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Charging to 100% and balancing... what's the benefit?

  • 09-10-2017 10:03pm
    #1
    n97 mini
    Registered Users, Registered Users 2 Posts: 8,295 ✭✭✭


    Warning: Leaf owner's perspective.

    My understanding of battery balancing is that it is when stronger cells are discharged to the same level as weaker cells, so that all cells are "in balance". So in essence you get the lowest common denominator, i.e. stronger ones are dragged down to match weaker ones.

    In intelligent batteries the stronger cells are discharged into the weaker cells, but in the Leaf stronger cells are discharged via shunts to resistors, so that little bit of energy is lost. The weaker ones are not "brought up".

    I had the Leaf at probably about 75% SOC (indicated) the other day and hooked up Leafspy. I had driven it 30 mins earlier (down from 80%). It showed, 30 mins after driving, that the majority of cells were shunting, i.e. they were being drained to match the weaker cells. (Bear in mind that energy is being lost as it's creating heat, and not going into weaker cells)

    So.... I've read on here about charging your battery to 100% regularly to balance the cells. But it appears the Leaf balances them anyway, regardless of SOC, and will always drag the stronger ones down to match the weaker ones.

    Am I missing something?


Comments

  • #2
    ELM327
    Registered Users, Registered Users 2 Posts: 22,635 ✭✭✭✭ELM327


    The balancing starts at 92-93%.
    Perhaps the BMS does some routine balancing also (it must keep every cell in similar range)
    I'm sure someone more technical than myself will be along to explain it better.


  • #3
    BoatMad
    Registered Users, Registered Users 2 Posts: 13,702 ✭✭✭✭BoatMad


    I had the Leaf at probably about 75% SOC (indicated) the other day and hooked up Leafspy. I had driven it 30 mins earlier (down from 80%). It showed, 30 mins after driving, that the majority of cells were shunting, i.e. they were being drained to match the weaker cells. (Bear in mind that energy is being lost as it's creating heat, and not going into weaker cells)

    how do you know they are shunting ? , I dont see that in LeadSpy. very unusual to try and balance while the cells are in use, dont know any BMS that does that

    Note that while driving , the stronger cells will discharge first , thats just electrical laws at work


  • #4
    n97 mini
    Registered Users, Registered Users 2 Posts: 8,295 ✭✭✭n97 mini


    BoatMad wrote: »
    how do you know they are shunting ? , I dont see that in LeadSpy. very unusual to try and balance while the cells are in use, dont know any BMS that does that

    Note that while driving , the stronger cells will discharge first , thats just electrical laws at work

    The red bars (cells) on Leafspy are shunting if I understand correctly?

    The cells weren't in use (other than to keep the car powered up so that the OBD dongle would work)... I had stopped driving 20 or 30 mins earlier.

    Had a look on other forums and this balancing/shunting seems to happen regularly, hence my question about charging to 100% as is common wisdom.


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