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BQ27421-G1: Accuracy of battery capacity prediction in SLEEP mode

Johannes Brecher
Prodigy 110 points
Part Number: BQ27421-G1

In the SLEEP mode the current consumption is much lower than in NORMAL mode.
(21µA in SLEEP mode compared to 93µA in NORMAL mode).
To reduce the systems energy consumption, I would prefer that the fuel gauge is in SLEEP mode as often as possible.

The default value for the Sleep Current is 10mA.
Imaging a system that uses roughly 3mA to 7mA - is it OK to leave the Sleep current at 10mA or will this drastically reduce the accuracy of the battery capacity prediction of the fuel gauge?

What would be a good value for the Sleep current in such a system?

  • 0
    TI__Guru* 76230 points
    if 3-7mA is the normal current range of your application, then sleep current has to be reduced to below 3mA if you want the gauge to be in normal mode otherwise the gauge will continuously be in sleep mode where it wakes once every 20 s to take measures and perform SOC calculations
    thanks
    Onyx
  • 0 in reply to Onyx Ahiakwo
    Prodigy 110 points
    Thank you for your fast reply.

    Do I understand this correctly: If the gauge switches to its SLEEP mode, the accuracy of the battery capacity prediction will be worse than in NORMAL mode, since all measurements are done in an 20s interval instead of an 1s interval?

    Or do you mean that in NORMAL mode the battery capacity value is refreshed every second and in SLEEP mode after 20s, but the battery capacity prediction would be the same and the only difference is that in SLEEP mode the prediction value is not updated that often?
  • 0 in reply to Johannes Brecher
    TI__Guru* 76230 points
    The first option. it wouldn't be terribly off because the current draw in sleep mode is so low that it wouldn't exert significant pull on the battery
    thanks
    Onyx
  • 0 in reply to Onyx Ahiakwo
    Prodigy 110 points
    There would be significant pull on the battery, since the system would still draw current from the battery.

    So, to clarify this: Can you give me an idea of the accuracy of the battery capacity prediction in the following scenarios:

    Scenario 1:
    1. The battery is full.
    2. The system is running for 5 hours
    3. The consumption of the system varies between 3mA and 7mA
    4. The sleep current is set to 10mA (=> the gauge would always be in NORMAL mode)
    5. The remaining battery capacity is read from the fuel gauge once. (after 5 hours of operation)

    Scenario 2:
    1. The battery is full.
    2. The system is running for 5 hours
    3. The consumption of the system varies between 3mA and 7mA
    4. The sleep current is set to 3mA (=> the gauge would be in SLEEP mode most of the time?)
    5. The remaining battery capacity is read from the fuel gauge once. (after 5 hours of operation)

    Thanks in advance!
  • 0 in reply to Johannes Brecher
    TI__Guru* 76230 points
    Johannes,
    You mixed up the modes in the scenarios.
    in scenario 1, the gauge will be in sleep mode since the curreent 3-7mA is below the sleep current.
    in scenario 2, the guage will be in normal mode since the current will be above the sleep current. I can say for sure that the gauge will be more accurate in scenario 2 than 1 since there will be no missed currents. To evaluate performance you will have to test this out as there is no way to put a number on the accuracy results.
    thanks
    Onyx