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BQ24103A: Charging a flat battery

Siegbert Brockmann
Prodigy 50 points
Part Number: BQ24103A
Other Parts Discussed in Thread: BQ24105

Hello,

I am using the BQ24103A as stand alone 2 cell Li/Ion charger. The charger work fine however I can not charge a flat battery where the protection circuit of the battery pack the shut off the cells. Normally, a small charge current is sufficient to reset the protection circuit. Whenever I power up the charger I do not see a voltage rise or a current flow at the charger output for a short time (observed by oszilloscope). I expect to be in precharge phase. The current I_set2 is set to 147mA.  The circuit of the design is similar to figure 16 in the data sheet.

What is wrong or do you have a work around?


Regards,

S.Brockmann

  • 0
    TI__Guru**** 166996 points
    Hi Siegbert,

    What is the state of the STAT pins? How much capacitance is on the output? Is it possible that the battery detection circuit as explained in datasheet section 8.3.16 is not functioning properly?

    Regards,
    Jeff
  • 0 in reply to Jeff F
    Prodigy 50 points
    Dear Jeff,
    all STAT pins are high impedance. The output capacity is higher than the recommended value. But I need higher capacities in this particular application. Is it possible to increase the detection current or time?

    Regards,

    Siegbert
  • 0 in reply to Siegbert Brockmann
    TI__Guru**** 166996 points
    Siegbert,

    When the sensed battery voltage is < VSHORT= 2V, the charger thinks the output is shorted and sources a fixed 35-65mA ino the battery until an independent timer expires. Taking TTC low does not disable that timer.

    One work around would be to add a series resistor and zener diode off of VTSB and then Schottky diode connect that directly to the FB pin. The zener voltage needs to be higher than the FB voltage would be when the battery is at VSHORTmax= 2.05V *no of cells + diode drop. This will disable the charger's short circuit protection and place the charger directly into precharge mode whenever the the battery voltage is below VLOWV.


    Regards,
    Jeff
  • 0 in reply to Jeff F
    Prodigy 50 points
    Dear Jeff,

    Thank you for your reply. Since the BQ24103A has no FB pin I am wondering how to source a current into that node. Is it possible to use the BAT pin including an additional series resistor to the battery in order to decouple the zener diode? Will be in this case the voltage regulation stable and accurate?

    Regards, Siegbert
  • 0 in reply to Siegbert Brockmann
    TI__Guru**** 166996 points

    Hi Siegbert,

    I apologize for the confusion with the BQ24105.  With the BQ24103A having CELLS instead of FB, my idea will not work.  I do not think driving into the BAT pin will work because of its relationship with the SNS pin.  Is it possible to change to the BQ24105?

    Regards,

    Jeff