Cell voltage jumps when balance switch is ON

[zedconnor]

It is a 1A Daly Smart Active Balancer.(Seperate balancer.)

Battery : 51.2V 280Ah
BMS : DALY Smart BMS

Everything is reading correctly everywhere. Cell voltages are being read correctly by the bms and the balancer as well.

One cell no.7 is quite above other cells and hitting high voltage cutoff when charging.

Lets say it hit 3.55V and i turned off charging. It drops a bit to 3.5V
Pack voltage : 53.5V


And then the Balancer balance switch turned on. (I set start voltage to 3.4V)
The moment it turns on cell no.7 will jump from 3.500V to 3.56V (sometimes about 100mV)

And pack voltage rise 0.5V (from 53.5 to 54V)

This is all only READINGS on the Active Balancer's Bluetooth app(or pc app Ive tried both and same results.)
All voltages on connectors, inverter + and - , bms + and - all show correctly, BMS's own bluetooth app also show correct voltages.
The balancer is the only one showing higher voltages.)

NOTE : Balancer shows correct voltage when NOT balancing.

I have tried 3 of these balancers now.(all brand new) and all 3 are making the same behavior.

Any idea why that is happening?

 

[Brett V]

Did you top balance the cells before assembly of the battery?

 

[zedconnor]

Did you top balance the cells before assembly of the battery?

No i didnt because it will take ages.
What I did is I bulk charge the pack using an inverter @3.45Vpc per cell with low current like 10A.

Now all other cells are over 3.33V and no.7 is over 3.4V

But my concern is not about balancing since I can easily get no.7 down using my EBC 40L charger/discharger.

My problem is the active balancer's bluetooth/pc app showing higher than actual voltages on cells.
(As a whole pack it is showing a whopping >0.5V difference. Actual 53.5V, balancer 54V)

And this is only when it's balancing is ON. When it's OFF all voltages in bluetooth/pc app of balancer are normal so I am not sure what is happening.

Could this be due to high wire resistance? If so, wouldnt it also affect the voltage readings while it is OFF?

Or does resistance only occuring when the 1A current is active?

NOTE
I already tried 3 brand new balancers of same type. Same results.

 

[zedconnor]

This is the setup. This is only for testing purposes and not for actual usage.

The harness wires from BMS and Balancer are put together into one side of the terminal block.
(The wire tips are all soldered)

BMS voltages are ok both when resting and during charging and discharging.

 

[Makeurown]

Hi, that is normal in all the units I have tested so far, especially if Pack and Cell over voltage settings are very close to the final charge state that you have selected and you are adding an Active balancer to the BMS.

Some of the balancer leads will have current flowing through them when Active balance is running, so voltage readings from the balancer are going to be jumping around as the current is switched. You can test by lowering the balancer current to see if it makes a difference.

As suggested, you may need to top balance the pack again, one or two cells may be reaching the steep charge knee before the others.

 

[Makeurown]

Just seen your last picture, that will make the voltage fluctuations appear worse as Active balance current will be also flowing through part of the Cell voltage monitor leads going to the BMS, due to mid point splicing of the looms.

EDIT: Just to make it clear, when you pull / push current through those leads during active balance there will be a voltage drop or boost across the balance leads and the cell readings will jump around, I simply ignore it.

 

[zedconnor]

Just seen your last picture, that will make the voltage fluctuations appear worse as Active balance current will be also flowing through part of the Cell voltage monitor leads going to the BMS, due to mid point splicing of the looms.

Would u suggest seperate cables going to the terminal for bms and balancer?

 

[Makeurown]

It will work both ways, it may help stop the BMS from tripping if you have O-V set a bit tight.

The longer leads may have a slight impact on the 1A current capability of the balancer, but really should be ok.

The only unknown is the firmware, and I would hope they sample the cell voltage between balance current flow, at least to get a more accurate voltage state. That also requires some allowance for cell variation right after a cell balance transfer - like ignore cell for X time, some of stuff I've seen makes me wonder though.

I do use these though, and I've had zero issues so far, and they really do work for me, even on a bad mix of aged cells thrown into a 48V 16s config, although I did top balance them first as they were seriously all over the shop before that.

 

[Brett V]

No i didnt because it will take ages.

Properly top balance the battery before assembly. Until you do, you're spending more time trying to correct self induced problems than it would have taken to do it right in the first place.