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I'm planning to build my 4S2P battery pack using 18650 lithium-ion battery cells. I need to make sure my connections and my battery pack is correct and safe before I make the connections. Here's the diagram for the battery pack:

14s2p Battery Pack

Each cell can be charged to 4.2 V maximum. Calculating the pack's specifications:

Capacity (Ah) = 8 x 3 Ah = 24 Ah

Wh = 14.8 V x 24 = 355.2

C-rate = 20 A / 24 = 0.833

Maximum discharge current (A) = 0.8333 x 24 ≈ 19.992 20 A

Are my connections and calculations correct?

Note that I labeled the connections for the BMS, B0, B1, B2, B3 and B4 which will be connected according to the diagram of the 4S BMS that I have.

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  • \$\begingroup\$ Welcome! Link to datasheet please. \$\endgroup\$
    – winny
    Commented Dec 23, 2023 at 11:37
  • \$\begingroup\$ As @Justme noted in their answer, all of the calculations are incorrect. \$\endgroup\$
    – StarCat
    Commented Dec 23, 2023 at 11:45

1 Answer 1

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I think the connections are correct.

But the calculations are not.

You have eight 3Ah batteries. Connected like that, it certainly does not make a 24 Ah battery, but a 6 Ah battery.

That's because you basically have two cells in parallel so that makes a 4s string of 6Ah batteries and if you draw 6Ah out of a series string then all 6Ah batteries are empty.

Only if you connect all 8 cells in parallel, you get a 24 Ah battery.

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  • \$\begingroup\$ I appreciate your answer. I still don't get it. Aren't all the cells connected in parallel? \$\endgroup\$
    – Tune333
    Commented Dec 23, 2023 at 12:37
  • \$\begingroup\$ @Tune333 No - you said it yourself, you are making a 4s2p pack. It means, you have four things in series, and each of the thing is a 2 cells in parallel, and 2 cells in parallel equals to 1 larger cell. That is also apparent as four 3.7V cells are needed in series to sum up to 14.8V \$\endgroup\$
    – Justme
    Commented Dec 23, 2023 at 12:45
  • \$\begingroup\$ @JustmeThis is where my confusion comes from. if two cells in parallel are one larger cell, then each cell has 3 Ah so that would make two cells in parallel having a capacity of 6 Ah? \$\endgroup\$
    – Tune333
    Commented Dec 23, 2023 at 12:50
  • \$\begingroup\$ @Tune333 Yes, two 3Ah cells in parallel are one 6Ah cell. But that was likely not what you are confused about, right? \$\endgroup\$
    – Justme
    Commented Dec 23, 2023 at 12:53
  • \$\begingroup\$ The way I look at the pack is as follows: each 2 cells connected in parallel have the capacity of 6 Ah, therefore I have 4 parallel connections. 4 * 6 = 24 Ah. \$\endgroup\$
    – Tune333
    Commented Dec 23, 2023 at 13:04

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