Ordinary lithium battery protection boards usually include control ICs, MOS switches, resistors, capacitors, and auxiliary devices FUSE, PTC, NTC, ID, memory, etc. Among them, the control IC controls the MOS switch to conduct under all normal conditions, making the battery cell and the external circuit conduct. When the battery cell voltage or circuit current exceeds the specified value, it immediately controls the MOS switch to turn off, protecting the safety of the battery cell.

Under normal conditions of the protection board, Vdd is high level, Vss, VM is low level, DO and CO are high level. When any parameter of Vdd, Vss, VM is changed, the level of DO or CO will change.

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 conductive state. The battery can freely charge and discharge. Due to the small conduction impedance of MOSFETs, which is usually less than 30 milliohms, their conduction resistance has little impact on the performance of the circuit.

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 (depending on the positive electrode material, some batteries require a constant voltage value of 4.1V), 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, leading to battery damage or safety issues.

In a battery with a protective circuit, when the control IC detects that the battery voltage reaches 4.28V (which is determined by the control IC, and different ICs have different values), the "CO" pin will change from high voltage to zero voltage, causing T1 to switch from conducting to off, thereby cutting off the charging circuit and preventing the charger from charging the battery, providing overcharging protection. At this point, due to the presence of T1's built-in body diode VD1, the battery can discharge external loads through this diode.

There is still a delay time between the control IC detecting that the battery voltage exceeds 4.28V and issuing the shutdown T1 signal. The length of this delay time is determined by C2 and is usually set to around 1 second to avoid misjudgment caused by interference.

3. Overdischarge protection

During the process of discharging external loads, the voltage of the battery 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 T2 to switch from conduction to shutdown, thereby cutting off the discharge circuit and preventing the battery from discharging the load again, providing over discharge protection. At this point, due to the presence of T2's built-in body diode VD2, the charger can charge the battery through this diode.

Due to the fact that the battery voltage cannot be further reduced in the over discharge protection state, it is required that the consumption current of the protection circuit be extremely small. At this point, 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 time the control IC detects that the battery voltage is below 2.3V and the time it sends a shutdown T2 signal. The length of this delay time is determined by C2 and is usually set to around 100 milliseconds to avoid misjudgment caused by interference.