminals of the high cell while all the cells are still connected in series to the BMS?

Yes, a regular incandescent bulb (it won't light but will use current).
A soldering iron or other dumb "heating element" would work too.

 

[MisterSandals]

Yeah I don't plan to use more than 50% of the capacity regularly, and I've lowered the voltage for the balancing parameter from the default.

I wouldn't lower the balance start voltage. This needs to be done at a high SOC.

Are you saying that the BMS should eventually balance the cells even if the cell voltages are imbalanced near full charge?

Will the BMS still balance in this situation?

Yes and yes. This is how you get your cells behave at a high SOC.

Yes because the BMS indicated that it's 239ah remaining and automatically cuts off charging.

This should not be happening with most BMSs. The charge controller should be controlling the charging. The BMS should be keeping the batteries from damage (and a small amount of balancing with small amounts of balancing current).
What charge controller do you have? Do you know the charge profile or is it simply a "lithium" battery setting?
If your charge controller can be configured, its likely you can make it work better for your battery back and use scenario.

 

[jeffc2021]

Yes, a regular incandescent bulb (it won't light but will use current).
A soldering iron or other dumb "heating element" would work too.

Does the regular bulb need to have a specific rated voltage since a cell is 3.2v? I'm just concerned about damaging the cell with an inappropriate voltage or current.

I have a soldering iron that's plugged into the outlet, and since it's AC then I can't connect to the cell right?

 

[Hedges]

You disconnect the bulb or soldering iron from AC or any other power source.
Use a "dumb" soldering iron, one that is just a resistive heating element and no thermostat or other controller.
Use a "dumb" incandescent light bulb, not LED, fluorescent, no electronics.

Measure the resistance with DMM, and you can calculate how much current it would draw from a 3.5V cell.

A light bulb changes resistance with temperature. If you use a 3V bulb, it will draw more current initially, less as it starts to glow.
If you use a 12V or 120V bulb, less change, more uniform.

Not many resistive elements that would draw too much current at 3.5V!

 

[Luxpower_Gilbert]

Check and re-tighten all connections…

 

[RCinFLA]

BMS should not be cutting off charging because its Columb counter says it greater than 100% of entered AH rating.

BMS should only terminate charging if 1) any cell exceeds cell overvoltage limit, 2) max set charge current limit is exceeded, or BMS gets too hot from current its passing.

In your case, by the first picture, cell #2 exceeded 3.500v maximum cell overvoltage limit. That is what shut down charging. That is likely because cells are not well balanced.

The fact that your AH remaining is greater than entered battery AH just says your Columb counter needs full charge reset referencing.

 

[jeffc2021]

I wouldn't lower the balance start voltage. This needs to be done at a high SOC.



Yes and yes. This is how you get your cells behave at a high SOC.


This should not be happening with most BMSs. The charge controller should be controlling the charging. The BMS should be keeping the batteries from damage (and a small amount of balancing with small amounts of balancing current).
What charge controller do you have? Do you know the charge profile or is it simply a "lithium" battery setting?
If your charge controller can be configured, its likely you can make it work better for your battery back and use scenario.

I think the BMS cut off charging due to over voltage protection or something, but I'm not sure since it does it automatically and unless I look at the logs.

I have a Epever Tracer 4215BN MPPT charge controller. I can configure it and change the settings. Should I set the charge cutoff voltage to 8*3.65v= 29.2v?

 

[RCinFLA]

I have a Epever Tracer 4215BN MPPT charge controller. I can configure it and change the settings. Should I set the charge cutoff voltage to 8*3.65v= 29.2v?

No, that will pretty much assure you get a BMS charging shutdown.

Your charge controller absorb voltage depends how much charge you want to subject battery to. Keep in mind balancing does not occur until a cell gets above 3.4v so you need absorb level to about 3.50 to 3.55v per cell, times 8 cells.

Absorb for about 2 hours at least once a month to keep up with balancing. Otherwise, they get more and more out of balance and it will take a lot more then 2 hours to rebalance them.


Raise your BMS cell overvoltage limit to 3.60 or 3.65v.

 

[MisterSandals]

Should I set the charge cutoff voltage to 8*3.65v= 29.2v?

There are gobs of EPEver settings in this thread:

epever 4210 settings

I have 2 200 watt 24v Rich solar panels, 5.32amp, 45.4 Voc, 37.6 Vmp, in series Epever Tracer4210an MPPT connected to PC 2 12.8v 200ah 2560wh Chins LiFePO4 in series Aeliussine 2000W 24v pure sine wave inverter I have read to much, my brain is full of mush and I just need to know what settings...

diysolarforum.com

 

[Levo]

Hi all, I seem to have a similar issue on my 280Ah 4S battery. I have gleaned what I can reading the above.

The fellow here in Alberta who assembled the battery answered my concern with a suggestion:

"Yes that is because of the charge curve of lithium, any imbalance will only show up at the very top end. the start mV should be set to 3420"

So it looks like the balancer does not start until the battery hits 14v right now.


I am trying to make that adjustment, but the Xiaoxiang app will not let me write the adjustment to the BMS and my supplier is not familiar with the app. Before I get into complexities of top balancing (if necessary), can anyone help me get by the error? How do I change the "cycle count" so I can write the revised start point to the BMS?

 

[rhino]

Hi all, I seem to have a similar issue on my 280Ah 4S battery. I have gleaned what I can reading the above.

Look at https://diysolarforum.com/resources/what-causes-runaway-cell-voltage.291/ and click on the DOWNLOAD button on top right to get the doc.

 

[MisterSandals]

I am trying to make that adjustment, but the Xiaoxiang app will not let me write the adjustment to the BMS and my supplier is not familiar with the app.

There are versions of the Xiaoxiang app that you have to buy ($6?) for read/write capability.
The OverkillSolar app is free and is fully functional. I use both and they are identical functionality-wise, slight color/UI differences only.

I seem to have a similar issue on my 280Ah 4S battery.

Which issue are you referring to?

 

[Levo]

Okay, firstly I need to adjust the "start voltage" on the app. I enter the revised start voltage on the balancer (3420mv) but the app will not let me write the value to the BMS - does not retain it. I don't know how to get by the error message. I did purchase the app for $10 CA (iphone). Last year I could only find the overkill app for android.

My problem may be just settings, perhaps not the cells.