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 battery can charge and discharge freely. 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. The current consumption of the protection circuit in this state is in the A range, typically less than 7A.
2. Overcharge protection
Lithium-ion batteries require a constant current/constant voltage charging method. Initially, charging is constant current. As charging progresses, the voltage rises to 4.2V (some batteries require a constant voltage of 4.1V depending on the cathode material), then switching to constant voltage charging until the current gradually decreases. If the charger circuit malfunctions during charging, the battery voltage may exceed 4.2V and continue constant current charging. In this case, the battery voltage will continue to rise. When the battery voltage exceeds 4.3V, the chemical side reactions in the battery will intensify, potentially leading 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 (this value is determined by the control IC, and different ICs have different values), its 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, 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 shutdown V2 signal, there is a delay period. The length of this delay period is determined by C3 and is usually set to about 1 second to prevent misjudgment due to interference.
3. Over-discharge protection
During the discharge process of a battery to an external load, its voltage will gradually decrease. When the battery voltage drops to 2.5V, its capacity has been completely discharged. If the battery is allowed to continue discharging to the load at this time, 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 will enter a low-power state, and the power consumption of the entire protection circuit will be less than 0.1A.
There is also a delay between when the control IC detects that the battery voltage is below 2.3V and when it sends the shutdown V1 signal. The length of this delay is determined by C3 and is usually set to about 100 milliseconds to prevent misjudgment due to interference.
4. Overcurrent protection
Due to the chemical characteristics of lithium-ion batteries, 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 cause permanent damage to the battery or safety problems.
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.
As can be seen from the above control process, the magnitude of the overcurrent detection value depends not only 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 is smaller for the same control IC.
5. Short circuit protection
During battery discharge to a load, if the circuit current becomes so large that U > 0.9V (this value is determined by the control IC, and different ICs have different values), the control IC determines that the load is short-circuited. Its DO pin will quickly switch from high voltage to zero voltage, causing V1 to switch 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, typically less than 7 microseconds. Its working principle is similar to overcurrent protection, only the judgment method and protection delay time are different.
The above details the working principle of a single-cell lithium-ion battery protection circuit. The protection principle of multiple series-connected lithium-ion batteries is similar and will not be elaborated here. The control IC used in the circuit above is the R5421 series from Ricoh Corporation of Japan. In actual battery protection circuits, there are many other types of control ICs, such as the S-8241 series from Seiko, the MM3061 series from Mitsumi, the FS312 and FS313 series from Fuchsun, and the AAT8632 series from Analog Technologies, etc. Their working principles are largely the same, with only differences in specific parameters. Some control ICs integrate the filter capacitor and delay capacitor inside the chip to save on external circuitry, thus requiring very little external circuitry, such as the S-8241 series from Seiko. Besides the control IC, another important component in the circuit is the MOSFET, which acts as a switch. Since it is directly connected in series between the battery and the external load, its on-resistance affects the battery's performance. When a better MOSFET is selected, its on-resistance is very small, the internal resistance of the battery pack is small, the load-carrying capacity is strong, and the power consumption during discharge is also less.
With the development of technology, portable devices are becoming smaller and smaller. As a result, the size requirements for the protection circuit of lithium-ion batteries are also getting smaller. In the past two years, products that integrate the control IC and MOSFET into a single protection IC have emerged, such as the DIALOG DA7112 series. Some manufacturers have even packaged the entire protection circuit into a small IC, such as the products of MITSUMI.
