

Extra cells to decrease degradation effect

Discussion in 'Model S: Battery & Charging' started by invisik, Oct 21, 2014.

  1. invisik

    invisik Supporting Member

    Mar 13, 2014

    Ok, crazy idea here. What if there was a small, isolated bank of cells that were always kept at a storage level charge. Over time, as the main pack started to degrade, the software could start to also use these cells with the main pack to make up for lost range. It might help minimize the visibility of degradation....

    Just a thought.... No idea how feasable it might be.

  2. yobigd20

    yobigd20 Well-Known Member

    Oct 28, 2012
    Skaneateles, NY
    that is not as optimal/efficient vs using the "extra cells" as normal with battery pack to minimize the depth of discharge across all battery cells.

    minimize the depth of discharge spread out across all battery cells instead of just 95% of them would minimize degredation more than an extra "unused" set of cells that only kick in when main pack cells start to degrade more. while none of us have knowledge of the exact algorithms tesla uses in their battery management software, what i've described is what I assume tesla already does. i know they've said they keep an "buffer" to protect the battery, and I think what we can assume what that means is exactly as I've described. its just that your idea keeps a subset ununsed while main battery cells have a larger depth of discharge for the same distance vs what I've describes as keeping a minimal buffer spread out across all cells.

    the best way to prolong lithium ion batteries is to minimize the depth of discharge (and of course avoid range charging and discharge to 0 rated range when neither is necessary). larger discharges/full cycles increase degredation which is what your algorithm would do. over equivalent time and miles, what I described would show less degredation than your idea. disclosure: i'm no battery expert, but i've researched enough to understand how these work.
  3. Saghost

    Saghost Well-Known Member

    Oct 9, 2013
    I'm not seeing how you would make this happen.

    The current 85 kWh battery is made up of 16 modules commenced connected in series, each of which contains 6 blocks connected in series and made up of 74 cells connected in parallel.

    The battery pack voltage is made up of the sum of the 96 block voltages, and the pack amperage is equal to the storage capacity of the smallest/most degraded block.

    So it seems like your spare battery might take one of two forms: it could either be a 400V device which was added to the pack as a whole, or it could be a replacement for a block or module.

    If it was connected up at full voltage, you'd be better off having it in line and cycling the whole time - because it would reduce degradation by making the pack cycle less.

    To have a replacement block or module, it'd have to be able to be connected to any of the blocks/modules - which would be a real challenge to do safely. Even if you accomplish that, it'll only help if one block/module is degraded a lot more than the rest.

    I'm not sure if I fully grasped your concept, but I'm not seeing sufficient value in any of the three forms I came up with to justify the cost and weight.
  4. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

    Mar 8, 2012
    This is sort of how Mercedes does it. They limit the range so it appears that the battery doesn't degrade.

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