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BQ40Z80: Internal Cell Balancing at Rest

Kiel Orwig
Prodigy 30 points
Part Number: BQ40Z80

Hello,

Is using the cell balancing at rest feature in an internally balancing design suitable for 2S,3S,4S, and 5S Li-Ion packs?

If so, what are the recommended values for Relax Balance Interval, min RSOC for Balancing, and min Start Balance Delta?

Regards,

Kiel

  • 0
    TI__Guru 61536 points

    Yes, as long as the balancing current is less than 10mA.

    The default values are recommended values.

  • 0 in reply to Dominik Hartl11
    Prodigy 30 points

    Page 74 of the Sluubt5a specifies the internal cell balancing FET as having an Rdson of 200 Ohms maximum.

    Page 34 of the BQ40z80 datasheet also states,

    "External series resistors placed between the cell connections and the VCx I/O pins set the balancing current magnitude. The internal FETs provide a 200-Ω resistance (2 V < VDS < 4 V). Series input resistors between 100 Ω and 1 kΩ are recommended for effective cell balancing."

    If a cell is at 4.2V with a resistance of (RVCx+Rdson) = 100+200=300, I would assume that (4.2/300) = .014 => 14mA might pass through the internal FETs.

    To protect the internal MOSFET, do you know what the minimum value for the RVCx resistors can be to ensure there is less than 10mA going through the internal FETs during cell balancing? 

    Is the 10mA limit a requirement for cell balancing during charging as well, or only during balancing at rest? 

  • 0 in reply to Kiel Orwig
    TI__Guru 61536 points

    For a 4.2V drop over VC(n+1) - VC(n), the total resistance must be >= 4.2V / 0.01A = 420 Ohm, so with the internal 200 Ohm resistance, the minimum external resistor is 220 Ohm.

    The 10mA is the limit that you can drive through the internal FETs. This is independent of when the balancing algorithm runs (charge or rest).

  • 0 in reply to Dominik Hartl11
    Prodigy 30 points

    Thank you for answering my question sufficiently. 

    The suggestion of 100-1k resistors in the BQ40z80 datasheet seems off to me, though.

    If the maximum current through the FETs can only be 10mA and I use the 100 ohms in series as suggested, then my maximum voltage allowed would be about 3.0V for the 300 ohms series resistance. If I'm using a Li-Ion pack this voltage seems too low to get accurate cell balancing. A note/update in the datasheet concerning this might prove useful.