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BQ40Z50-R2: IMP and CB PF questions

Statham Su
Genius 4990 points
Part Number: BQ40Z50-R2

Hello Sir:

We have a few questions as below please kindly provides your recommendations for us.

a. IMP PF function, please refer NO1_T00013920080000233.gg file.

1067.NO1_T00013920080000233.gg.csv

  1. Can we use the updated range of the Ra table to estimate the range value set by the IMP protection?

  2. Can you let us know delta meaning in the formula of Trip?

  

  3. IMP delta threshold meaning, does IMP use to detect the impedance difference between two cells? does it not one cell exceeding 150%, am I right?

  4. Does TI recommend using IMP for applications with multiple strings? For example, it is not recommended to use less than 2 strings, etc.

b.CB PF function, please refer No10_0754.gg file.

No10_0754.gg.csv

  1. The original design only set CB, without CBR setting, so the main protection active by the long operation time, what is the mechanism for judging the cell balancing start counting time?

  2. According the No10_0754.gg file setting that cannot finish the cell balancing requirement, would you please give us some recommendations? which may help us to optimize our balancing operation.

Looking forward your feedback.

Thanks a lot.  

  • 0
    TI__Guru** 105600 points

    Hello Statham,

    I'm not sure I understand your first question, if you have an aged pack with gauge then you should be able to look at the aged Ra table to determine how you want to set the impedance imbalance PF.

    The equation takes the delta of resistance between all the cells at a specific grid point to use as comparison, if the delta is larger than delta threshold * the design resistance for a specific number of cycles than the PF will trigger.

    It uses the largest delta between all the cell at the reference grid.

    I'm not sure I understand your last two questions, if cell balancing at rest is needed then it should be enabled, especially for applications where it may sit for longer periods of time.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Genius 4990 points

    Hi Wyatt:

    Sorry, it's my mistake, I re-asked these questions.

    1. As the usual condition, if battery was used after 500 cycles which DCR should be increased and capacity should be decreased around 20% to 30% off. According this theory should we set IMP threshold to over 120% to 130%? Is it a better choice? If it's not better choice, please recommend how to set it will be better?

    2. As the usual condition, should we turn on the IMP function on 2s to 4s application? please let me know your recommend.

    Looking forward your feedback.

    Thanks a lot.

  • 0 in reply to Statham Su
    TI__Guru** 105600 points

    Hello Statham,

    This percentage of degradation would vary depending on the cell. and so the threshold may need to be adjusted. Also the IMP is the impedance imbalance, not how much the cells impedance has increased.

    IMP is rarely used, but if it is required for a system then it would only make sense to use it for 2s-4s, 1s cannot have cell impedance imbalance.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Genius 4990 points

    Hi Wyatt:

    Thanks your feedback.

    Still has questions as below.

    1. May I ask what was the consideration when IC designer designing the IMP function?

    2. About cell balancing time, please let me know my understanding are correct or not.

    Please refer my example as below.

    example all QMAX = 2000mA, SOC% and DOD% as below

    Cell n

    SOC%

    DOD%

    dQ

    Cell 4

    91%

    9%

    = CellnDOD- CellLowest_SOC_DOD x CellnQmax ≒ 2000mAh x 91%- 89% x 2000mAh= 40mAh

    Cell 3

    92%

    8%

    = CellnDOD- CellLowest_SOC_DOD x CellnQmax ≒ 2000mAh x 92%- 89% x 2000mAh= 60mAh

    Cell 2

    90%

    10%

    = CellnDOD- CellLowest_SOC_DOD x CellnQmax ≒ 2000mAh x 90%- 89% x 2000mAh= 20mAh

    Cell 1

    89%

    11%

    = CellnDOD- CellLowest_SOC_DOD x CellnQmax ≒ 2000mAh x 89%- 89% x 2000mAh= 0mAh

    According to my example we know that cell 3 dQ is higher than others, so cell 3 should be the first one to start cell balancing, am I right?

    Please let me know your recommendation.

    Thanks a lot.

  • 0 in reply to Statham Su
    TI__Guru** 105600 points

    Hello Statham,

    Many of the features are specified by working with our customers. This was most likely a customer request to have a protection against high cell to cell impedance variation.

    This is not exactly correct, it is not based on SOC, it is based on the DOD, so the range would be smaller than 1% since DOD is measured from 0 to 2^14.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Genius 4990 points

    Hi Wyatt:

    Can you help to make a simple example for us?

    We are not 100% understand it.

    And also provide more explanation on CellLowest_SOC_DOD?? what is it?

    Looking forward your feedback.

    Thanks a lot.

  • 0 in reply to Statham Su
    TI__Guru** 105600 points

    Hello Statham,

    An example would be if it is a system requirement to disable the pack permanently if the cell to cell resistance variation is greater than 200% for more than 10 cycles, and if it reaches 300% deviation it should immediately trip you would set the following registers:

    IMP:Delta Threshold = 200%

    IMP:Max Threshold = 300%

    IMP:Ra Update Counts = 10

    We do not know all the system requirements, so this is decided on a customer by customer basis, we cannot help create them.

    The calculation for cell balance time is usually not helpful for debug since the gauge will always have variations based on internal registers you will not have access to. The lowest SOC DOD is essentially the comparison between the remaining capacity of the cells, so we take the difference between the current DOD0 and the DOD at EOC along with Qmax to calculate the remaining capacity for all the cells and use this for the comparison.

    Sincerely,

    Wyatt Keller