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Now before everyone start saying how bad or dangerous it is to connect used and new 18650 Li-Ion Cells together - I would like to keep it technical and fact based.

I (like many others) have many used cells ling around and would like to be able to utilize them the best way possible on high current / voltage device (scooter / e-bike etc).

Connecting in series is clearly a bad idea as when one cell voltage drops below the lower limit it will 'permanent damage' point.

Now in parallel (assuming I pre-charged them to a similar voltage ) - what would really happen?

Used batteries will have less capacitance and their voltage will drop more rapidly but they can't because they are connected in parallel so defectively what I would expect is that "each cell will supply whatever current it can" until they all drop together to the minimal voltage allowed (where I would stop the discharge) . I will connect 5, 10, 20 cells in parallel and use a DC2DC Step up to convert them to the needed voltage.

The only thing I can think off is if the internal parallel resistance of one cell is reduced over time - self discharging and leading all other cells to discharge through it. I really want to find a way to maximize my scooter and use so many old yet good cells (ranging from 1200 to 2200mAH left in them).

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  • \$\begingroup\$ It is definitely a bad idea to use them in a high current application. The reason is that you don't know how much current is being supplied by each cell. Maybe some cells are exceeding their maximum and others are not. Some cells have higher or lower series resistance. You just don't know what will happen. \$\endgroup\$ – mkeith Mar 10 '18 at 18:58
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    \$\begingroup\$ Also, if one cell goes bad, it may become a sink for all the other cells. If that happens it will get very hot very fast and maybe the whole pack will vent. \$\endgroup\$ – mkeith Mar 10 '18 at 19:00
  • \$\begingroup\$ If you use temp sensing on each cell if not thermally bridged ok but current sharing is compromised with depleted cells resulting in accelerated aging of remaining cells. Get a LiPo capacity Cell tester and match each group of parallel cells \$\endgroup\$ – Sunnyskyguy EE75 Mar 10 '18 at 19:08
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    \$\begingroup\$ Welcome to the site. I'm afraid your post is just a 'sea of words', making it very hard to understand your situation and question. Please edit your question to split your text into paragraphs (with a blank line between them) that each cover a distinct point. Make your question clear. Please add a circuit diagram. Otherwise, you're asking people to piece together your circuit in their head from all this text. The schematic editor here is a breeze to use. The better the quality of your question, the better the quality of the answers it will attract. Again, a warm welcome to the site. \$\endgroup\$ – TonyM Mar 10 '18 at 19:52
  • \$\begingroup\$ Thanks . Good points . recommendation taken ...(I cant press ENTER) it doesn't move to next line but rather submits the comments . in any case - it would be great to find a 'smart' way of using different capacity cells together ..tx \$\endgroup\$ – Oren W Mar 11 '18 at 21:30

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