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BQ76930: This is a question about the bq76930 evm kit.

kim myoungsoo
Prodigy 65 points
Part Number: BQ76930
Other Parts Discussed in Thread: BQ76940,

Hi...

I want to use BQ76940 EVALUATION SOFTWARE. When trying to activate the CELL BALANCE function of this tool,

 it is okay to fill in CELL BALANCE1,2,3(0x1,0x2,0x3) - CB1,CB2,CB3...CB15 with DATA "00",

but enter a value other than DATA "00". fill As the "DEV_XD" BIT of the "SYSTEM STATUS(0x0)" REGISTER is set to "1",

 the "DSG_ON", "CHG_ON" BIT of the "SYSTEM CONTROL2(0x5)" REGISTER changes to 1->0, making FET OFF.

 How should I set the CELL BALANCE REGISTER when this problem occurs?

 Also We ask if the functions of CELL BALANCE 1,2,3, (0x1,0x2,0x3) are necessary for cell balance.

 I would like to know how to set the voltage difference between each cell necessary for cell balancing.

Is the BQ76930 CHIP automatically balancing by default? I ask you

  • 0
    TI__Guru 54715 points

    Hi Kim,

    Yes setting the cell balancing register values to 00 (the default) is correct for normal operation of the board. 

    The BQ76930 and BQ76940 EVMs have a resistor divider made up of 200 ohm resistors.  When a cell balancing bit is set in one of the registers the inputs monitoring that cell will pull together loading the resistor and shifting the voltage on all cells.  If you set one bit at a time the board should continue to operate and you should see altered voltages.  If you set multiple bits at the same time you will likely get an XREADY fault since the voltage will collapse on one of the upper cell groups.  If the voltage collapses on the bottom cell group you will likely need to reboot the part. To recover from the condition boot the part if needed, clear the cell balance registers (0x00), clear the XREADY if present and re-enable the FET outputs.

    To demonstrate cell balancing you will need multiple sources.  One of the simplest methods is to use one source or supply for the board, and a floating supply set at the cell voltage to connect to the cell you want to balance.  The floating supply will hold the voltage across the resistor for that cell constant when the extra current is drawn for the balancing action. 

    The BQ76930 provides the mechanism to activate balancing of the cells, it does not have an algorithm for balancing and can not balance automatically.  It can not turn off balancing automatically, so the host which is controlling the balancing must remain active to turn off the balancing current.

    The balancing algorithm implemented in the host is your choice.  Many designs will balance only during charge because that is when charge is flowing into the battery.  However the charge current can influence the observed voltage, so some designs will balance when the battery is idle but near fully charged.  A simple balancing method is if the host sees a cell higher than the others by some threshold, it will balance that cell.  Balancing should normally run some seconds.  The BQ76930 duty cycles the balancing current to continue measurement every 250ms. Since one cell may be lower than the others, all the high cells will need to be balanced down to match.  The host should use some algorithm to select which cells to balance, that may be determined by the heat allowed on the board, the BQ76930 architecture allows balancing up to every other cell in a group.  See section 4 of the application note https://www.ti.com/lit/pdf/slua749 for more information on current paths and an issue which can occur due to the shared path at the cell group boundary.  Also know that cell balancing can influence the voltage readings due to large filter time constants, particularly when balancing multiple cells, that can be observed on the EVM.