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I have disassembled an 18650 e-bike battery pack and have removed its BMS. I am looking to salvage the cells. I now have the battery pack looking like so:

My question is whether it matters which nickel strips are cut first?

I know I should cut the strips attached to the negative ends of the cells, to avoid shorting the positive ends.

I am wondering whether cutting a cell or a set of cells from the pack might result in an abnormal voltage passing through the remaining cells that haven’t been cut out yet? In other words: if I remove three 3.7 V cells from a 13S-pack running at 48 V (E.G.), this would mean that this is now a 10S-pack running at 37 V. Would the remaining P-packs now be trying to deliver 48 V to this 37 V P-pack? I hope this makes sense, and I am assuming this isn’t the case but I would like to check and be sure. Also I’m giving example voltages, this pack might be different.

Is there a good place to start to tackle cutting this pack apart? Any other general tips for the cutting procedure? I have a pair of straight-cut tin/aviation snips I plan to use.

Any other safety tips to take care of whilst cutting? I plan to do this outside with a hose nearby in case of fire. I will try to find a metal container filled with water sand, in case the worst happens and I need to throw it in quickly. I know to use plastic pry tools rather than metal, and to remove all jewellery etc.

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  • \$\begingroup\$ voltage passing through the remaining cells ... voltage does not pass through components ... current passes through components ... voltage is applied across components \$\endgroup\$
    – jsotola
    Oct 3, 2021 at 17:01
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    \$\begingroup\$ Water to put out a lithium fire? \$\endgroup\$
    – Transistor
    Oct 3, 2021 at 17:04
  • \$\begingroup\$ draw the schematic diagram of the battery ... it may make it easier to determine where to cut it in half and then in quarters ... etc \$\endgroup\$
    – jsotola
    Oct 3, 2021 at 17:05
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    \$\begingroup\$ At highish voltage and substantial short circuit current like yours, I try to cover all other cells when I cut and disassemble to not accidentally slip and short circuit something. secondlifestorage forum might be a better place to ask in general when it comes to battery harvesting. \$\endgroup\$
    – winny
    Oct 3, 2021 at 17:09
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    \$\begingroup\$ @winny That’s a good idea, I’ll do that! I’ll take a look at that forum too, thanks! \$\endgroup\$ Oct 3, 2021 at 17:13

2 Answers 2

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In the past, I built a lot of batteries with these cells to experiment with my spot welder before getting into larger packs. Once I had welded a bunch of cells, I had to take them apart again and re-use them, so here is what I did.

A couple of things first, as others have said, water is not a good idea if the cells catch on fire, although what is more likely to happen if you do anything wrong (like touching across a don't touch zone with the pliers) is that you will get a big spark and the accompanying sound will scare you enough to pull away. If you can, heat-shrink your pliers' legs leaving only the tips exposed, that way, it is harder to touch across plates when you work. Safety glasses are a good idea as well.

In the battery you are trying to disassemble, notice the continuous sheets of nickel - as you probably know, these connect the negative of one cell to the positive of the next. What I would do, is put the tip of the pliers under the spot welds and push down on the pliers so the tips are pushing the nickel strip away from the cell, much like using a crowbar to dislodge something, or using a hammer to pull out a nail. If the welds are good, the nickel will tear and it can be separated by the cells, probably leaving some nickel on the cell. That's perfectly fine. If the welds aren't good, the nickel will just snap and separate. Either way, keep doing this, i.e., use the pliers as a crowbar and pull/tear away the nickel from each cell. As you do that, roll the nickel sheet much as you roll the top of a can of sardines till you have removed an entire nickel sheet. Be careful towards the end to make sure it doesn't touch the adjacent sheets - if you have a good pair of sheers, you can also cut it away once it has detached from the cells. In fact, for the cells at the top of your photo, where they are adjacent to each other, I would probably just cut the nickel with sheers then separate it from the cells with the pliers.

Once you have a nickel sheet removed, flip the battery and do the same but this time, hold each cell as you pry away the nickel strip on the other side. That way, when the other side of the cell is free, you are holding it as opposed to it rolling away. Put away each cell as it becomes free, cutting the nickel strips if they get big.

Once you have disassembled everything, you can use a Dremel to carefully grind away any remaining bits of nickel that remain attached to the cells. This is definitely not a standard procedure but it gets easier as you do it. If the Dremel runs away from you and cuts away the insulation of a cell (there may be a spark, don't ask how I know), resist the urge to keep it - that cell is probably best to be recycled properly.

I've taken enough of my "test" batteries apart that way, some cells multiple times, and it works for me. Hope it works for you as well.

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whether it matters which nickel strips are cut first?

Yes.

  1. The connection in the middle of the battery, which immediately halves the danger by dividing it into two strings of half the voltage.
  2. The connection in the middle of each half battery, to halve the voltage once more
  3. And so forth, in a binary fashion, until down to a single block of cells

Then:

  1. The connection in the middle of each block, which immediately halves the danger by dividing it into two blocks of half the current capability.
  2. The connection in the middle of each half block, to halve the current capability once more
  3. And so forth, in a binary fashion, until down to a single cell
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