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I'm looking into building my own li-ion battery pack. I read about linking the cells in parallel and in series, and how to add a BMS. I would like to build a 12V pack with a very large capacity (mAh). So I need to build a 3S pack and then link the 3-series connected cells in parallel.

What I don't understand is: does the BMS need to know about the number of parallel connected cells, or can I simply keep adding them up?

For example, one BMS I was looking at had this illustration:

enter image description here

It shows 2 parallel connected packs, but can I also make it 3P, or 10P, or 150P?

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    \$\begingroup\$ CV time sensitive limits would make this slow and unwise for battery aging. This would become like float charging which is not allowed on li-ion \$\endgroup\$
    – Tony Stewart EE75
    Apr 6, 2019 at 3:06
  • \$\begingroup\$ @SunnyskyguyEE75 I've read in an article in here. It says, Float charging Li-ion cells is possible, but the voltage-temperature-lifetime-capacity trade-off is complex. And Broussely warns: “Certainly discuss this with your battery manufacturer. It is very application dependent.”. Do you agree? \$\endgroup\$
    – Unknown123
    Apr 6, 2019 at 3:51
  • \$\begingroup\$ you can CV at 3.8V but not get full capacity but if you CV at 4.2 you will much reduced life cycles \$\endgroup\$
    – Tony Stewart EE75
    Apr 6, 2019 at 4:20
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    \$\begingroup\$ electronics.stackexchange.com/questions/270292/… \$\endgroup\$
    – Tony Stewart EE75
    Apr 6, 2019 at 4:20
  • \$\begingroup\$ Depends on the specs of the BMS. I am sure it has some limits on the total capacity of cells it can handle. \$\endgroup\$
    – RoyC
    Jun 8, 2019 at 8:41

1 Answer 1

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There are two reasons you need to be concerned about the number of parallel cells in your battery pack

a) The current cutoff threshhold when doing the final CV charge is dependent on the battery capacity, which is dependent on the number of parallel cells. You simply need to know the numbers, and program your charger correctly.

b) In the event of a failure of one cell, the other undamaged cells in parallel with it may discharge into the damaged one, prolonging and exacerbating any 'vent with flame' event, turning a mess into a catastrophe, turning a damaged battery into a damaged vehicle or building. I talk from experience, my brother lost an eBike and the van in which it was being charged, and would have damaged any adjacent vehicles as well but for being parked in a remote corner of the carpark.

You can mitigate this latter problem by connecting a fuse in series with (ideally) each cell, or at a minimum with each small group of parallel cells. The smaller the group of cells, the closer it can be fused to protect it.

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  • \$\begingroup\$ I've seen people using fusewire instead of nickel strip so that you don't have to put fuse in series on each cell. What do you think? \$\endgroup\$
    – Unknown123
    Apr 6, 2019 at 7:19
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    \$\begingroup\$ @Unknown123 In theory, good idea. I don't know how well fuse wire spot welds to cells. I have known people solder wires to cells, which is definitely not recommended unless they're tagged cells. Spot weld a nickel tag, then hook up with fuse wire would be the cheapest correct way to go. \$\endgroup\$
    – Neil_UK
    Apr 6, 2019 at 8:26
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    \$\begingroup\$ You don't need to spot weld wire. I saw this amazing solution instead batteryhookup.com/collections/cell-level-nickel-fuse @Neil_UK \$\endgroup\$
    – JaySabir
    Aug 1, 2020 at 22:51

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