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BQ27441-G1: Remaining Capacity goes to zero prematurely

Andy Sturman
Prodigy 75 points
Part Number: BQ27441-G1

I am using the BQ27441-G1A with a 3400mAH lithium ion battery.    The battery drives an LED that draws approximately 3 amps when the battery is fully charged and ends up at 400mA when the battery is at 2.9 volts.    I set up the parameters according to the quick-start guide:

design capacity = 3400

design energy 3400 * 3.7 = 12,580

terminate voltage = 2900

taper rate = 3400 / (.1 x 100mV) = 340

 

I am confident these are getting set because I can read them back at a later time.

When the battery is fully charged, I can read back the full charge capacity and remaining capacity and they seem reasonable.

When I discharge, the remaining capacity starts to go down as it should.  The problem I’m having however, is that it reaches zero much too early.    If my discharge cycle takes 100 minutes, the remaining capacity drops to zero around 75 - 80 minutes.    And it doesn’t drop gradually, it goes from around 350 mAH to zero suddenly.   There is still 3.1 volts on the battery at that point.

After running a dozen charge-discharge cycles the problem is not better, in fact it seems a little worse.

Questions:

1.       Am I doing anything obviously wrong ?

2.       Has anyone else seen this ?

3.       I looked at someone else’s code who worked on this before me and he was setting the R_a0 table with numbers based on a golden cell.    The problem I’m having happens whether or not I set these numbers.

4.       Are there any other parameters I must set that are important ?

5.      Is there errata on this part ?    Could I have an old rev ? 

 

Thanks!

  • 0
    TI__Expert 4450 points

    Hello Andy,

    The unaccuracy of RM and FCC may caused by the unright OCV, unupdate of Qmax and Ra table, or larger change of temperature.

    First, bq27441 is a fixed chemID IC, make sure you have choose the right chemID. Different chemID may response to different termination voltage for bq27441.

    Then, do self-learning cycle to make sure Qmax and Ra table have been updated, check your update status register.

    Hope this could help you. Thanks.

    Best regards,

    Sammi Shang

  • 0 in reply to Sammi Shang
    Prodigy 75 points

    Dear Sammi,

    Thank you for your fast reply.

    What do you mean by  "unright OCV" ?      Do you mean the BQ chip is not accurately measuring OCV or the battery is not putting out the right OCV ?

    I checked the battery chemistry and it is LiCoO2.

    I am not doing any self-learning or setting Qmax and Ra.   Where do I find out the exact procedure how to do this?    Can I use my board or do I have to get the evaluation board ?

    I am only setting up Design Capacity, Design Energy, Terminate Voltage, and Taper Rate.   With these 4 settings shouldn't the gauge have enough information to learn by itself ?  How many charge-discharge cycles does it need ?   I have run about 12 and it seems to get worse not better.

    As far as temperature,  the chip is near the load, and it gets pretty warm. Could that be the problem ?

    Thanks,

    Andy

  • 0 in reply to Andy Sturman
    TI__Guru 62386 points

    Andy,

    The bq27441-G1A uses built-in tables (OCV, impedance) for a model of a typical LiCoO2 cell with a 4.2V max. charge voltage. If your cell isn't a good match (the LiCoCO2 chemistry isn't necessarily a guarantee for a good match) then the simulations that the gauge runs with a (badly matching) model may cause errors like you observe.

    Qualifying charge/discharge cycles will adjust the impedance table (over time) to better match your actual cell (it will also adjust for aging of you cell) but if the OCV table isn't a good match then accuracy can be permanently off.

    A qualifying cycle consists of minimum charge/discharge amounts and relax times. If your application regularly discharges and charges the cell (and gives it time to relax) then learning is automatic. An ideal cycle is described in: Note that your application doesn't have to go through the full learning cycle from this document. The default configuration will let the gauge update it's impedance tables and chemical capacity with less tight cycles.

    You can use your board to accomplish a learning cycle automatically if you charge/relax/discharge/relax/charge/relax... the cell. Please make sure that your application relaxes the cell (current < quit current) for approx. an hour and that you charge and discharge the battery from 100% to 0% SOC.

    It will update its model paramters after each cycle - you don't need 12 or more. The gauge uses filters to make sure that model parameters don't jump so it may take a few cycles if the initial settings (e.g. design capacity) are off by a large amount.

    Temperature will affect the result. The gauge runs simulations to determine the remaining capacity (and SOC). Temperature is one parameter that will affect the simulation. If the temperature sensor reports higher temperatures than at the cell the gauge will over-estimate remaining capacity and if the actual cell voltage drops sharptly below the termination voltage (because the cell is cold) then the gauge will make remaining capacity jump to 0mAh.