• State TI Thinks Resolved
  • Locked Locked
  • Replies 9 replies
  • Answers 4 answers
  • Subscribers 65 subscribers
  • Views 201 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

BQ76952: Using very low sense resistor value for high current application

Mett
Intellectual 340 points
Part Number: BQ76952
Other Parts Discussed in Thread: BQ34Z100-G1, BQ34Z100

Tool/software:

Hi TI Team,

We have recently initiated a new 48V battery project, utilizing the BQ76952 and BQ34Z100-G1 ICs on the same BMS board. The pack charge current is 240A continuous, and the discharge power is approximately 1200W (around 30A). Due to the significant difference between charge and discharge, we have separated the charge and discharge paths.(high side drive)

Given the high power dissipation on the sense resistor when using a high-value resistor, we are considering using a resistor value between 0.1 to 0.25 milliohms based on the thermal calculations of the pack. In our previous project, we used a 0.5 milliohm resistor with the same ICs and did not encounter any issues related to calibration, noise, or measurement tolerances.

Could you please advise if there are any potential risks associated with using a resistor as low as 0.1 milliohms in terms of SoC accuracy, measurement errors/tolerances, and noise?

Thanks

  • 0
    TI__Mastermind 19550 points

    Hi,

    The recommended sense resistor size for the 34z100 is 5mohms-20mohms.

    For BQ34z100 the smallest sense resistor I have seen used is 500uohms.

    Is it necessary to have a sense resistor this small?

    What is the capacity of the cells being used?

    What is the max charge/discharge current?

    Regards,

    Diego 

  • 0
    TI__Genius 11045 points

    Hello Mett,

    The sense resistor being around 0.1 to 0.25 mOhms should be okay for the BQ76952, however, there will definitely be a larger room for error. The main concern with the current sense resistor is having a temperature coefficient no greater than 50 ppm in order to minimize current drift with temperature. This BQ76952 How to Calculate Coulomb Counter Current Error FAQ may also be useful.

    Best Regards,
    Alexis

  • 0 in reply to Alexis_H
    Intellectual 340 points

    Hi Alexis,

    Thanks for the answer on BQ76952 IC. We will consider temperature coefficient while selecting shunt resistor.

    Do you have any suggestions on BQ34Z100-G1 behavior if we use sense resistor <=0.25mohms? What would be the impact on SoC accuracy for BQ34Z100-G1?

    Best Regards,

    Mett

  • 0 in reply to Mett
    TI__Genius 11045 points

    Hello Mett,

    Unfortunately, I’m not familiar with the BQ34Z100-G1. Diego will be able to assist more.

    Best Regards,
    Alexis

  • 0 in reply to Alexis_H
    TI__Mastermind 19550 points

    Hi Mett,

    The BQ34z100 is specced for a sense resistor range of 5mohms to 20mohms. I have seen customers go as low as 500uohms without issue. 250uohms I think will also be possible. 100uohms will require a nontrivial work around. It should be noted the smaller the sense resistor less accurate the current reading will be, which can affect SOC accuracy, which is why we recommend customer stay within spec.

    I really recommend you use as much of the voltage range on the SRN/SRP pins as possible. (+/- 125mv)

    Is it necessary to have a sense resistor this small?

    What is the capacity of the cells being used?

    What is the max charge/discharge current?

    Regards,

    Diego

  • 0 in reply to Diego
    Intellectual 340 points

    Hi Diego,

    Thanks for the answer. Apparently we will use 250uohms in this project due to having thermal constrains in the battery pack. Pack capacity is 49Ah, max charge current is 240A and max discharge current is 35A(Peak 65A, 1s)

    Best Regards 

  • 0 in reply to Mett
    TI__Mastermind 19550 points

    Hi,

    I am currently looking into this, please allow me sometime.

    Regards,

    Diego

  • 0 in reply to Diego
    Intellectual 340 points

    Hi Diego,

    Sure, Looking forward to hear from you.

    Regards,

  • 0 in reply to Mett
    TI__Mastermind 19550 points

    Hi,

    I am currently talking with Elliot, please talk with him for faster communication.

    Regards,

    Diego