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Original question:

BQ40Z80: Ext. balancing for 5S BQ40Z80

BQ40Z80: Cell 6 external balancing FET not turning on.

Daniel Librio
Intellectual 390 points
Part Number: BQ40Z80
Other Parts Discussed in Thread: BQSTUDIO

Hi,

I was wondering if you could please confirm my circuit configuration for external balancing, specifically around the 6th cell?

I am experiencing an issue where Q3A is not turning on fully or at all, reducing balance current significantly on this cell.

In this first oscilloscope capture for Cell 4, CH1 (Yellow) is the gate of Q4A, and CH1 (Blue) is the node between R21 and Q4A.  You can see that Q4A is clearly turning on under control of the BQ40Z80, as evidenced by CH2 (Q4A Drain) being pulled low by around 4V, causing balance current to flow through the 51R resistor.  (Note this was captured in Sleep mode, with CBS=1 hence the longer pulse times):

This is how I'd expect it to look for all cells undergoing balancing.

In the second capture for Cell 6, the same nodes are sampled around Q3A. Note that, despite the gate being toggled, the node at Q3A and R19 only drops by around 430mV, and thus a fraction of the desired balance current flows:

On a second battery pack I tested, the FET for Cell 6wasn't turning on at all.

These FET's have a very low VGHth of Max. 1.2V, so I would expect them to be fully on with 2V at the Gate.  I suspect a biasing issue of some sort.  

Note that the oscilloscope was grounded to Batt-.  I plan to repeat the test with a battery powered scope grounded to the Source of each FET to confirm the Gate-Source voltage.

Any thoughts would be greatly appreciated!

regards,

Daniel

  • 0
    TI__Guru** 105770 points

    Hello Daniel,

    It looks like the schematic shared did not get uploaded correctly, can you try re-uploading the image?

    Can you confirm that cell 6 is the one the gauge intends on balancing? Some of the cell voltages on the gauge side will fluctuate when the gauge is balancing other cells, that is why we must do one cell at a time for external balancing, and also cell voltage measurements.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Intellectual 390 points

    Hi Wyatt,

    Thank you for the prompt reply.  Yes, in both cases cell 6's bal time was higher than all other cells, as was the cell voltage (this is what prompted my investigation).

    I've attempted to attached the schematic again, please let me know if it appears.  If not I could PM you a pdf perhaps?

    Did the oscilloscope captures appears correctly?

  • 0 in reply to Daniel Librio
    TI__Guru** 105770 points

    Hello Daniel,

    It looks like all the configurations are the same between cells, so I don't think the issue would be with the schematic, it could be with the layout, but this is such low current I don't think that would be the case.

    Can you share the bqStudio log file and .gg from before the log? I'd like to take a look at the configuration and what the gauge is estimating for cell balancing.

    Our gauges mostly use a delta-capacity measurement for cell balancing between cells, so if the gauge thinks the 6th cell had more charge or the battery actually does have more charge the gauge will keep trying to balance that cell.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Intellectual 390 points

    Hi Wyatt,

    Thank you.  I've checked over the PCB layout again and can't see any issues... trackwork reflects the schematic, and the balance FET/resistor are located close to the Cell 5 and 6 nodes, with traces <10mm long.

    I've attached configuration and log files.  Whilst the log was being taken, Cell 6 Bal time was equal highest with Cell 5 and 2, and CB was set, so I'd expect it to be balancing.  In any case, I've observed Cell 6 balancing at the drain of Q3A over many hours, and attempting to change settings to ensure it was balancing (eg. turning CBM off, charging the pack, etc), and so far I've not seen anything indicating significant current through the FET (ie. voltage drop across the 51R resistor).  

    /cfs-file/__key/communityserver-discussions-components-files/196/SN011-Configuration-15-Feb-2022.gg.csv

    /cfs-file/__key/communityserver-discussions-components-files/196/SN011-15-Feb-2022.log

    I've also attached an oscilloscope capture demonstrating the Q3A Gate voltage of 2Vp-p (The oscilloscope was grounded to node 5P (ie. Q3A Source), with the probe tip at the Gate of Q3A.  2V seems a little low (I believe I've seen 4Vp-p on other cells balance FET Gates), however it should still be sufficient to turn the FET on.  I've purchased some alternative FET's (EM6K7T2R) which have a lower VGSth to trial, however this test will have to wait till I have time to build a new pack.

  • 0 in reply to Daniel Librio
    TI__Guru** 105770 points

    Hello Daniel,

    With the CBM enabled you should not see 4Vp-p between the FETs Vgs voltages, with the external cell balancing enabled it should only control one balancing at a time. If the CBM = 0 with external FETs it could lead to differences in the Vgs between all he external FETs with some being higher if they're overlapping.

    The Vgs of the external FETs should ideally be as low as possible, since the external balancing basically uses a resistor divider to create the Vgs, if the cells are at 3V and need balancing, the Vgs would be approximately 1.5V (with the default 1k ohm resistors in the external network).

    Sincerely,

    Wyatt Keller