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BQ78350-R1: About capacity configuration

Vinicius PW
Prodigy 75 points
Part Number: BQ78350-R1

Hello!

My pack has 2300 mAh, from which I want to set 2100 mAh as Design Capacity and 200 mAh as Reserve Capacity. For the estimation method, I want to use the simplest available, described in SLUA924:


My questions are:

1 - EDV0 should be the voltage which my pack would still be at 200 mAh, my Reserve Capacity?
2 - EDV1 and EDV2, therefore, should be at 263 mAh and 347 mAh, respectively?
3 - The parameters Learned Full Charge Capacity and Battery Low should be initialized? If so, how to I calculate them?

Thank you!

  • 0
    TI__Guru* 81034 points

    Hi,

    The answers to questions #1 and #2 is yes. 

    For question #3, the Learned Full Charge Capacity should be initialized to the same value as your Design Capacity parameter. For Battery Low %, this might be tricky. The TRM says to subtract the percent of the Reserve Capacity of the total capacity. In your case, you are proposing setting 200 mAh which is about 9% of the total capacity. 

    This is high compared to what I have normally seen used for Reserve Capacity so it might have issues.

    Regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 75 points

    Hi, Matt!

    My project has a need for a considerable reserve. In section 9.1.8, the TRM says that if I use 1% of reserve, the Battery Low parameter should be 6%. So, if I change my reserve to 100 mAh, I will have to use 2.65% in Battery Low? There's a problem of this number being less than 3%? I would miss the EDV1 threshold?

    Thanks!

  • 0 in reply to Vinicius PW
    TI__Guru* 81034 points

    The biggest problem with a high reserve capacity is that it moves the EDV0, EDV1, and EDV2 points to a less optimal part of the battery discharge curve. Gauging is most accurate when the EDV points are below the knee of the curve. The majority of the discharge curve is pretty flat. So changing your Reserve Capacity to 100 mAh might give better gauging accuracy than using 200 mAh.

    I checked with the original system engineer for this device. Since you are using Fixed EDV and modifying the EDV values to account for the Reserve capacity, the Battery Low % should not need to change. The Battery Low % setting is really setting the voltage at which Battery Low % (in mAh) + Reserve Capacity is reached.

    Best regards,

    Matt