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Original question:

BQ34Z100-G1: design energy scale value setting problem

BQ34Z100-G1: Correct values for battery pack 13S16P Li-ion

Dario Fresu
Prodigy 11 points
Part Number: BQ34Z100-G1

Dear Ti team,

Here are the parameters of the Li-ion battery pack I am working with:

Single-cell capacity   3500 mAh
Single-cell voltage   4200 mV
Number of cells in series   13 Cells
Number of cells in parallel   16 Cells
Real design capacity   56000 mAh
Maximum voltage   54600 mV

Maximum current around 100 A.

I have read the SLUA760 and found no troubles with even cells number in series with lower capacity.

I have some doubts regarding the following:

1. Should I calculate design energy using the single-cell voltage or the pack voltage?

2. Since the design energy is higher than 32767mWh, the scale factor should be 8?

Real Design energy
(56 000 mAh * 4.2V)		   235200 mWh

235200 / 32767 = 8?

3. If so, should the value be set as follow?

CC Threshold   6300 mAh
Design capacity   7000 mAh
Design Energy   29400 mWh

4. Should I use the design energy factor for the current related parameters?
such as Taper current, Quit current, Qmax Cell 0, etc?

4. When calibrating the current should I input into the Coulomb counter value: (Real value of the current / Design energy factor )?

Thank you for the support.

Dario

  • 0
    TI__Expert 8315 points

    Hey Dario,

    I'll go ahead and answer this question by question Slight smile

    1. For calculating the design energy you should use the pack voltage

    2. The scale factor should indeed be 8 here because as you mentioned the ceiling of 235299/32767 = 8

    3. These values seem to be correct

    4. Not 100% sure what you are asking here but if you mean should you use the scaling factor of 8 for the current related parameters rather than a scaling factor based on current then I would say yes as your current based scaling factor would be (100,000mA /32767mA=4). You always use the greater of the two scaling factors for both current and capacity based factors and since 8>4 you should use 8.

    5. Yes, you should divide the current you use for input for calibration by 8.

    For more info you can also refer to this app report (I found it very helpful myself as well)

    https://www.ti.com/lit/an/slua760/slua760.pdf?ts=1628782754166&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FBQ34Z100-G1&_ticdt=MTYzMDQyMzczMnwwMTdhOWIxMTczZGQwMDE3NTMwZTczMjBjYzk2MDMwNzMwMDFiMDZiMDBiZDB8R0ExLjIuODk3MzA4Mjg5LjE2MjYwOTkyNTA

    Thanks,

    Jackson

  • 0
    Prodigy 11 points

    HI Jackson,

    Thank you for your reply. 

    If as you mentioned the Design energy should be calculated with the pack voltage then:
    Design Energy (mWh) = 56 000 mAh * 54.6 V = 3,057,600 mWh

    then in this case the Scale Factor would be = 3,057,600 mWh / 32767 = 93.32

    How should I proceed from here? 

  • 0
    Prodigy 11 points

    HI Jackson,

    Thank you for your reply. 

    If as you mentioned the Design energy should be calculated with the pack voltage then:
    Design Energy (mWh) = 56 000 mAh * 54.6 V = 3,057,600 mWh

    then in this case the Scale Factor would be = 3,057,600 mWh / 32767 = 93.32

    How should I proceed from here? 

  • 0 in reply to Dario Fresu
    TI__Expert 8315 points

    Dario,

    I believe there was a bit of confusion here. I was meaning design capacity not design energy for scaling so my apologies there. I would take a read of that document I attached in the previous reply and go from there because design energy is not mentioned there and a scaling factor of 94 seems quite high.

    I believe for voltage scaling you may run into issues because you have an odd number of series cells so I would make sure to set the VOLTSEL bit in the pack config register. 

    For current scaling you use either current or capacity for scaling taking whichever one is the greater value. This is what I was intending to explain in my previous reply but was not so clear.

    Sorry for the confusion here but let me know if you have further questions. Again, the document attached should be able to provide all the info you need as well.

    Thanks,

    Jackson

  • 0 in reply to Jackson DiGiovanna
    Prodigy 11 points

    Jackson, 

    Thank you for your support. The SLUA760 unfortunately is not that clear on how to handle battery packs with high energy.
    For high current and high capacity, there are no problems and I found the document helpful. 
    The problem comes in when you try to set a battery pack not only with high capacity and high current but also high energy (and sometimes an uneven number of cells) in particular when the registers:

    Design Energy and Design Energy scale need to be set properly, otherwise, the readings from the Gauge are wrong.


    For those of you coming across the same issue, here is a link that helped me:

    Thomas Cosby 


    Here are the steps I took to solve the problem:

    1. The energy for the Design Energy register should be calculated at the single-cell voltage level and not with the total voltage of the battery pack.
         So, even if in my case the configuration of the battery pack is 13S16P, the energy is calculated as: 4.2V * (3500mAh * 16 ) =235 200 mWh. 
    2. Since the calculated energy is higher than the register 'Design Energy' max value (32767), then scaling should be applied.

                     In this case the Scale factor energy factor calculated is : Scale factor energy = 235 200 mWh / 32767 = 8.

    1. Since the maximum current for my application is only 50 A, the scale factor calculated for the current is 

                  Scale factor current = 50A/ 32 = 2.

    1. The total capacity of the pack is calculated as:

                 Total capacity = cells in parallel * single cell capacity(mAh) = 16 * 3500 mAh =   56 000 mAh. Since the value is higher than the max value (29 000                 mAh), then scaling should be applied. In this scale the Scale factor for the capacity is:

                  Scale factor capacity = 56 000 mAh / 29 000 = 2.

    1. The scale factors calculated are:

                   Scale factor current = 2.

                   Scale factor capacity = 2.

                    Scale factor energy = 8.

    Since the Scale factor energy is the highest, then this scale factor should be applied for almost all the current and capacity-related parameters. The exceptions are listed in the presentation from Thomas Cosby.

    Here are the values after scaling has been applied with a scale factor of 8:

    Scaled MAX current

     

    6.25

    Ah

    Scaled Capacity

     

    7000

    mAh

    Scaled Energy

     

    29400

    mWh

    Values inserted in the registers:

    CC Threshold

     

    6300

    mAh

    Design capacity

     

    7000

    mAh

    Design Energy

     

    29400

    mWh

    SOH load I

     

    -6300

    mA

    Qmax Cell 0

     

    7000

    mAh

    Design Energy scale

     

    8

     


    When calibrating, the current should also be scaled by the same value. So if the current applied to the load is -8000 mA, then the value -1000 mAh should be reported to the Gauge in the Applied Current field. 

    If you try the same steps, do it at your own risk. This is only informative.

    Also, I should mention that the voltage was not scaled in my case as I have chosen a resistor for the voltage divider (and the Voltage divider register value) so that max voltage does not exceed 1V. Hence, the voltage is not scaled and the register Number of cells in series is set to 13.

    Hope this helps.

    Dario