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BQ24120 - Over discharged battery is not indicating a fault condition.

Using a power supply to replicate an over discharged battery (0v), when this apparatus has been applied to the charger that uses the BQ24120 chip, no fault condition is being detected. The battery charger should not try and charge this battery and voltages at STAT1 and STAT2 (page 17) should be pulled LOW but this is not the case. What does happen is that STAT 1 constantly pulses from LOW to HIGH. The operational flow chart (page 13) indicates that a timer starts when Vbat < Vlowv a timer starts for 30 minutes, I have waited longer than 30 minutes and still there was no change to STAT1 pulsing from LOW to HIGH.

I have ruled out that the setup is incorrect as when the power supply has been set to replicate an over charged battery (>4.8v), the fault condition has been established and the charger does not try and charge it. Instead STAT1 and STAT2 remain LOW.

Any thoughts of why this is happening and how to allow me to enter the fault condition for an over discharged battery cell?

  • Hello Micheal,

    Have you tested this set-up with the simulated battery above the Vshort voltage threshold of 2V but still below the precharge threshold of about 75% of the regulation voltage?

    I am thinking that you are not reaching the precharge timer since the device is testing to see if the battery is shorted.

    Try testing with a simulated battery voltage above 2V and let me know what you see.

  • Hi Ryan,

    Thanks for your quick reply and your ideas,

    Unfortunately one of the initial tests I had carried out was when the voltage was set at 2.3v. This voltage is still Higher than VSHORT, yet Lower than 71% OF VOREG (2.98V) and as presumed, the outcome was the same as before.

    Any other suggestions Ryan would be much appreciated.

    Micheal

  • Hello Micheal,

    How fast is the pulsing of the STAT pins?

    Are you just connecting a power supply straight to the BAT terminals for your testing? What I am thinking is that the supply that you have connected to the BAT is holding it to firmly in a voltage range where it is trying to detect and charge a battery and since it is not accepting current and not dropping when current is pulled the detection algorithm by cycling between attempting to charge and a battery absent fault.

    A number of the other EVMs provide an example battery simulation schematic like the one I have attached below.

    I will have to check with some of my colleagues to see what there thoughts are for this case.

  • Ryan,

    see below for the outcome when STAT1 was connected to a scope. Does this help describe what may be happening? 

    Can you explain the difference in the BQ24187 and the bqSWITCHER as the bqSWITCHER is descrived as the Evaluation board for the chip in question.

    Also can you confirm that this circuit could be used to help give a better representation of a battery load and if so, can you point me in the direction to purchase it.. And if not can you direct me to a suitable piece of test equipment that can be purchased to help replicate a battery.

    but again, i would still like to understand why the power supply can be a suitable piece of test equipment to simulate a battery and detect over charged but not an over discharged.

    Thanks Micheal

  • Ryan,

    Any update on this matter?

    Micheal