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BQ77905: bq7790503 , 4 stacked togther, not giving any output

user 1234
Prodigy 185 points
Part Number: BQ77905

1. I have stacked 4 bq7790503, to make 100Amps of board.

Intially the boards worked fine, now suddenly I have there is no output from circuit at pack+ & pack-.

I have traced back & checked that on both charge & discharge N-channel mosfet , there is zero voltage.  So thats why there is no output.

How to track which bq7790503 out of four, is making entire outout zero.

  • 0
    TI__Guru 54715 points
    Hi User,
    I would suggest measuring the CHG(U) and DSG pins of each device with respect to its reference cell (or device DVSS). Avoid the loading shown in www.ti.com/.../slua774 figure 10, measure on the "upper" part in the figure.
    The bq7790503 requires load removal for both under voltage and current faults. You may need a charger to bring PACK- sufficiently low to clear all devices.
  • 0 in reply to WM5295
    Prodigy 185 points
    For 20 series cell i.e 4 stacked bq77905 what value of resistor u suggest to use for cell simulatoin.
    I am using 1ohm,20 in series they got very heat up.

    What value u suggest
  • 0 in reply to user 1234
    TI__Guru 54715 points
    Hi User,
    The bq77905 EVM user guide suggests 200 ohm for cell simulation resistors. Some users will select 1k ohm, but voltages can move a lot with any disruption.
    Some users will use 10 ohm 3W resistors to hold the voltage more stable if there are disruptions from measurement equipment, but this dissipates a lot of heat. As well as being uncomfortable, check surface temperatures of the resistors and surrounding surfaces when using small resistor values.
    A 10M ohm meter or scope probe will affect a high resistance simulator more than a low resistance one. Also be aware of the impedance of the nodes you are probing, for example CTRC and CTRD are high impedance signals as mentioned above.
    Select a resistor value which supports your equipment, intended tests, and provides a safe operating condition.