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BQ76952: Protecting/managing 4S battery

Eddy Bergers
Prodigy 20 points
Part Number: BQ76952

Hi,

We are evaluating the use of the BQ76952 for protecting a series combination of 2 batteries (LiFePo4 in a 4S configuration). No balancing is required. Can we trick the cell inputs on the BQ with a voltage divider so it will correctly evaluate the 2 batteries over- and under voltage conditions?

Thank you.

  • 0
    TI__Guru 54715 points

    Hi Eddy,

    It sounds like you have an 8S assembly with your two 4S batteries in series.   The BQ76952 can monitor 8 cells and control protection FETs.  If the cell taps are available you should be able to hook this up.  Since there are extra cell inputs on the BQ76952 one of the extra cell inputs could be used to monitor the interconnect of the batteries if desired.  If the individual 4S batteries have protection this will be a challenge, the switches in the cell stack would need to be shorted  and control given to the BQ76952 circuit since the BQ76952 inputs can take up to 85V, but not reversed cell or pack voltages.  For example if your 4S batteries have high side protection and the switch between the 2 batteries opened with the cells at 3V and a load attached, there would be -24V across the "interconnect" cell input.

    If you don't have access to the cells, for short term monitoring you could divide the battery voltages back to cell voltages with a resistor divider.  The input resistors to the BQ76952 should be about 100 ohm maximum, so the cell divider resistors should be small and would be a significant drain on the batteries.  With the high drain on the cells this method would be useful only good for a short term trick.  Protection switching in the batteries would still be a damage risk to the BQ76952, switching in the stack should be bypassed, the control for the switching should be sent to the properly configured CFETOFF/DFETOFF inputs of the BQ76952 to respond to conditions not recognized with the average voltage monitoring.

  • 0 in reply to WM5295
    Prodigy 20 points

    Hi,

    Thanks for the quick response.

    Your description of our setup is correct. We don't have access to the individual cells, only to the central connection between the 2 '4S' batteries. Later on in the project we want to move to controlling individual cells (replace the 2 batteries with a cell stack). We would like to integrate the BQ76952 with its protection features already at this point.

    The batteries (Super-B Andrena 12V 5Ah) have only a simple equalisation on board, no protection or turn off devices.

    Is it necessary to go as low as 100R for the resistor value? We only want to monitor the voltage on the batteries to detect over/under voltage.  I can't imagine that the sense input current of the mux has any meaningful value since it is always present. The spec reads 2MOhm of effective differential resistance for the sense inputs.

    As indicated call balancing should be inactive when we use the '4S' batteries. I hope the BQ76952 can be programmed as such. Did not work through the complete documentation yet so your help is greatly appreciated to get a kickstart in this.

    Eddy

  • 0 in reply to Eddy Bergers
    TI__Guru 54715 points

    Hi Eddy,

    Eddy Bergers said:
    I can't imagine that the sense input current of the mux has any meaningful value since it is always present.

    Correct, the differential input is high, but there is sampling current when the input is periodically measured which will vary with the common mode voltage and can be observed in the measurement with larger resistors. You might set up the board with the resistors you expect to use, then create your divider with the largest resistors you can tolerate for your test time.  Adding caps across the divider resistors will help stabilize the voltage during measurement.  I don't have good recommendations since we don't do this, a few hundred ohms and 1 uF may be a good starting point.