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BQ27Z746: 1S Parallel Cells Configuration Understanding

rimark
Prodigy 65 points
Part Number: BQ27Z746

Tool/software:

Hi, 

May I get some advise on whether connecting 2 x Lithium Ion Cells in parallel (1S) would be suitable for this device?
How would you go about connecting the 2x NTC thermistors to the TS pins?
The cells are not part of the same pack in my application. 
Any further guidance/tutorial on battery charging from TI is kindly appreciated. 

  • 0
    TI__Mastermind 24931 points

    Hello, 

    Connecting two cells in parallel should not be an issue, although you would not be able to connect two different thermistors to the gauge. 

    Regards, 

    Robert. 

  • 0 in reply to Jonny
    Prodigy 65 points

    Hi Robert, 


    Thanks for the kind response. In that case would it be better to select a suitable 2S charger to allow for measuring the temperature from two individual dedicated thermistor pins? Or would you recommend having 2 x BQ27Z746 (one for each cell)?
    Reducing cost is also key to my design.

    Deeply appreciate your time,
    Richard

  • 0 in reply to rimark
    TI__Mastermind 24931 points

    Hey Richard, 

    You should be able to use a multi cell gauge to utilize multiple thermistors.

    Regards, 

    Robert. 

  • 0 in reply to Jonny
    Prodigy 65 points

    Thanks for your help Robert! Deeply appreciate it. Can i consider the following as well:

    Since the battery is connected in parallel separately, have each battery with its on protector/gauge IC with temperature sensing input.
    The output for each battery with its protection PCBA then goes onto an i2C interface to the host MCU which also communicates with this 1S battery charger IC via I2C. 

    Is this architecture recommended? The thermistors are not directly to the battery charger now but to the bms protector chip for each cell. 


    Regards,

    Richard 

  • 0 in reply to rimark
    TI__Mastermind 24931 points

    Hey Richard, 

    You should be able to do this, one thing to consider though is you will have to have your host MCU interpret the gauge parameters (i.e. State of Charge and State of Health) from each of the gauges, and report them in such a way that makes sense to the end user. Essentially you would have to determine your own way to average the SoC and SoH and other values reported by the gauge so that these would not exceed 100%. 

    Regards, 

    Robert. 

  • +1 in reply to Jonny
    Prodigy 65 points

    Hi Robert, 

    I assume in this configuration, the SOC will be quite similar from both fuel gauges?
    From my very basic understanding of battery charging, both batteries connected in parallel tend to balance each other out as the voltage is constant.
    (Hence cell balancing tend to be considered in series configuration rather than parallel)

    I definitely agree that SOH will be different overtime from both cells even if they came from the same part number and manufacturing batch. 

    Please see my attached system diagram to confirm what we have discussed. 

    Thank you!
    Richard


  • 0 in reply to rimark
    TI__Mastermind 24931 points

    Hello Richard, 

    I think you misunderstood what I am saying here. You can use the two gauges, but each gauge will report their own SOC for each of the cells, It is possible, for example, that one is at 98% SoC and the other is at 97% SoC. It is up to you and your MCU for how you want to interpret this. 

    Regards, 

    Robert.