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bq34z100 minimum sense resistor.

Masami Akazawa
Intellectual 505 points
Other Parts Discussed in Thread: BQ34Z100

Hello,

I'm developing a 10 cell LIB battery pack using the bq34z100.
Since I'm worried the heat, I want to use the 1mohm to sense resister.
Is it available under the following conditions?

- sense resister : 1mohm
- maximum current : 50A (duration 3sec)
- normal current : 4A
- using X10 mode

Also, please teach me the next.
 1. In the above case, How much does it reduce the accuracy compared to using 5-20mohm resister?
 2. Minimum resistance that can be used for sense resister
 3. Do you have any use case of 1mohm?

  • 0
    Intellectual 505 points

    Does anyone develop a battery pack with using less than 1mohm sense resister?
    Please teach me whether accuracy of sense resister of 1mohm is practical, or where can I get the document about it.
    No matter how small information, I would like some information as soon as possible.

    thanks.

  • 0
    Intellectual 505 points

    Thank you for reading.

    Please let me know if you find some deficiencies in question...

    Because I can't write English well.

  • 0
    TI__Guru 73830 points

    The measurement accuracy should be the same for a 1 mohm resistor as it is for a 5 mohm resistor. The problem that you will have with smaller sense resistors is with the ADC offset. The accuracy with detecting smaller current will be reduce. The typical input offset error is 10uV, so the device can detect down to 10uV/1 mohm = 10mA with a 1mohm sense resistor. This should be okay for your application. We have seen users use a 500 uohm sense resistor in some applications.

  • 0 in reply to ThomasCosby
    Intellectual 505 points

    Thank you for your answer.

    I would like some information about the impact to the degree of accuracy.
    (Such as how to calculate the error of capacity measurement)

    you seems to have seen the users who used the 500uΩ in some applications, but whether there was a problem of accuracy this user.
    Did TI make any guarantee when there is a problem? (The user's own risk?)

    If you have any information at that time something, please teach me.

    In addition, There is also a proposal to use the 2 mohm.
    So, if you have any infomation about the 2 mohm apprication, please teach me.
    (If I use the 1mohm resister and the threshold of sleep 1.25mV, it will be ignored 1.25A, influence on the capacitance measurement is too large)

    Thank you.

  • 0 in reply to ThomasCosby
    Intellectual 505 points

    Let me further questions.

     

    I want to understand the operation of sleep mode.

    Please Let  me know the follows.

     

     1. Conditions for the start of sleep mode

    The 23 pages of the data sheet, it is written as follows. 

    SLEEP mode is entered when (1) AverageCurrent() is below a programmable level Sleep Current and (2) if the [BUSLOW] bit of Pack Configuration() is set and the data bus (both SCL and SDA low pins) is low for 5 s.

    I want to know about time threshold. AverageCurrent() is below Sleep Current even momentarily, the device is in the sleep mode?

    Sleep Current is written in the above and  Sleep Current is Data Flash Summary (data sheet page 41) is the same?

     

     2. Conditions for release of sleep mode

    The 24 pages of the data sheet, it is written as follows. 

    The wake up comparator is used to indicate a change in cell current while the bq34z100 is in SLEEP mode. PackConfiguration() uses bits [RSNS1-RSNS0] to set the sense resistor selection. PackConfiguration() uses the [IWAKE] bit to select one of two possible voltage threshold ranges for the given sense resistor selection.　An internal interrupt is generated when the threshold is breached in either charge or discharge directions. A setting of 0x00 of RSNS1..0 disables this feature.

    So interrupt, can the device be release from sleep mode within several hundred milli seconds?

    If RSNS = 0x00 then how the device release from sleep mode?

      

     3. What is the difference between normal mode and sleep mode?

    The 23 pages of the data sheet, it is written as follows. 

    In SLEEP mode, the gas gauge exists in a reduced-power state, periodically taking measurements and performing calculations.

    Does the device include the result of measurement in sleep mode to coulomb count?

      

    thank you for reading.

  • 0 in reply to Masami Akazawa
    TI__Guru 73830 points

    1) You must be using an older version of the datasheet, because the BUSLOW bit is no longer a feature. I found that the device will enter the Sleep state whenever the Current parameter is below the Sleep Current threshold.

    2) The device will still enter and exit the Sleep state if the RSNS0:1 bits are set low, but it will take longer. I found that it takes about 14 seconds for the current to increase.

    3) The difference between Normal mode and Sleep mode is the frequency of the Current, Temperature and Voltage measurements. They will update every one second in Normal mode and typically every twenty seconds in Sleep mode. The device will not be in Sleep mode if current is present, so there should be no contribution to Passed Charge.

  • 0 in reply to ThomasCosby
    Intellectual 505 points

    Thank you for your answer.

    I've found a lot about sleep mode.
    Just in case, the following is correct?

    1) Conditions for the start of sleep mode
    AverageCurrent < Sleep Current

    2) Conditions for release of sleep mode
    - Iwake < Current(charge or discharge)
    - Sleep Current < AverageCurrent

    3) The difference between Normal mode and Sleep mode
    - the frequency of the Current, Temperature and Voltage measurements
    - AverageCurrent which is measured in sleep mode is no contribution to Passed Charge.

    In addition, because there is no confidence about FULLSLEEP mode, please let me know whether the following is correct.

    1) Conditions for the start of Full Sleep mode
    Duration of sleep mode > Full Sleep Wait Time

    2) Conditions for release of Full Sleep mode
    - Iwake < Current(charge or discharge)
    - Sleep Current < AverageCurrent
    - Receiving of HDQ or I2C

    3) the difference of Full Sleep mode from Sleep mode.
    - the high frequency oscillator is turned off in Full Sleep mode

  • 0 in reply to Masami Akazawa
    TI__Guru 73830 points

    Your assumptions look correct.