Battery discharge - cells discharged to different voltages

[Just]

Hi all,
I've just fully discharged my 48V LiFePo4 battery (with BMS) and measured the cell voltages.
Here are the results:
Cell#01  2.33V
Cell#02  2.40V
Cell#03  2.47V
Cell#04  2.55V
Cell#05  2.63V
Cell#06  2.70V
Cell#07  2.77V
Cell#08  2.85V
Cell#09  3.03V
Cell#10  3.06V
Cell#11  3.09V
Cell#12  3.11V
Cell#13  3.13V
Cell#14  3.17V
Cell#15  3.20V
Cell#16  3.22V
So, why the cells were discharged to the different voltages even they are connected to BMS? Where is a problem - in BMS? How to know? How to fix? The cells are listed as they are connected.

Thank you

[truly_bent]

Does look like half your cells are being allowed to discharge at a different rate, now don' it just Bubba? I'll wager Brownie Points on the BMS.

Possibly a different R value was subbed into a voltage reference for one side. This would lead to the assumption that the BMS core handles 8 cells and that you'll likely find duplicate chip sets on the board, or two separate boards. Probable limitation being the analog IO count. But, I digress.

If the assumption is true, look for a different R value between the two patterns. Getting a resistor off a SM board is something i'd be asking Dennis how to best tackle though.

This all presumes you want to repair it yourself - not a task to be undertaken lightly. The voltages are not high in a system like this, so your pacemaker is unlikely to zap out (i dunno, I don't wear one), but the current can be a life altering experience. Getting down to the BMS board assumes an understanding of the risks involved.

Best of luck.
Jeff

[truly_bent]

Just thought of something. If there are two separate boards, there might be a wire break linking the voltage reference. One side's reference may be left floating.

Might be worth a look.

[Just]

"If there are two separate boards" - the BMS is a single board, but the cells are organized in two bunches of 8 cells in each one.
"might be a wire break linking the voltage reference. One side's reference may be left floating" - are you about the wires connecting the cells to BMS?

[Bikemad]

It's my understanding is that the BMS balances the cells during charging, but only monitors the cells while discharging.

So it is likely that the cells will discharge to different voltages, unless the capacity of each single cell has been acccurately measured and the packs then constructed using similarly matched cells, which I think is unlikely with these packs.

If you measure the cell voltages when the pack is fully charged, and they are all pretty similar, I don't think there is a problem with your pack.

Alan
 

[Just]

If you measure the cell voltages when the pack is fully charged, and they are all pretty similar, I don't think there is a problem with your pack.

Yes, when the battery is fully charged then all the cells receive the same voltage level - about 3.33V on each cell. So, is the problem in the BMS, which doesn't balance cells during the discharge? Should it? Must it?
I always thought that BMS MUST balance the cells during their discharge, otherwise we would pay in the overall battery capacitance (in fact that some cells in the pack were discharged to 2.33V while others to only 3.22V)...
Thank you

[Morgen 3Eman]

Hi Folks,

I must apologize in advance for my silly answers, but I just cannot resist such easy jokes:

    Your battery is mounted with the cells stacked vertically, and the weight of the electrons above are making the lower cells higher voltage...

     Your battery is mounted vertically, and the electrons at the bottom get out of the cells easier....

Ok, I just had to do that.   

So, seriously now, I see two patterns to the discharged voltage readings.
  1.  The first 8 are less than 3 volts, the second 8 are more than 3 volts.

  2.  The cells are increasing in voltage from the first reading to the last reading, with two subsets, again broken down by first/second 8.
         The first 8 readings increase by .07/.08 volts per cell, while the second 8 readings increase by .03/.04 volts per cell.

I must now apologize for being a complete idiot about the  actual implementation of BMS in the battery, and don't really know what to do with that information.  From what I do know, the BMS does not try to balance the battery during discharge, it is only active during the charge cycle. But that doesnt necessarily mean that the BMS circuitry is not connected during the time between charges.  It appears that the BMS is functioning properly during the charge, because all the cells are charged to the same voltage, which is the purpose of having a BMS.

  A part of me wants to find a leakage current path that would cause such a pattern, because I wouldn't expect normal usage to yield such a linear bifurcated pattern.  Part of me  is concerned that the discharged voltage of cell#16 is so very close to the fully charged reading.  When you open the throttle, you will draw an equal current from each of the cells in series circuit.  So each of the cells should decrease in voltage by a pretty similar number, and your list says that cell #1 discharged to about 67% of full charge while cell #16 only discharged to 97% of full charge.   

And part of  me is suspicious of the measuring device or technique. Did you make the measurement by connecting one lead to one end of the battery and use the other lead to move from cell to cell?  Or did you move both leads from cell to cell?  If you used the first method, a non-linearity in the A/D convertor of the DVM could cause such a pattern, but I don't want to believe anyone sells a DVM with such a large inaccuracy.  Was there large amounts of alcohol involved with the measurement?    (Sorry, I'm just a smart mouth old fool)
   

 What kind of range did you get from this battery?  Could you just let it sit for a couple of days after fully charging it, and monitor the cells for self discharge? What caused you to make the measurement? 


