BQ76940: AFE Cell Balancing

Hi Nayab,

The BQ78350-R1 cell balance test toggle sets alternate bits in the balance registers.  The part can't measure balance current, that must be done by test equipment to check the board inputs. Another aspect of the test feature is that it is alternate physical cells, it does not account for use of the AFE Cell Map which may indicate a cell is unused and that 2 odd cells are now adjacent. The test feature may still be used at board level if the cell voltage can be changed by the tester to 0 so that the adjacent cell can be checked for proper balance operation. 

When cells are attached the test feature still works, but there would be no measurement port for current from the cells. It would be possible to monitor the voltage across the resistors on the board to tell if the circuit was working, but it is expected that access to the circuit is limited once cells are attached.  Also as a test mode it will operate until turned off, so cells could be drained if the test is not terminated appropriately.  So the user is cautioned not to use it with cells.

Cell balancing operates at a relatively low current compared to most cell capacity, it is intended to make small adjustments to the cell over time. It is difficult to see once cells are attached.  Testing the cell balance circuit operation before cells are attached is suggested.

Hi Nayab,

While we know testers will vary, in our estimation most  test systems will have a variety of power sources available to supply the board.  These might be dedicated or switched as needed.

The challenge is really where you have reduced cell count in a group by 1 cell, so typically 14, 13, or 12 cells. In this case the odd cells turned on by the BQ78350-R1 are adjacent and there is not sufficient control voltage on the central resistor to turn on the external balance FET.

If you have one supply per cell you can set the cell voltages to any value desired and monitor the current from the individual supplies to see if balance performs properly.  Considering 14 cells in a battery with BQ76940, cells 13 and 15 are adjacent physically. For test you can set cell 14 to 0 and cell 13 to 4V to test cell 13 balance, then set cell 13 to 0 and c3ll 14 to 4V to test cell 14 balance. The other cells would be set to some nominal voltage. 

If you have 3 supplies and 14 cells (VC13 and VC14 are connected), the cells 13 and 14 of the system will be on cells 13 and 15 of the monitor (BQ76940) and the BQ78350-R1 cell balance toggle will balance both cells on the "odd" balance condition.  Normally this will not work properly for balancing.  As an example if your supplies are single ended and can source and sink current your could set cell 14 input = 50V, cell 13 input = 46V, cell 12 input = 46V for a net 0V on system cell 13. You should be able to see balance current from the cell 14 supply into the cell 13 supply.   Once that is confirmed you can set cell 14 input = 50V, cell 13 input = 50V, cell 12 input = 46V to test the cell 13 cell balance function. If the other cells are provided by a resistive divider the balanced cells will collapse during their balance time. 

If you have a floating supply that you can connect to any cell, you could short the adjacent cell, that is perhaps set cell 14 input to 50V with a resistive divider for the other cells.  Then set the floating supply on cell 14 at 40V.  Short cell 13.  Set the odd cell balancing condition, confirm current from the floating supply for the balance condition.  Then remove the short, connect the floating supply to cell 13, short cell 14, and confirm balance current into cell 13. 

Consider the resources you have in your test system and design a test algorithm which checks the appropriate signals.  Use as much parallelism as your system allows or you desire. Remember that each cell group in the BQ76940 must remain above VSHUT, so operate, so you must have some nominal operating voltage for the BQ76940, then at least one source (if floating) to provide current for the balanced cell under test.  The other inputs would need to maintain the operating group battery voltage during transient.