I am designing a battery management system for lithium ion batteries. I am referring a schematic that is already available.

I do not understand the function of the two MOSFETs marked in the schematic.

enter image description here

  • 1
    \$\begingroup\$ Have you looked at the datasheet of the chip that is driving those MOSFET gates? If so, was there commentary in the datasheet that you need help interpreting? \$\endgroup\$
    – nanofarad
    Dec 17, 2021 at 4:31
  • \$\begingroup\$ I can understand one is for charging and the other for discharging. How do they work here? \$\endgroup\$
    – BlackHawk
    Dec 17, 2021 at 4:40
  • \$\begingroup\$ They either conduct or they don't. You need two back-to-back because of the body diodes (on mobile so no time to write full answer) \$\endgroup\$
    – nanofarad
    Dec 17, 2021 at 4:57
  • 1
    \$\begingroup\$ A BMS does not charge a battery. It is not a battery charger. As explained, the 2 Fets are here to disconnect the battery to protect it (under-discharge, over-charge, over-temperature, overcurrent etc). 2 Fets are needed because Mosfets have a parasitic body diode that will let the current flow from Source to Drain. A second Mosfet in the opposite direction is necessary to interrupt the current in both directions. \$\endgroup\$
    – romain145
    Dec 17, 2021 at 10:11
  • \$\begingroup\$ The CFOUT and DFOUT pin descriptions in the datasheet should have all the info you need. \$\endgroup\$
    – user16324
    Dec 17, 2021 at 14:56

1 Answer 1


They together work as a bidirectional relay.

It's a well known circuit in literature.

The states of this solid state relay are: ON or OFF.

When the relay is in the ON state, current can flow in both the directions.

  • \$\begingroup\$ What is the purpose of a MOSFET switch when the fuse has blown off? \$\endgroup\$
    – BlackHawk
    Dec 21, 2021 at 9:20
  • \$\begingroup\$ I can't answer. I would have measured the output and input currents and would have turned off the MOSFET's under overcurrent conditions instead of using a slow fuse. \$\endgroup\$ Dec 21, 2021 at 16:04

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