TTFN,
Dennis

[Just]

Okay, Dennis, your jokes a re really smart... But, unfortunately, alcohol was not involved to the measurements...
The cause of the measurement was a shorter distance (than expected), which I was able to ride with the battery...
Actually the battery is 48V 10Ah (should be), but when I charge it with 5A charger from a fully discharged state (cells have voltages as described above), it receives only 6.85Ah. This is the reason why I decided to check what's going inside the battery...
As for the discharge process, the battery was fully discharged during my ride, then I just brought it home, open it and made the measurements.
As for the measurement itself, I've actually measured the cells by moving both leads of DVM.
As for the voltage levels of the Cell#0 and Cell#16, they are exactly as they were measured. As for the rest of the cells, they actual voltages may differ from the "real" measurements by 0.1-0.3V (firstly I measured them one by one and only then I wrote their values down from my memory).
Anyway, it really looks like the BMS doesn't balance the cells during their discharge... But should it? Do you know 48V BMS, which support 65A of continuous discharge and also balance the cells? If you know then I want such one!
Anyway, I really don't understand the phenomena so that a discharge rate of the cells, which are connected to the positive pole, is higher than ones, which are connecter closer to the negative pole of the battery. Any explanation (without jokes)?
Thank you!

[Morgen 3Eman]

Hi Dmitryl

I can make up a scenario to cause the voltage readings.  But I don't believe that normal usage discharge should cause the pattern you found.

To make the readings fit a cause,  we need  a different discharge path for each cell.  A BMS balances the cells by switching on a current path around each cell it controls as individual cells reach the target voltage. The current path typically is formed by a resistor and an optical isolator switch controlled by a voltage comparator.  When the cell is charging properly, the opto switch is open, forcing the charge current to flow only thru the cell.  When the opto switch is on, the resistor forms a parallel path around the cell, reducing the charge current for that cell, while keeping the overall current through the string of cells the same.    So in fact we do have such a path.  As I understand it, the BMS is powered up by the charge port supply, not the battery.    I can understand having one physical BMS split into two eight cell electronic sections, as there are lots of chip sets that can be purchased in Byte sizes.  If we pretend that the BMS is still powered up by the battery, not the charge port, the BMS would still draw power, and try to balance the cells. But without the charge current, the resistor would become  a load on the cell, discharging it.

Now all we have to do is make the comparators turn on, one after the other, with some time in-between, and you get the readings you found. 

So to test the idea, can you charge the battery, and see if it discharges without being connected to the load?

TTFN,
Dennis

[Just]

So to test the idea, can you charge the battery, and see if it discharges without being connected to the load?

Okay, I'll do so and let you know the results later. How long time would you suggest to give the battery to be discharged by itself?
Again, if each cell has its own discharge path, why BMS doesn't manage to balance them? Should it?
I'm attaching the BMS's data sheet and connection schema.

[truly_bent]

As I understand it, the BMS is powered up by the charge port supply, not the battery

Just curious Dennis, but if the BMS is only powered by the charge port, then it would be Off during normal discharge use. That would render it useless for any discharge balancing, which I always assumed a BMS was doing.

Have I missed something?
Jeff

[Morgen 3Eman]

Sorry, guys, but I guess I should have said."I would have the BMS powered by the charge port. "  I just checked my GM 48V battery, and the charge port has 57 volts on it all the time.   Which means the BMS is drawing power all the time.  But I did warn you that I was an idiot about the implementation.  I have no idea why it would be left under power, unless it is just for under voltage protection.  But that should only need to be active when the battery is in use, and the key switch is on. 

From what I have read, BMS's only balance the charge, and should have no effect during discharge.  But some of this thread make me think that the BMS is doing something.   

Hey, Alan,  is it possible to easily and safely disconnect the BMS from the cell string to see what the effect would be? 

I really don't like power vampires like that.

Wouldn't it be nice if GM China read and responded to this?  And made us all wiser....

I really need to finish a painting for display next weekend, but I'll do some more thinking...

My guess is that a couple of days of sitting idle will cause the voltage across cell#1 to drop off noticeably. 

TTFN,
Dennis

[Just]

BMS's only balance the charge, and should have no effect during discharge

\
During discharge, BMS should protect the cells from over-discharge and disconnect the battery from the load once it happen.
But, how to force the cells being discharged equally? In the situation, which I described in the  thread, only 50% of the actual capacity is used. Is this normal? Could someone measure cell voltages on the fully discharged battery and let us know the results?
My guess is so that in a "normal" battery with a "normal" BMS all the cells should be discharged almost equally to the same voltage level... Am I wrong? Please share your knowledge.
Thank you!

[Just]

If I connect a discharge port of BMS to its charge port, then will it balances the cells? Will it not destroy/burn the BMS and cells?
Thank you!

[truly_bent]

Ummm... I wouldn't be doing that without a better understanding of the BMS itself.

Color me yellow.
Jeff


The professor and the emperor are probably off for the weekend. Wait for their feedback before trying to commit BMS murder.