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BQ76940EVM: 9 external cells- over and under voltage values

Part Number: BQ76940EVM
Other Parts Discussed in Thread: BQ76940

Hi :) 

Im using the BQ76940EVM with 9 external cells. 

BATT+ and BATT- are being supplied by the external cells. 

The cells are using a 36V supply - each cell can range between 2 to 4V

I want to know the following: 

1) Is the use of a 36V supply okay for this application? 

2) At what point does the BQ76940EVM determine the cell is under voltage or overvoltage? 

3) What setup is required to get the BQ76940EVM to determine the battery is charging or discharging

4) what register must be read on the SMBus/I2C line to read whether the device is sensing a charging or discharging battery? 

Thank you so much

  • Hi Natasha,

    1. 36V with 9 cells would give your 4V you mention.  If using the on-board cell simulation resistors they have tolerance so voltages will vary some.  You may want a supply with more range to drive cells clearly to overvoltage.

    2. The BQ76940 determines OV and UV from the ADC value.  See the data sheet section 10.3.1.2.1 Integrated Hardware Protections. You can calculate the voltage from the gain and offset stored in each part, or from a desired voltage setting you can calculate the register value to enter.  Unused cells which are shorted are not included in the UV check, see UVMINQUAL.

    3. The BQ76940 does not know if the battery is charging or discharging in terms of a bit or status.  It can measure the current for the host to recognize charge or discharge.  The CC is an integrating converter so it will show the net effect of both charge and discharge if the battery has both during the conversion time.  In general:

    1. Set OV and UV levels
    2. Check and clear any fault status
    3. Set registers as desired including CC_CFG, ADC_EN, FET enables, and CC_EN
    4. Read the CC_HI and CC_LO for the current value.  CC_HI will indicate if current is positive or negative and show if current is charge or discharge.

    4.  CC_HI will indicate if current is positive or negative and show if current is charge or discharge.  Since systems will have some variation you may want to collect the full CC value from CC_HI and CC_LO and set a threshold or thresholds to recognize charge or discharge condition from idle.