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BQ40Z50-R2: Cell Balancing for LiFePO4

Yoshinobu Kagawa
Expert 2010 points
Part Number: BQ40Z50-R2
Other Parts Discussed in Thread: BQ40Z50

Hello Kang-san,

Question from Customer(TOKADO Energy) about bq40z50-R2 cell balancing.

【Background】
A prototype pack (connection: 4S2P) was charged at a charge voltage of 14.4V (3.6V/cell) using a lithium iron phosphate battery (full charge voltage: 3.6V / cell), but passed 3.4V / cell It turned out that the balance of the cell collapsed from around and it stopped by overcharge.
The difference between Max and Min of the cell was Δ100mV. (IC used: bq40z50-R2)

[Cause]
The charging characteristics of the lithium iron phosphate battery are characterized by a charging curve as shown in the graph below, with the voltage rising from around 3.4 V. Individual differences in each cell are prominent at this point, which seems to disrupt the balance. You.

[The figure has already been sent to you.]

[Countermeasure]
Therefore, this time, the prototype pack will charge the battery before charging the voltage rise point on the graph.
However, since the capacity will decrease, we want to charge at 3.6V / cell, which was originally planned.

[Request for confirmation]
Is there an effective charge control method (bq40z50-R2 setting) when the balance of each cell starts to collapse after 3.4V/cell as described above?

Best regards,
Kagawa

  • 0
    TI__Genius 12215 points

    Hi, Kagawa San

        How do you think about using the JEITA feature which is supported by bq40z50 to address this issue?

        For example, you can define a charging current for the cell voltage range from 3.4v to 3.5v or 3.45v, which is lower enough for the balancing to take effect and not to cause too much voltage discrepancy between the individual cells.

        Have you used external balancing circuit to balance the cells? This can be also considered to use to ease such problem.

        For the details on how to define the charging current per JEITA spec, please refer to the details in section 4.1 of bq40z50 TRM