Analysis of the working principle of lithium-ion battery protection circuit.
Working principle of lithium-ion battery protection circuit
This circuit has functions of overcharge maintenance, overdischarge maintenance, overcurrent maintenance, and short circuit maintenance. The working principle analysis is as follows:
1. Normal state
Under normal circumstances, both the CO and DO pins of N1 output high voltage in the circuit, and MOSFETs are in a conducting state. The battery can be freely charged and discharged because the conduction impedance of MOSFET is very small, generally less than 30 milliohms, so its conduction resistance has little impact on the functionality of the circuit. In this case, the current consumption for repairing the circuit is Class A, generally less than 7kvA.
2. Overcharging maintenance
As a rechargeable battery, lithium-ion batteries require constant current/constant voltage charging methods. During the initial charging stage, constant current charging is required. During the charging process, the charging 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. During the charging process, if the charger circuit loses control and the battery voltage exceeds 4.2V, it will continue to charge at a constant current. At this point, the battery voltage will continue to rise. When the charging voltage of the battery exceeds 4.3V, it will intensify the chemical side reactions of the battery, leading to battery damage or safety issues.
In the battery maintenance circuit, when the control integrated circuit detects a battery voltage of 4.28V (the selected value is the operation of the integrated circuit, and different integrated circuits have different values), the company's pin conducts from high voltage zero voltage, V2 to shutdown, and then cuts off the charging circuit. The charger can no longer charge the battery, and the charging maintenance effect is improved. Now, thanks to V2's own diode VD2, the battery can release external loads through it.
There is also a delay time between the battery voltage of 4.28v detected by the control IC and the signal to declare the shutdown of V2. The delay time is selected by C3 and is generally set to around 1 second to prevent misjudgment due to interference.
Excessive discharge maintenance
During the discharge process, the voltage of the battery will gradually decrease. When the battery voltage drops to 2.5V, its capacity has been fully released. At this point, assuming the battery will continue to discharge, 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, the selected value is the operation of the integrated circuit, and different integrated circuits have different values. The pin is made to conduct from high voltage zero voltage V1 to shutdown, and then the discharge circuit is cut off. The battery can no longer maintain the effect of discharging the load. Now, due to V1's own body diode VD1, the charger can charge the battery through the diode.
Due to the fact that the battery voltage cannot decrease further during over discharge maintenance, the current consumption requirement for maintaining the circuit is very small. At this point, the control IC will enter a low-power state, and the power consumption of the entire maintenance circuit will be less than 0.11a.
When the control IC detects that the battery voltage is below 2.3V and declares the V1 signal off, there is also a delay time. The delay time is selected by C3 and is generally set to around 100ms to prevent misjudgment due to interference.
4. Overcurrent maintenance
Due to the chemical properties of lithium-ion batteries, battery manufacturers stipulate that the maximum discharge current of lithium-ion batteries cannot exceed 2C (C=battery capacity/hour). When the battery is discharged beyond 2C, permanent damage or safety issues may occur.
During normal load discharge of cells, the discharge current behind a series of two MOSFETs is stimulated by MOSFETs, and the impedance attacks the voltage at both ends. The voltage U=I * RDS * 2, RDS is stimulated by one MOSFET, and the impedance controls the V-pin test voltage value of the IC. If the load is abnormal for some reason, causing the circuit current to increase, when the ocean current gives U>0.1V (the selected value is the operation of the integrated circuit, different ICs have different values), the pin will switch from high voltage to zero voltage, causing V1 to turn on and off, and then cut off the discharge circuit to make the current in the circuit zero, which has the effect of maintaining overcurrent.
