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Original question:

BQ35100: BQ35100 Accumalator mode question

BQ35100: Accumulator mode operating philosophy.

Sam Pattemore
Intellectual 480 points
Part Number: BQ35100

Hi, we would like to use BQ35100 but after reading all that we can about the unit there is still a big unanswered question.

Our application uses about 7uA  for the majority of the time and has pulses up to a few hundred mA our hardware is designed to operate for around 10 years on a primary Li-SOCL2 battery.

We wish to utilised BQ35100 in Accumulator mode to monitor battery usage.

The datashseet states average current for accumulator mode of 0.3uA, this sounds great but we are not sure what this actually means.

The datasheet also states current for accumulator mode of 130uA (about 10x too much for our  applications)

We have done some interpretation on the datasheet and come up with 3 contrasting understandings of how the device operates.

Option 1.

- host initialises the BQ35100 and tells it to start accumulating

- BQ35100 accumulates continually (at an average of 0.3uA)

- during this accumulation, the BQ35100 may or may not be able to "capture" very short pulses (datasheet mentions 1sample/second

- the host can access the accumulated value as infrequently as required 


Option 2.

- BQ35100 uses 130uA whilst accumulating.

- Host needs to continually start and stop BQ35100 accumulating to perhaps focus on active power states like Tx, Rx or MCU Processing to be able to get low average consumptions like the stated 0.3uA

Option 3.

-BQ35100 in accumulator mode simply monitors current,voltage,temperature and the host must continually read this data and implement some algorithm to determine battery usage 

- the host would monitor accumulator only during periods of peak power states to ensure that average  consumption  for battery monitoring tasks is minimised

Option 1 is how we hope the BQ35100 works and makes sense for a device designed to gauge consumption on long-life/ IOT applications

Option 2 is inferred from the two very different accumulator current requirements which are not explained.

Option 3 comes mostly from this description of accumulator mode.

"7.3.2.1 In this mode, the BQ35100 device measures and updates cell voltage, cell temperature, and load current every 1 s. This data is provided through the I 2C interface while ControlStatus()[GA] is set. To begin accumulation, the GAUGE_START command should be sent, and when accumulation ends, the GAUGE_STOP command should be sent. To ensure that no data is lost, the host should wait until G_DONE is set before powering down the device."

We have been reviewing TI's documentation for some time now and have not been able to fully understand how this works, any assistance would be greatly appreciated.

TIA

  • 0
    TI__Genius 10370 points

    TIA,

    As you state the bq35100 should not be awake for the majority of the time. It's average consumption of 130uA is not useful when your base application is 7uA. Instead it should be woken up to gauge the mA pulses of your application. The 7uA should be estimated by your host, using the bq35100 periodically to make sure the 7uA is your correct average. The bq35100 has a continuous coulomb counter sampling 65k/sec so it should be able to capture even the smallest of pulses. 

    for LiSOCL2 cells the other option would be EOS. This requires alot more host intervention and the battery to be at a state of OCV, but it will give you an indication when you have reached the exponential growth part of the impedance curve. It looks for a change in the batteries internal impedance to indicate end of life.  

    Thanks,

    Eric Vos

  • 0 in reply to Vos
    Intellectual 480 points

    Thanks Eric, 
    That partially answers my question, please allow me to ask a couple of follow up questions.

    1. Do we need to write over I2C every time we want to start/stop accumulating (I.e whenever the device wakes and sleeps) or do we just need to set the enable pin.

    2. After 1, do we need to wait for some startup period before we do work (ie. start accumulating some period before entering higher energy states? 

    3. we assume that the host doesn't need to continually monitor the accumulator but could check the ACC/SOC when we want to update the battery state variable (say 1/week or 1/month). is this correct?

  • 0 in reply to Sam Pattemore
    TI__Genius 10370 points

    Sam,

    1) Yes after the GE goes high you need to issue the Gauge_Start command. Then If you want you can issue the gauge stop to commit the captured capacity to flash memory

    2) There is some minor command processing time, but it should be in the mSec range. 

    3) SOC is not updated in Accumulation mode. Only the other modes SOH/EOS offer an SOC. You would need you host to do the math to figure out how much it has left

    Thanks,

    Eric Vos