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BQ769142: cell balancing: wave of balancing!

hamzeh ahangari
Prodigy 111 points
Part Number: BQ769142

Hello,

I was testing cell-balancing feature using a battery simulator (a group of seri resistors). I saw an interesting behavior which can cause problem. I noticed when cell balancing is activated, the read cell voltage of adjacent cell raises to a level that even can cause pass COV level. For example, all my cells are around 3.7-3.8V. When balancing on one is activated, the adjacent cell even may pass 4.2V and trig my COV condition. Even if COV is not triggered, this deceives the balancer, where after a while, it switches to adjacent cell. This even can repeat where balance can have a repeated switch over all cells while they are not really un-balanced. I used a 1K resistor on each cell wire.

thank you

  • +1
    TI__Guru* 81035 points

    Hi Hamzeh,

    Yes, this is a common issue when users try to test cell balancing with a resistor divider. You will not see this issue when you are using actual battery cells. I plan to explain this in the next revision of the Cell Balancing document since it can be confusing.

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 111 points

    Thanks Matt,

    I want to ask a question, as I am curious about something. When balancing is activated around one cell and internal FET turns ON, there is some mA current on two external resistors surrounding that cell. then there are two voltage drops. How BMS still can read the exact value of cell, despite having two voltage drop on external resistors? it should normally read Cell_voltage - 2 x resistor voltages. does balancer estimate the external resistor values and somehow calculate and compensate those drops?

  • 0 in reply to hamzeh ahangari
    TI__Guru* 81035 points

    Hi Hamzeh,

    That is a good question. The device will still measure cell voltages but will use a modified schedule so balancing is temporarily disabled on the cell being measured by the ADC. There is a much more detailed description of this in Section 10.2 of the TRM.

    Regards,

    Matt