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Comment by dreamcompiler | Hacker News Reader

    +  Charger  -
    |           |
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |           |
    +   Sense   -

What is being sensed here and why is it wrong? If voltage is being sensed and the busses are fat chunks of copper, this seems entirely proper. What am I missing?

EDIT: Wait, I think I know. You also need independent current measurement at each cell to detect internal shorts. And probably a way to automatically isolate such cells.

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"Charger" would of course also be for discharge. "Sense" can either be charge sense, but that's not very common (due to CCCV charging), or monitoring. In either case, in this way of wiring things up the uppermost cell has a lower resistance connection to the charge/discharge port compared to the lowermost cell. So that uppermost cell will handle higher currents and overall will degrade more quickly. (This frequently applies when connecting "identical" parts in parallel in power electronics, e.g. capacitor banks).

If the voltage feedback (assuming there is any) is taken from the lower side, then a reduced voltage (by U=Rbus×Ibus) will be measured, which is lower than the voltage present at the cell terminals. Siblings mentioned that you want protection against internal cell shorts, using fuse wires or similar.

A better configuration would look like

    +  Charger
    |
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
    |-   Cell  -|
                |
       Charger  -

As you can see the total length of wire/bus between the pack's poles and each cell is the same length, so has the same resistance.

If each cell is connected to the bus bars with fuse wire (like in the Tesla battery pack for instance) this connection diagram seems valid. Same way it is connected in the car/powerwall packs that tesla makes at least.

Short detection < bingo.

Technically it depends on the cell chemistry and how they fail.

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