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Working principle of lithium battery protection:

1. Normal state

Under normal conditions, both the "CO" and "DO" pins of N1 in the circuit output high voltage, and both MOSFETs are in a conducting state. The battery can freely charge and discharge. Due to the small conducting impedance of MOSFETs, usually less than 30 milliohms, their conducting resistance has little impact on the performance of the circuit.

The consumption current of the protection circuit in this state is μ A level, usually less than 7 μ A.

2. Overcharge protection

The charging method required for lithium-ion batteries is constant current/constant voltage. In the initial stage of charging, it is constant current charging. As the charging process progresses, the voltage will rise to 4.2V (some batteries require a constant voltage value of 4.1V depending on the positive electrode material), and then switch to constant voltage charging until the current decreases.

During the charging process of the battery, if the charger circuit loses control, it will cause the battery voltage to exceed 4.2V and continue constant current charging. At this time, the battery voltage will continue to rise. When the battery voltage is charged to exceed 4.3V, the chemical side reactions of the battery will intensify, which can cause battery damage or safety issues.

In a battery with a protective circuit, when the control IC detects that the battery voltage reaches 4.28V (this value is determined by the control IC, and different ICs have different values), the "CO" pin will change from high voltage to zero voltage, causing V2 to switch from conducting to turning off, thereby cutting off the charging circuit and preventing the charger from charging the battery, providing overcharging protection. At this time, due to the presence of the built-in body diode VD2 of V2, the battery can discharge external loads through this diode.

There is a delay time between the detection of battery voltage exceeding 4.28V by the control IC and the issuance of the turn off V2 signal. The length of this delay time is determined by C3, usually set at around 1 second, to avoid misjudgment caused by interference.

3. Overdischarge protection

During the discharge process of the battery to external loads, its voltage will gradually decrease. When the battery voltage drops to 2.5V, its capacity has been fully discharged. If the battery continues to discharge the load at this time, it will cause permanent damage to the battery.

During the battery discharge process, when the control IC detects that the battery voltage is below 2.3V (this value is determined by the control IC, and different ICs have different values), its "DO" pin will change from high voltage to zero voltage, causing V1 to switch from conducting to turning off, thereby cutting off the discharge circuit and preventing the battery from discharging the load again, providing over discharge protection. At this time, due to the presence of the built-in body diode VD1 of V1, the charger can charge the battery through this diode.

Due to the fact that the battery voltage cannot be further reduced under over discharge protection, it is required that the consumption current of the protection circuit be extremely small. At this time, the control IC will enter a low-power state, and the power consumption of the entire protection circuit will be less than 0.1 μ A.

There is also a delay time between the detection of battery voltage below 2.3V by the control IC and the issuance of the turn off V1 signal. The length of this delay time is determined by C3, usually set at around 100 milliseconds, to avoid misjudgment caused by interference.

4. Overcurrent protection

Due to the chemical characteristics of lithium-ion batteries, battery manufacturers have specified that the maximum discharge current cannot exceed 2C (C=battery capacity/hour). When the battery discharges beyond 2C, it will cause permanent damage to the battery or safety issues.

During the normal discharge process of the battery on the load, when the discharge current passes through two series connected MOSFETs, a voltage is generated at both ends due to the conduction impedance of the MOSFETs. The voltage value U=I * RDS * 2, which is the conduction impedance of a single MOSFET, is detected by the "V -" pin on the control IC. If the load is abnormal for some reason, causing an increase in the circuit current, when the circuit current is high enough to make U>0.1V (this value is determined by the control IC, and different ICs have different values), the "DO" pin will change from high voltage to zero voltage, causing V1 to turn from conduction to turn off, thereby cutting off the circuit. The discharge circuit is adjusted to zero current in the circuit, providing overcurrent protection.

There is also a delay time between the detection of overcurrent by the control IC and the issuance of the turn off V1 signal. The length of this delay time is determined by C3, usually around 13 milliseconds, to avoid misjudgment caused by interference.

In the above control process, it can be seen that the magnitude of its overcurrent detection value not only depends on the control value of the control IC, but also on the conduction impedance of the MOSFET. When the conduction impedance of the MOSFET is larger, the overcurrent protection value for the same control IC is smaller.

5. Short circuit protection

During the discharge process of the battery on the load, if the circuit current is large enough to make U>0.9V (this value is determined by the control IC, and different ICs have different values), the control IC will judge it as a load short circuit, and its "DO" pin will quickly change from high voltage to zero voltage, causing V1 to turn from conducting to off, thereby cutting off the discharge circuit and providing short circuit protection. The delay time for short-circuit protection is extremely short, usually less than 7 microseconds. Its working principle is similar to overcurrent protection, but the judgment method and protection delay time are different.

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