I want to create a circuit that has input 12V DC and uses this voltage both charging two 2S (7.4V Li-Ion without balancing wire with BMS Circuit on the pack) batteries and at the same time supplying energy to the load.

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When 12V DC is not connected to the system, I want to use these two 2S battery pack as 4S battery and with buck boost converter decrease the voltage to 12V DC. As you guys know I can't connect 2S batteries with series without any protection circuit.

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Can you help me with this application? The charger IC can be changed.

  • 1
    \$\begingroup\$ You have designed yourself into a problem here. Running two separate 2S packs in problematic in itself. Using 2S without balancing or protection is problematic. Reconfiguring your pack between charging and discharging is possible but creates more problems than it solves. Is the end user OK with being unable to use the device while charging? How come you are not using a 4S1P or 1S4P configuration from the start? \$\endgroup\$
    – winny
    Commented Dec 21, 2022 at 11:57
  • \$\begingroup\$ @winny Batteries it self has bms circiut. So, no problem in there. for the last question, I have these battery pack in my hand a while. I need to use them :) \$\endgroup\$ Commented Dec 21, 2022 at 12:14
  • \$\begingroup\$ Ah! There was a nuance to “7.4V Li-Ion without balancing wire”. Still problematic design since there are so many edge cases to cover. Is the end user ok with being unable to charge and use the device at the same time? \$\endgroup\$
    – winny
    Commented Dec 21, 2022 at 12:27
  • \$\begingroup\$ @winny I edited the question. Thanks for the warning :) Yes the end user is ok with that. Since 12V entering the whole circuit, we can able supply both battery charging and to the laod. \$\endgroup\$ Commented Dec 21, 2022 at 12:31
  • \$\begingroup\$ You need to have an isolated DC/DC to make this happen and the batteries would be connected in 4S configuration. \$\endgroup\$
    – winny
    Commented Dec 21, 2022 at 12:33

1 Answer 1


Solutions from absolute best to absolute worst:

  1. Buy a ready-made 12 V power bank: safe, guaranteed to work, well-enclosed, cheaper in the long run
  2. Use 4 LPF cells in series (3.2 V each) to make a 12 V battery, use a current limited, step-up/step down, 12 V to 12 V converter to charge it (it will charge regardless of the actual input voltage and regardless of the battery's state of charge)
  3. Place all the cells in parallel (3.7 V); use two DC-DC converters, one to charge and one to power the load; The first one must be CCCV (current limited, adjusted for 4.2 V); the secnd one must have a 3 V low-voltage cut-off
  4. Connect half the cells in one parallel block, the other half in another parallel block, connect the two blocks in series (2S); add a BMS able to shut off the battery current if required to protect the cells; use two DC-DC converters, one to charge and one to power the load
  5. Your approach

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