Principle of integrated protection circuit for lithium-ion batteries
Overcharging, over-discharging, and overcurrent are significant factors affecting the lifespan and performance of lithium-ion batteries during use. Integrated protection circuits for lithium-ion batteries effectively monitor and prevent damage through various protection units. A typical lithium-ion battery charge/discharge protection circuit consists of two field-effect transistors, a control integrated circuit, and several resistors and capacitors.
(1) Normal state
Under normal conditions, both the CO and DO terminals of N1 output high voltage in the circuit, and fets are in the conducting state, allowing the lithium-ion battery to charge and discharge freely. Because the conduction resistance of the field-effect transistor is very low, typically less than 30 meters, its conductivity has almost no impact on circuit performance. In this state, the current consumption of the protection circuit is in microamps, generally less than 7A.
(2) Overcharged state
Lithium-ion batteries require constant current and constant voltage charging. Initially, charging is done at a constant current. As charging progresses, the voltage gradually increases to 4.2V (depending on the cathode material, some batteries require a constant voltage up to 4.1V). If the charger circuit malfunctions during charging and the voltage exceeds 4.2V, the lithium-ion battery will continue charging at a constant current, causing the voltage to rise further. When the lithium-ion battery voltage exceeds 4.3V, the chemical byproducts of the lithium-ion battery will intensify, potentially leading to battery damage or safety issues.
In the lithium-ion battery protection circuit, when the control integrated circuit tests the lithium-ion battery voltage at 4.28V (the value is determined by the control IC; different integrated circuits have different values), the voltage will eventually transition from high to zero, causing VT2 to conduct to the off state, cutting off the charging circuit. The lithium-ion battery charger cannot charge, thus achieving the charging protection effect. At this time, due to the body diode VD2 of VT2 itself, the lithium-ion battery can discharge to the external load through the diode. There is a delay between the control integrated circuit detecting that the lithium-ion battery voltage exceeds 4.28V and sending the offVT2 signal. The delay time is determined by C3 and is usually set to around 1 second to prevent interference from causing false alarms in the protection circuit.
(3) Over-discharge state
During discharge, the voltage of a lithium-ion battery gradually decreases. When the voltage drops to 2.5V, its capacity is fully exposed. Continuing to discharge will cause permanent damage. During discharge, when the battery voltage detected by the control integrated circuit (IC) falls below 2.3V (the value varies depending on the IC), it will switch from a high potential to zero, causing VT1 to turn off, cutting off the discharge circuit and preventing further discharge, thus providing discharge protection. At this time, due to the presence of the body diode VD1 of VT1, the charger can charge the battery through the diode. Since the battery voltage cannot drop further under over-discharge protection, the current consumption of the protection circuit must be minimized. The control IC will then enter a low-power state, with the total power consumption of the protection circuit less than 0.11A. There is also a delay between the control IC detecting the battery voltage below 2.3V and sending the offVT1 signal. This delay is determined by C3 and is typically set to around 100ms to prevent interference from causing misjudgments in the protection circuit.
(4) Overcurrent state
Due to the chemical properties of lithium-ion batteries, manufacturers limit their discharge rate to below 2 degrees Celsius. Discharging at a rate exceeding 2C can lead to permanent damage or safety issues.
When a lithium-ion battery discharges normally to a load, the discharge current will cause a voltage drop across the series FET due to the conduction impedance of the FET.
