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BQ24735: is necessary 4s balancing with 18650 battery?

Stefano Pelizzari
Prodigy 40 points
Part Number: BQ24735
Other Parts Discussed in Thread: BQ29209, BQ77915

Hello everybody.

I'm evaluation this application:

  • battery powered board
  • 4x 18650 cells
  • single cells has PCM
  • I need to put these cells in series
  • BQ24735 for charger
  • I've onboard microcontroller

Questions:

  • is strictly necessary to do cells balancing with 18650? there's a simple IC like BQ29209 that do that for 4 cells?
  • what happen if I don't balance cells? problem with operating time? danger behaviour of cells?

I two commercial application I retrofit I found 18650 (with single PCM every cell) placed in series without balance.

So I'm not sure if I need it or not

I didn't find simple balancers for 4 cells in TI portfolio. Something like BQ29209.

So I'm trying to design a MCU managed balancer. Here you are my draft.

I can read single cell voltage (here you don't see analog frontend) and I can apply a discharge resistor to any cell.

Thanks for any reply to my questions.

Thanks

Stefano

  • 0
    TI__Mastermind 19090 points

    Hi Stefano,

    Over time cells will become more and more imbalanced as they charge, discharge, and age. As they become more imbalanced, you lose out on more and more capacity (your charge and discharge are limited by the highest and lowest voltage cells). I'm not an expert on cell chemistries, so I couldn't tell you exactly what that would look like for your particular cells.

    This presentation from last year's BMS seminar explains in more detail: https://www.ti.com/lit/SLYP943.

    For balancing 4 cells I would recommend the bq77915. It has a lot more functionality than the bq29209 and natively supports 3-5s, so you won't need to implement a stacked solution.

    Regards,

    Max Verboncoeur

  • 0 in reply to Maxwell Verboncoeur
    Prodigy 40 points

    Hello Max,

     thanks for reply.

    Very useful your link on BMS seminar.

    Great suggestion with BQ77915, I lost it in my previous searches in TI catalog.

    Regards

    Stefano