Cell voltage fluctuation

[kent2ben]

Hi

I have a jkbms setup with 8 cells.
In this setup I have problems with the voltage measurement on specific cells (measured by the bms). When I measure with a meter the voltage is stable.

Below is a picture of cell 7 and 1 voltages over 20min. As you can ser cell 7 is all over the place when load rises.


Because of this fluctuation I have problems when charging because the voltage of cell 7 "rises" too quick above 3.45v.

I have gone through all voltage sense cables and redone them all.

As a bonus info my bms log (in the app) reports “abnormal current sensor”

JK: Abnormal Current Sensor

I could not make out what this "Abnormal current sensor" issue is all about. Any idea? It comes and goes and I have no idea what is happening. Thanks Dan

diysolarforum.com

Info about my bms:


What can be wrong? Any ideas?

 

[Pierre]

Do a decent top balance of all the cells and install an active cell balancer.

 

[kent2ben]

The BMS is equipped with a 2A active balancer.
I am not concerned about the deviation between cell 1 and 7 in my picture a top balance can probably fix that.

My concern is cell 7 is fluctuating from 3.04v - 3.21v at about 06:04 (in my picture).
Can that be fixed with a top balance?

 

[uksa007]

active cell balancer.

The JK-BMS has a active balancer.

 

[uksa007]

In this setup I have problems with the voltage measurement on specific cells

This would suggest you have a bad connection in either the balancer leads, or the bus bars.

Did you crimp or solder the balancer leads ring lugs, I find crimp and soldering them is a better connection.

 

[kent2ben]

This would suggest you have a bad connection in either the balancer leads, or the bus bars.

Did you crimp or solder the balancer leads, I find soldering them is a better connection.

Yesterday I redid the balance leads. I crimped & soldered the copper lugs.
I will try changing the bus bar ?

 

[uksa007]

copper lugs.

Copper lugs are prone to oxidation, I use tin plated, which is usually what the bus bars are plated with, so there are no dissimilar metals to cause electrolysis and high resistance over time.

 

[kent2ben]

Copper lugs are prone to oxidation, I use tin plated, which is usually what the bus bars are plated with, so there are no dissimilar metals to cause electrolysis and high resistance over time.

My bad - it is copper tin plated lugs.

 

[kent2ben]

When I turen off my inverter and only charges (from solar) the fluctuation disappears?!

 

[uksa007]

When I turen off my inverter and only charges (from solar) the fluctuation disappears?!
View attachment 139104

How much current are you charging with?
Less than 30A like when under load?
The larger current will show up the high resistance in bad connections

 

[kent2ben]

I am charging with the around 35A and just turned off the inverter?
The inverter was pulling 10-30A.

 

[kent2ben]

It looks like the inverter is sending somekind of noise back to the BMS ?

 

[mikefitz]

Renogy inverter/ charger?

 

[kent2ben]

The inverter is a Victron Multiplus ii 24/3000.

 

[Supervstech]

Post pictures of the build.
Close up of connections on that cell.

 

[Supervstech]

Try moving that cell to a different location. See if the problem moves also.

 

[JJJJ]

Is the cell recently received? Sometimes IR in a cell out of long term storage can be a factor until it gets a few cycles in.

 

[kent2ben]

Is the cell recently received? Sometimes IR in a cell out of long term storage can be a factor until it gets a few cycles in.

All 8 cells Are one year old.

 

[kent2ben]

Try moving that cell to a different location. See if the problem moves also.

I will try moving the cell but the problem seems tied to to inverter? Now with the inverter off it it charging perfect with no fluctuation.

 

[JJJJ]

I have the Multiplus 24v previous generation. Likely won’t work but perhaps drop charging voltage by 50 percent just to see?

 

[kent2ben]

I have the Multiplus 24v previous generation. Likely won’t work but perhaps drop charging voltage by 50 percent just to see?

The problem is visible both when the Multiplus is charging and inverting.

When you say drop charging voltage by 50 % do you mean I should try charge my 24v pack with 12v?

 

[JJJJ]

The problem is visible both when the Multiplus is charging and inverting.

I think I have a better understanding. I was suggesting lowering the charge rate from the inverter.

My Multiplus only sees the battery, not the individual cells. My BMS sees the cells and you provided the screen shot for that.

Given that the problem is not visible from the solar end might be a difference in charge rates. Lowering the Multiplus charge rate might provide a clue. I can do this via the Mk3-Usb connection from my computer.

I would still think that the issue is with the bus bar connection on the battery. Perhaps some corrosion over time? Switching cells would help isolate this.

On the other hand perhaps a bus bar and terminal cleaning with some scotch brite and a good wipe down would clean up the connection?