The protection circuit typically consists of a control IC, a MOSFET switch, a fuse, resistors, capacitors, and other components, as shown in Figure 2. Under normal circumstances, the control IC outputs a signal to turn on the MOSFET, connecting the battery cell to the external circuit. When the battery cell voltage or circuit current exceeds a specified value, it immediately turns off the MOSFET to protect the battery cell.
1. Normal state
Under normal conditions, both the CO and DO pins of N1 output high voltage, and both MOSFETs are in the ON state. The lithium-ion battery can be freely charged and discharged. Since the on-resistance of the MOSFETs is very small, typically less than 30 milliohms, their on-resistance has little impact on the circuit performance. In this state, the current consumption of the protection circuit is in the μA range, typically less than 7μA.
2. Overcharge protection
Lithium-ion batteries require constant current/constant voltage charging. 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 cathode material, some batteries require a constant voltage of 4.1V), and then switch to constant voltage charging until the current becomes smaller and smaller.
If the charger circuit malfunctions during the charging process, the battery voltage may exceed 4.2V and continue to be charged at a constant current. At this time, the battery voltage will continue to rise. When the battery voltage is charged to more than 4.3V, the chemical side reactions of the battery will intensify, which may lead to battery damage or safety issues.
In a battery with a protection circuit, when the control IC detects that the battery voltage reaches 4.28V, its CO pin will switch 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 lithium-ion battery, thus serving as overcharge protection. At this time, due to the presence of the body diode VD2 built into V2, the battery can discharge to an external load through this diode. Between the control IC detecting that the battery voltage exceeds 4.28V and issuing the V2 turn-off signal, there is a delay time. The length of this delay time is determined by C3 and is usually set to about 1 second to prevent misjudgments due to interference.
3. Over-discharge protection
During the discharge process of a lithium-ion battery to an external load, its voltage gradually decreases. When the battery voltage drops to 2.5V, its capacity has been completely discharged. If the battery is allowed to continue discharging to a load at this point, it will cause permanent damage to the battery.
During battery discharge, 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 to the load, thus serving as over-discharge protection. At this time, due to the presence of the body diode VD1 built into V1, the charger can charge the battery through this diode.
Since the battery voltage cannot drop further under over-discharge protection, the current consumption of the protection circuit must be extremely low. At this time, the control IC enters a low-power state, and the entire protection circuit consumes less than 0.1μA. There is also a delay between the control IC detecting that the lithium-ion battery voltage is below 2.3V and issuing the shutdown signal V1. The length of this delay is determined by C3 and is typically set to around 100 milliseconds to prevent misjudgments due to interference.
4. Overcurrent protection
Due to the chemical characteristics of lithium-ion batteries, lithium-ion battery manufacturers stipulate that their maximum discharge current cannot exceed 2C (C = battery capacity/hour). When the battery discharges at a current exceeding 2C, it will lead to permanent damage to the battery or safety issues.
During normal discharge of the battery to the load, when the discharge current passes through the two MOSFETs connected in series, a voltage will appear across them due to the on-resistance of the MOSFETs. This voltage value is U = I * RDS * 2, where RDS is the on-resistance of a single MOSFET. The V-pin on the control IC detects this voltage value. If the load becomes abnormal for some reason, causing the loop current to increase, when the loop current becomes so large that U > 0.1V (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 change from on to off, thereby cutting off the discharge circuit and making the current in the circuit zero, thus serving as overcurrent protection.
There is also a delay between the control IC detecting an overcurrent and issuing a shutdown signal for V1. The length of this delay is determined by C3 and is usually around 13 milliseconds to prevent misjudgment due to interference.
