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I've read many question on here about how to connect multiple BMS controlled batteries in parallel to step up amps. Is this necessary to step up amps as opposed to running all the cells in series and stepping down the voltage instead?

For instance 4 X 18650 3.7v cells connected with a target voltage of 7.4v and the highest amps possible.

  • Considering cost and safety
  • Would you get similar amps from a single BMS that connects all 4 cells in series and is then run through a buck converter to get 7.4v?
  • Or running 2 x 2 cell BMS producing 7.4v each, then run in parallel and charged in parallel?

It just seems like a lot of cost, frustration, and safety issues can be resolved by using a single BMS and a buck converter to achieve this goal, provided they produce similar amps.

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  • \$\begingroup\$ Use a single 7.4V BMS. But connect pairs of cells in parallel to each of its cell terminals. This configuration is called 2S2P. \$\endgroup\$
    – user16324
    Commented Jul 11, 2021 at 18:44
  • \$\begingroup\$ @user_1818839 Thank you. This should work with any BMS as the single or parallel cell voltages are the same? \$\endgroup\$
    – Mikeologist
    Commented Jul 11, 2021 at 18:54
  • \$\begingroup\$ Yes............ \$\endgroup\$
    – user16324
    Commented Jul 11, 2021 at 18:54
  • \$\begingroup\$ @user_1818839 Do you think there is anything original about this question, or should I delete because of redundancy? \$\endgroup\$
    – Mikeologist
    Commented Jul 11, 2021 at 18:57
  • \$\begingroup\$ It depends how well balanced each cell is for V and Ah and risk of shorted cells without fuses and current sense in each shared parallel pack with a voltage booster vs weak link concept in series and pack fails from 1st weak cell. For parallel no BMS is needed but a boost converter capable of max current \$\endgroup\$
    – D.A.S.
    Commented Jul 11, 2021 at 19:30

2 Answers 2

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If you are using a battery with a built-in BMS, unless you are willing to tear the battery apart, you don't have access to the individual cells, hence the issue of paralleling BMSs. If you are building the battery yourself, or are willing to tear up an existing battery and re-wire the cells, you can then put them all in parallel and use an appropriate sized BMS.

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  • \$\begingroup\$ Thank you, I decided to go with Li-Po off the shelf. Designed a nice MOSFET power latch for it. Then I just remove the battery pack and use a half-decent charger. Too many questions that I can't answer otherwise. \$\endgroup\$
    – Mikeologist
    Commented Sep 3, 2021 at 15:33
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If you run in series then one poor cell can drop the total output.

In parallel, then the other strings can cope when one is poor. One of the functions of the bms.

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  • \$\begingroup\$ So, one answer to consider is that paralleling BMS circuits can provide longer reliability, provided you don't mind the potential for a loss of amps. Conversely, if the amps are a requirement of the machines functions then a bucked single BMS will prevent the machine from running when the pack is no longer capable of providing the proper amps because of a dead, diminished, or otherwise damaged cell? \$\endgroup\$
    – Mikeologist
    Commented Jul 11, 2021 at 17:10
  • \$\begingroup\$ "One of the functions of the bms" In parallel each BMS is not aware of the others, or am I misunderstanding? \$\endgroup\$
    – Mikeologist
    Commented Jul 11, 2021 at 18:40

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