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BQ76PL455EVM: Balancing time and current

Riswin MH
Prodigy 30 points
Part Number: BQ76PL455EVM

Hi,

We are using BQ76PL455AEVM for developing our BMS.

We are using EVM GUI. These are our observations

1.We tried to balance our cells( note that charger was not connected) and after 2 hours of balancing, the total voltage reduction was 5mV, balance current was 50mA.

2.There was an offset in the adjacent cells while balancing. We were balancing cell2, so the offsets were +10mV in cell1, -40mV in cell2 and +30mV in cell3.

Following are our doubts:

1. Can we balance the cell at any time or do we need to connect the charger to balance?

2. Can we balance at any voltage and at any voltage difference?

3.Is the time for balancing is expected like in our observation?

  • 0
    TI__Genius 11480 points
    Hi Riswin,

    What is the capacity of your battery pack?

    If you balance at 50mA for 2 Hours, you only use 100mAh capacity from your cell. For an EV this is going to be almost negligable, depending on the size of your pack. The offset is also expected... you can calculate it with fundamental circuit analysis like Kirchoff's Current Law and Ohm's Law.

    To directly answer your questions:
    1) Yes you can balance at any time. Depending on your settings, a fault could stop balancing. As you noticed, you will see offsets when measuring. The pl455 must also be active and not in sleep mode.

    2) The part must have proper power applied, and a minimum of 1.8V Vcell to balance

    3) Yes, this seems to be expected with 50mA balance current
  • 0 in reply to David Wiest Jr
    Prodigy 30 points
    Hi David,

    Thanks for the reply.

    Answering your question, we are now using a 66Ah pack for testing purposes.

    How can we increase balancing current, since 50mA seems negligible in this scenario?
  • 0 in reply to Riswin MH
    Prodigy 30 points
    One more thing David, Do we need to disconnect the charger while balancing when charging?
  • 0 in reply to Riswin MH
    TI__Genius 11480 points

    Hi  Riswin,

    You can decrease the balance resistor value to increase the  balancing current (ohm's  law - Cell Votlage/Sum of  resistances in balance path). Alternatively, you  could switch to an  active balancing  solution to achieve balancing  currents >1A. Keep in mind  that this high performance comes with increased costs and complexity.

    You  should  not  have to disconnect the  charger while balancing.