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Cell Balance adjacent problem?

Matt Tsai
Intellectual 330 points
Other Parts Discussed in Thread: BQ76930

I use bq76930 AFE and cell connection is 7cells.
VC0-VC1: CELL1
VC1-VC2: CELL2
VC2-VC3: CELL3
VC3-VC4: short
VC4-VC5: CELL4

VC5-VC6: CELL5
VC6-VC7: CELL6
VC7-VC8: short
VC8-VC8: short
VC9-VC10: CELL7

As described at datasheet,
The host controller must ensure that no two adjacent cells are balanced simultaneously
within each set of the following:
VC1–VC5
VC6–VC10

For 7cells connection,
CELL3 and CELL4 is adjacent? Can I enabled cell balance of CELL3 and CELL4 simultaneously?
CELL6 and CELL7 is adjacent?


Thanks

  • 0
    TI__Guru 54715 points
    Hi Matt,
    Yes, with the cells shorted as shown cell3 and cell4 are adjacent. Cell6 and cell7 are also adjacent. These should not be balanced simultaneously.
    If balancing is turned on simultaneously on these cells, check that the abs max level for the pins and adjacent cell pins are not violated. Also if using external balancing there will be an approximate zero voltage on the common filter resistor, so one of the external paths will not turn on.
  • 0 in reply to WM5295
    Intellectual 330 points
    Hello,
    the abs max level for the pins and adjacent cell pins are not violated
    ==>Could you explain it more detail?
    I don't real understand what is the meaning.

    Thanks
  • 0 in reply to Matt Tsai
    TI__Guru 54715 points
    Hi Matt,
    This may be explained better in some apnote, but:
    As an example, if cell 2 and cell 3 which are adjacent are balanced, VC1, VC2 and VC3 will all try to pull to the same voltage, which if the cells are 4V each would be 8V. The abs max single ended voltage for VC1 is 7.2V which would be violated. If you have low voltage cells it may not be violated. Each condition and combination must be checked, some will be OK while others will not. Rather than try to describe an algorithm to check each condition the data sheet does not recommend using adjacent balancing.
  • 0 in reply to WM5295
    Intellectual 330 points
    Hello,
    The abs max single ended voltage for VC1 is 7.2V which would be violated.
    ==>Do you mean that the cell 1 cell voltage value will become 7.2V (register VC1_HI, VC1_LO)?

    Thanks
  • 0 in reply to Matt Tsai
    TI__Guru 54715 points
    Hi Matt,
    The voltage on the VC1 pin with respect to VSS would be 8V during balance exceeding the 7.2V abs max. During measurement the voltage would return to normal, although with the larger voltage excursion it will take longer to settle to the lower voltage. See www.ti.com/.../slua775, particularly figure 5, and envision not VC0 swinging up 1/2 the cell voltage as shown in the figure, but VC1 swinging up a full cell voltage to meet the VC2 normal voltage. The differential voltage for cell 1, VC1-VC0 would be 8V which is ok.
    If the host were to try to balance cells 2, 3, and 4 simultaneously, the pins would pull higher and both the VC1 single ended limit and the cell 1 differential limit would be exceeded.
    Again the recommendation is to not balance adjacent cells.