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BQ29706: BQ29706 for lithium iron phosphate battery in 2S configuration, and cell balancing

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Replies: 7

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Part Number: BQ29706

Hello,

I want to use  BQ29706 and BQ29200 for 2S1P configuration of lithium iron phosphate (LFP) battery. I have few questions regarding this:

Q1: As a LFP has upper voltage limit of 3.65V, can I use BQ29706 (OVP=3.85v) for cell protection?

Q2: If yes, can I scale this configuration to 2S1P by adding two similar configurations as used in Q1?

Q3: Can BQ29200 (2-cell balance IC)  be used in cell balancing of configuration used in Q2? As the battery I want to use is  LFP battery whose nominal voltage is 3.2v, however, the IC BQ29200 is for 4.2v. Since the IC BQ29706 has already over-voltage protection, can this BQ29200 be used for balancing without using its over-voltage protection feature?

Thanks!

  • Hi Dilip, 

    To stack up 2 BQ29706 for a 2P config is tricky. You need to consider how to deal with V- of each device and how to combine the outputs. We haven't seen this implementation before. Voltage wise 3.85V is good for LFP. 

    The BQ29200 could balance the unprotected cells (cell side of the protector), so it has solid ground. 

    Thanks and regards, 

    Sandy 

  • In reply to Sandy Xu:

    Thanks for your reply!

    Do you mean 2S instead of 2P in the reply sentence "To stack up 2 BQ29706 for a 2P config is tricky."?

    Dilip.

  • In reply to Dilip K:

    Hi Dilip, 

    Yes I meant to say 2S...

    Thanks, 

    Sandy 

  • In reply to Sandy Xu:

    Hi Sandy, Dilip, 

    I'm looking at doing exactly the same, providing balancing and protection for 2s.  When you say it is tricky to handle outputs and V-, is this because you anticipate trying only to use one set of FETs?  if each circuit had its own pair of FETs wouldn't each circuit just work independently., unaware of the other cell?

    many thanks

    David

  • In reply to David Bleloch:

    HI David, 

    Do you mind providing a drawing? Both sets of FETs would need to be on the ground line. 

    Thanks and regards, 

    Sandy 

  • In reply to Sandy Xu:

    Hi Sandy,

    sorry for the delay - please see a modified version of your simplified layout from the datasheet.  Each circuit appears to be self-contained.  I might have missed something really obvious...

    The layout below shows the proposed topology where the two left-hand cells are Lithium Sulfuryl Chloride or Lithium Thionyl Chloride primary cells.

    Our load is placed across the red and black terminals.  Yellow terminal is for voltage monitoring only.  We would not require cell balancing as the LiSCl cells would determine the terminal voltages on the high current cells.

    many thanks

    David

  • In reply to David Bleloch:

    Hi David,

    I suppose if you are attaching the cells individually, this may work. I am concerned about what would happen if one device was in fault mode and the other was not. 

    Have you looked into some of our 2S protectors? These may be a better fit. I would recommend looking at the BQ2961 or BQ2962 families. Additionally, you could use a BQ29200 and disable cell balancing.

    Thanks,

    Shawn

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