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BQ77915: External cell balancing wire

Silvan Sommer
Intellectual 320 points
Part Number: BQ77915

Tool/software:

Hello

While making the schematic, a question came up to me.

I wanna use external cell balancing for my 13S18P battery pack.

As you can see in the image, there are 3 different lines going to the same potential. 

- The line from the source of the external FET

- the line from RINE (for voltage sense I think)

- and the line from the lower RCB resistor

Now in my battery pack I soldered for each cell sense voltage only one wire to connect to the BMS.

But now I ask myself, do I need 3 different wires? Or is this working fine with just 1 wire.

Thanks

Silvan

  • 0
    Prodigy 80 points

    Hi Silvan.

    These 3 points are connected to the same line in the schematic. You can use 1 wire. But you should not use passive balancing for a battery pack of 13 in series and 18 in parallel. -Because the recommended current range for cell balancing is battery capacity/10. The minimum capacity of 18 parallel batteries (if you use 1000mA batteries) is 18.000mA. This means that your cell balancing current must be at least 1,800mA. If you balance with less current, the balancing time will be very long and the system will probably not work properly. To do passive balancing with 1,800mA current, you should use approximately 2.2R balancing resistor. The energy you spend on each resistor will be approximately 8W. Since there are 13 series batteries, 8 x 13 = 104W energy will be released. This means a lot of heat.Thinking

    My recommendation is to use active cell balancing.

  • 0 in reply to Ahmet Burma
    Intellectual 320 points

    Hello Ahmet

    As far as i read all information, the balancing current on passive system should be C/100 or C/500. 

    So im using 5.9Ah battery cells. 5900mAh / 100 = 59mA or when calculating with 5900mAh / 500 = 11.8mA

    So i have 18 in parallel -> i have a resistor of 3.6Ohm. When i have 3.8V cell the current = 3.8V/3.6A = 1.0555A total current.

    Now this current is divided by the 18 parallel cells = 58mA each cell is getting. And as the cell voltage rises, the current even gets higher.

    I want to charge my batterys with round about 1.2A per cell. So for me and for what i read, this should be okay. 

    Thanks 

    Silvan

  • +1 in reply to Silvan Sommer
    TI__Mastermind 42060 points

    Hello Silvan,

    Typically you would want to separate the high current of the balancing cell from the sensing lines. To make sure that there are no IR drops during balancing that could affect the cell measurements.

    However, It should be okay to connect to a single wire. The part will stop balancing to take measurements, so the IR drop effect should not affect the measurement accuracy.

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Intellectual 320 points

    Hello Luis 

    Okay thank you. 

    The replies from above, what do you say to that in case of the balancing current? 

    Should a balancing current rather be C/10 or C/100 or C/500.

    What I read is that when you have a 100Ah battery pack, the balancing current is C/100 = 1A 

    Thanks

    Silvan

  • 0 in reply to Silvan Sommer
    Prodigy 80 points

    Hi Silvan.

    I am using 4S1P 3000mA battery package and balancing current ratio is 1/10 (300mA balancing current).

    I made a new circuit using "BQ7791501" and I am working on it. To understand how cell balancing works, I manually discharged the cells.

    Cell voltages is C1 = 3.8V, C2 = 3.8V, C3 = 4.1V and C4 = 3.8V.

    And later I discharged the battery pack to 13V with a 4R resistor and charged it again with 2A 16.8V. Cell balancing start voltage is 3.8V. I wait for all cells to balance until the battery pack is charged or until the C3 cell reaches 4.25V (i.e. until the charge is cut off).

    However, the 300mV voltage difference between the cells never closes with a one charge. 300mA balancing current is not enough to balance cells with a voltage difference of 300mV between them with a one charge.

    I have been trying for 2 days and the voltage difference between the cells decreases by approximately 20mV with each charge. To balance the cells with a one charge, I must either increase the balancing current or decrease the charging current or decrease the charging voltage (4.1V per cell).

    If the balancing current were C/100, the cells would probably never be balanced.

  • 0 in reply to Ahmet Burma
    Intellectual 320 points

    Hi Ahmet

    Yeah when you have such a big delta voltage between the cells, it's clear. 

    But my cells are new, so each cell is exactly 3.51V before first charge. So unless one cell is destroyed, the cell voltage delta should not go too high.

    Here is a table which shows how big balancing current should be:

    And remember, this is for gross balancing so when the cells have big delta voltage.

    Thanks 

    Silvan