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Original question:

BQ27441-G1: External resistance

BQ27441-G1: Sense Resistor

Carlos Herrera M.
Prodigy 50 points
Part Number: BQ27441-G1

Hi,

I have a question about the sense resistor that I have to use with the BQ27441.

I have a LiCoO2 battery with a maximun charge voltage of 4.2[V] and 4000[mAh] of capacity. The load consumes about 6[A] for 3[ms] and then go to rest for about 2[s].

Im not sure about the value of the sense resistor that I have to use because I put the recommended 10[mOhm] value, but sometimes BQ reports me 0% of SoC but battery keeps running for about one hour more until voltage reaches 3.0[V].

BQ's Datasheet on point 8.1 (page 7) Integrating ADC (Coulomb Counter) characteristics, says that a typical Vsr is Bat+-25[mV]. I asumed that maximun differential voltage is 50[mV] so, with a resistor value of 10[mOhm] the highest posible current to measure is 5[A]. In consequence, Im not covering my full range of current (6 amps peak).

Anybody could give me any direction about that. Is the differential voltage of 50[mV] right?. Do I have to use a 8[mOhm] sense resistor instead of 10[mOhm]?

Thanks in advance

  • 0
    TI__Guru* 76230 points

    Hi Carlos

    Your gauge reporting 0 is not the fault of the  sense resistor you used. It is likely due to your settings in data flash.

    Did you run gpc chem to determine if the hard coded chem id on the device are a good match for your cells? Did you configure the gauge settings properly following the quick start guide?

    thanks

    Onyx

  • 0 in reply to Onyx Ahiakwo
    Prodigy 50 points

    Hi Onyx,

    thanks for your answer.

    I did not run gpc chem, just followed the quick start guide instructions to set the registers. Maybe some value in data flash is wrong.

    How ever, this is not a regular issue. I mean, in some discharging test I've been able to observe the problem (see the picture below)

    However, I will run gpc chem to be sure and double check the settings in data flash.

    But, still my question is do I need to calculate the sense resistor for a better accuary?. It is because, if I'm not wrong, the full load current that I can measure with a 10 miliOhms resistor is (differential_voltage/Rs)=50[mV]/10[mOhm]=5[A]. But for my devices I need to sense 6[A] at full load.

    Also, for BQ27441 is 50[mV] the right value for differential voltage?

    Thanks for your advices.

  • 0 in reply to Carlos Herrera M.
    TI__Guru* 76230 points

    Hi Carlos

    I don't believe you can use this gauge. The minimum sense resistor recommended is 5 mohm and max is 20mohm

    The max current the gauge can support is actually 25 mV. So with a 5mohm sense that puts you at 5A. You need a gauge with 0.125mV like the bq27520.

    thanks

    Onyx

  • 0 in reply to Onyx Ahiakwo
    Prodigy 50 points

    Hi Onyx,

    Yeah... I guessed something like that but I was not sure at all.

    Thanks a lot  for your collaboration. I will check Bq27520.

    Kind regards