I am trying to design a BMS for 4 18650 batteries in a 2S 2P configuration.

Some of the requirements are:

  • I would like to charge and discharge from the same connectors
  • I would like to implement a protection circuit to prevent over/under voltage and over/under charge
  • I would like this system to be completely autonomous aka does not need a MCU

I have looked at numerous ICs and have found that I am struggling to understand how to connect these up to one another. For instance I have looked at the MCP73842 and S-8252 and tried to implement a circuit but I think the implementation is incorrect.

Could somebody look over the circuit and tell me if I am on the right track? Could someone also explain how bms circuits are usually connected together?

Cheers enter image description here

  • To add, the provided system has one connection for charging (DC + and DC -) and another for discharging (Eb+ and Eb-)

1 Answer 1


On the one hand, you say "Designing a BMS for 2S2P 18650 batteries".

On the other hand, you're using the MCP73842, which is a Charge Management Controller (not a BMS).

Given that you don't know the difference between charging and protecting, that implies that you may be a perfect candidate for using off-the-shelf solutions. (Especially given the serious consequence with playing with Li-ion without a sufficient understanding of the technology.)

  1. Buy an off-the-shelf 2S Li-ion BMS, single port
  2. Connect it as instructed to the Li-ion cells
  3. Buy an off-the-shelf 2S Li-ion charger
  4. Connect the charger to the BMS's port
  5. Connect your load to the same BMS port
  6. Done.
  • \$\begingroup\$ Hi there, thanks for commenting. I appreciate your point but I am trying to learn how to design a system that can allow for charging and discharging from a single port whilst providing sufficient protection for the load. \$\endgroup\$
    – scelectron
    Commented Feb 18 at 21:38

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