BQ35100: Sense resistor value check?

Julio Apps

Part Number: BQ35100

The datasheet in the 8.2.2.5 Current Sense Resistor Selection says "The BQ35100 device calculates current through measuring a voltage across a small resistor in series with the battery. The default value is 100 mΩ. To maximize current measurement accuracy, the ideal value is calculated as:

RSENSE (mΩ) = V(SR)Max / Peak Load Current (mA)
Where V(SR) MAX = 125 mV"

I am concerned that we may not have a sufficient voltage drop to make this chip work correctly as we have a buck/boost converter between the battery and the comms module that shapes the current we sample. We are transmitting for 80-100ms with ~190mA current sourced from the buck/boost and its Vout caps. The buck/boost will reduce this current value on its Vin input which is connected to the battery. What minimum voltage drop across the sense resistor would still work for the BQ35100 chip? Shall we use an op-amp to boost the Rsense volage or stay with 100 mΩ ?

over 5 years ago

0 Vos over 5 years ago

TI__Genius 10370 points

Julio,

There is no magic number we need to see because as you say the senseR value can change and we can calibrate to it. What is the current that you expect to see across the senseR through the buck/boost?

ideally we would like to size the senseR to exercise as much of the ADC's full range as we can. I would not add an op-amp to the RSense at this point. you should be able to size correctly to measure.

Thanks,

Eric Vos

0 Julio Apps over 5 years ago in reply to Vos

TI__Expert 3665 points

Thanks Eric,

We see 80mA to 100mA peak current. See the image below. Is 0.1 ohm sensing resistor going to work then?

0 Vos over 5 years ago in reply to Julio Apps

TI__Genius 10370 points

Julio,

The amplitude should be fine. At 80-100mA the gauge will be able to measure that accurately. The issue with the image you provided is the duration. The load must be at least 32mSec long.

It is also critical the battery voltage drops at least 50mV under load

Thanks,

Eric Vos

0 Julio Apps over 5 years ago in reply to Vos

TI__Expert 3665 points

Eric,

Thanks for the feedback, it will use the following battery : https://www.ultralifecorporation.com/ECommerce/product/er34615m/batt-li-socl2dsshr

The load will jump from 0.15mA to the pulse shown in the plot above. It appears will be ok with the 50mV drop.

The load

0 Julio Apps over 5 years ago in reply to Julio Apps

TI__Expert 3665 points

Hi Eric,

Would holding the current sense peak voltage and the battery valley voltage in an external circuit make the BQ35100 algorithm work correctly? Is really all they need to do is have GE (active) duration for 32ms in the impedance measurement mode to make this method work?

0 Vos over 5 years ago in reply to Julio Apps

TI__Genius 10370 points

Julio,

You can use a power supply to simulate the gauge. However it would be very difficult to do manually. the battery voltage needed to change under the pulse load during the gauge start section. if they are not causing the drop in the middle it will not work.

Thanks,

Eric Vos

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BQ35100: Sense resistor value check?