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BQ27421-G1: Valid SOC issue

Eran Dahan
Prodigy 70 points

Part Number: BQ27421-G1
Other Parts Discussed in Thread: BQSTUDIO

We are trying to get SOC of our battery with the following config:

Design Capacity =  2350mAH

Design Energy = 3.8 * Design Capacity = 8930

Terminate Voltage = 3050mV

Taper Rate = Design Capacity / (0.1* Taper Current)   //  Taper Current=50mA

sometimes the fuel gauge reports wrong SOC.

one case is when the battery was almost full but SOC reports around 60% 

1. If we are using the configure of the parameters mentioned above, Do we need to charge the battery to full for one time for each pcb/device before fuel gauge will report valid SOC?

2. If we will run the learning cycle - Do we get valid SOC immediately after connecting the battery or we need charge to full also before get valid SOC?

  • 0
    TI__Guru** 107655 points

    Hello Eran,

    The gauge only reports incorrectly sometimes on the first initialization? Was the battery fully charged when you plugged it in? When the battery is inserted the gauge will take an OCV reading to try to estimate the SOC. After the gauge is cycled it should update it's RAM with battery specific information like Qmax and Ra tables to more accurately gauge the battery.

    Because this is a ROM based gauge, if the power is disconnected the gauge will lose it's Qmax and Ra table from learning and will have to relearn them.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 70 points

    Hello,

    Answer 1: Yes, the gauge reports incorrectly sometimes on the first initialization.

    Answer 2: No. we are getting the battery from manufacturer charge to ~60% 

    I would like to clear the issue of Valid SOC:

    consider that are going to production phase (no matter if we are using golden file or the minimum parameter config method),

    Is it mean that we need to charge each product (PCB and its battery) to full before sending it to customer to get the accurate SOC? 

    I'm asking this because I found in some of your application note that 100% is the sync point of the fuel gauge.

  • 0 in reply to Eran Dahan
    TI__Guru** 107655 points

    Hello Eran,

    If the battery is inserted near the flat region of the battery voltage curve the OCV measurement taken upon insert may have a hard time to correlate the voltage to an SOC reading. 

    For best gauging on upon insert the battery should be relaxed and outside of the flat region of the battery voltage curve.

    If you could insert the battery at 100% and relaxed that would be ideal for the gauge.

    Which app note are you referencing? 

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 70 points

    Hello,

    The app note which I'm referring is SLUUAH7B - Quickstart Guide for bq27421-G1:

    1. for the meaning of "insert battery at 100% relaxed" - can you specify some threshold for discharge current or it should be 0[A] to be called relaxed? for how long relaxed? is this any flag in the gauge that say that we are ready with valid SOC the we can based on? can you advice for some an automatic method with some logic criteria ? it is essential to define clear procedure during production phase.  

    2. Can you advice for an app note which guide how to perform the learning cycle automatically using the BQstudio app ?

    Thanks,

    Eran

  • 0 in reply to Eran Dahan
    TI__Guru** 107655 points

    Hello Eran,

    I think I see what you are referring to, this is for setting the taper current during charging, you want the gauge to know the battery is fully charged before the charger stops charging so you can gauge the full range of the battery.

    Before you insert the battery wait for the battery to be relaxed and outside of the flat region for voltage. If you could charge the batteries and let them rest for a couple hours before insertion the gauge would take a OCV from the beginning outside the flat region and it can make a better SOC prediction, otherwise to get accurate gauging you may have to cycle the battery.

    100% SOC and relaxed, for the battery to become relaxed it can be left disconnected for a couple hours after charging.

    Currently there is no app note to show how to set up an automatic learning cycle.

    Sincerely,

    Wyatt Keller