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BQ40Z80: bq40z80 balancing problem

markand
Prodigy 65 points
Part Number: BQ40Z80 
Hi
I am having balancing problem with bq40z80.I am using 4s LFP cells.
When external balancing fets are available and external balancing is active, 3 series are balanced, but 1 series becomes cov very quickly.
When I remove the external balancing fets and use only internal balancing, the 3 series is still balanced, but 1 series cannot exceed 3600mv. 
This also applies when the balancing function is completely turned off.

You can find the balancing schematic and charging log records in the appendices.

MPTS_VOK_EN_NOEXTERTNALFETS.log
cb_disable.log
  • 0
    TI__Guru** 105760 points

    Hello Markand,

    From the logs it looks like the gauge is trying to balance the cells when CB is enabled, I'm not sure if this is a cell balancing related issue. If one of the cells has a higher internal resistance or trace resistance, when charging this will lead to something like you see, where the voltages are close, but when current is removed the voltages deviate from each other.

    The cell balancing the BQ40z80 uses is based on the capacity of each cell, this means it might balance each cell to a different voltage since it is not voltage based balancing. For LFP batteries this could cause problems since the OCV is so flat, to perform cell balancing we require on OCV to calculate each cells DOD and use this to balance the cells.

    Sincerely,

    Wyatt Keller