What Are the Functions of MOSFETs in Lithium‑ion Battery Protection Boards?

1. Over‑charge protection

When the battery cell voltage rises to the over‑charge threshold after full charging, the protection IC sends a signal to turn off the charge MOSFET.

The charging path is physically disconnected, and external power cannot continue to charge the battery. This prevents risks such as battery swelling, thermal runaway caused by over‑charging. The discharge MOS remains on, and the battery can still supply power normally to the load.

2. Over‑discharge protection

When the battery power is exhausted and the cell voltage drops below the over‑discharge threshold, the protection IC turns off the discharge MOSFET.

The discharge loop is cut off, and the battery stops outputting current outward. It avoids permanent capacity attenuation or cell damage triggered by deep over‑discharge. At this time, the charge MOS is ready, and normal charging can resume once an adapter is connected.

3. Over‑current & short‑circuit protection

During discharging, if load over‑current or output short‑circuit occurs, huge current will flow through MOSFET. The protection IC quickly detects the abnormal current and shuts down the discharge MOS within microseconds.

It isolates the battery from the short‑circuit load, avoids excessive heat generation, smoking or even fire hazards brought by large short‑circuit current.

4. Charge over‑current protection

When the charging current is too large (for example using a mismatched high‑power charger), the IC will turn off the charge MOS.

It limits excessive charging current, inhibits lithium dendrite precipitation inside lithium‑ion cells, and extends battery service life.

5. Bidirectional blocking (eliminate body‑diode risk)

Two NMOS are connected source‑to‑source. Their intrinsic body diodes face opposite directions.

When both MOS are off, neither charging direction nor discharging direction can conduct through body diodes. It prevents unwanted reverse leakage, and realizes true bidirectional cut‑off, which single MOS cannot achieve.

6. Low‑resistance conduction under normal working status

When charging and discharging are in normal condition, both MOSFETs are fully turned on. Thanks to low \(R_{DS(on)}\), the conduction loss and heat generation are kept low.

Battery energy can flow in or out efficiently without large power waste.

Summary

Charge MOS mainly manages the input of charging current, responsible for over‑charge and charging over‑current protection.
Discharge MOS mainly controls output of load current, responsible for over‑discharge, discharge over‑current and short‑circuit protection.
Under normal conditions, MOS acts as a low‑loss conductor; under fault conditions, it acts as a high‑speed electronic switch to disconnect the circuit for battery safety.