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BQ76940: bq76940 general queries

vaibh P
Intellectual 260 points
Part Number: BQ76940
Other Parts Discussed in Thread: TIDA-010030, TIDA-00792, TIDA-00255

Hi , 

We were working on BQ76940 from last few months and there is XREADY Fault on previous boards which we had already discussed here.

So now we have  a revised version and following are the some problems as mentioned in the attached file.

Note : All the three boards have same components.

Cell voltage readings and Battery pack reading are coming to be same but CC reading on each board differs .

BMS_v1.0_Queries_1_28_June.pdf

  • 0
    TI__Guru 54715 points

    Hi Vaibh,

    1. CC Readings: 

    There is no known reason for the noise or variaiton of the CC readings to increase suddenly, with time, or differ significantly from board to board.  Offsets with or without current may occur if the CC_CFG REGISTER (0x0B) register value is overwritten from its recommended value.  Also the CC is powered by the REGOUT supply, you might check for differences in the effectiveness of the capacitor on that pin, or loading differences which might somehow feed back into the IC affecting the CC readings.

    2. Balancing currents:

    Yes, balance current can be changed.  Smaller resistor values or more resistors in parallel will increase the balance current.

    3. Current while charging causing OV:

    If higher charge current causes an OV fault, check for unexpected resistance between the cells or in the cell.  You might check with your cell supplier to understand if there is some limiting behavior of the cells. On the board check where the top and bottom cells are sensed to see if interconnect resistance is contributing to the apparent cell voltage.

    4. Connecting parallel BQ76940 modules:

    Usually cells are paralleled for more capacity before connecting to a battery management electronics system, for example a 13 cell battery with 1 cell might be referred to as a 13S1P battery while a battery with 2 parallel cells would be a 13S2P battery.  The cells in parallel are seen by the battery management circuit as a single cell with higher capacity.  When considering paralleling modules you may have to deal with the modules being at different voltages and protecting at different times. Some ORing controller would need to decide which module should be loaded or charged at what time. The BQ76940 does not have features to provide this, those would need to be provided by some other subsystem.  Each module may need its own FETs to avoid high currents if 2 modules of different voltage were connected, or to stop use of the module when a single cell goes high or low in voltage. 

  • 0 in reply to WM5295
    Intellectual 260 points

    hi , thank you for your suggestions.

    For 4th point , Kindly please suggest any reference design for Battery Management System using Lithium ion cells of capacity each 3.7V/2600mAh for battery pack of 48V/20A.

  • 0 in reply to vaibh P
    TI__Guru 54715 points

    Hi Vaibh,

    I don't know of a reference design for parallel modules, such implementations are unique to the system. In general ORing might be achieved with diodes or ORing controllers. You might look at products starting at http://www.ti.com/powerswitches 

    For reference designs for 48V/20A you might look at the reference designs in the BQ76940 product folder, TIDA-00255, TIDA-00792, and TIDA-010030.  You may need to make adjustments for your particular system needs.