I want to build a custom 4S battery pack using 21700 batteries. The battery needs to be charged via USB and the output needs to just be straight 4S from the batteries.

I have a 4S balancing BMS and a 4S 5 V step up voltage regulator.


For charging do I wire the 5 V step up regulator to the balancing BMS?

Adding pictures of the components I have.

4S balancing BMS:

4s Balancing BMS

4S 4 A charger:

4S 4A charger

  • \$\begingroup\$ Does the step up regulator have circuit facilities that can correctly and safely charge your battery? Please link pdf data sheets because the pictures don't really reveal anything useful. Do the parts come from a reputable source (rather than eBay or such like)? \$\endgroup\$
    – Andy aka
    Feb 28, 2023 at 11:49
  • \$\begingroup\$ What does the BMS do, exactly? (Not a rhetorical question. You tell me - you chose the part and you know what it is) \$\endgroup\$
    – user253751
    Feb 28, 2023 at 13:59
  • \$\begingroup\$ The BMS is chosen to charge balance the LiIon cells and to protect the batteries from overdischarge. The USB Charger would be used only to feed the BMS the right voltage for 4S. \$\endgroup\$ Mar 28, 2023 at 7:09

1 Answer 1


If your step up has no surge with a smart soft-start and a 4A Type C charger, it appears to be a good start.

But the real answer will be in the missing details.

I imagine it will need free air cooling.


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