Analysis of the working principle of lithium battery protection circuit

The working principle of lithium battery protection circuit The protection function of lithium battery is usually completed by the protection circuit board and PTC. The protection board is composed of electronic components, which can accurately monitor the voltage of the battery cell and the charge and discharge circuit at all times under the environment of -40℃～+85℃ The main purpose of PTC is to protect the battery in high-temperature environments to prevent vicious accidents such as burning and explosion of the battery. The protection circuit is usually composed of control IC, MOs switch tube, blown fuse, resistor, capacitor and other components, as shown in Figure 2. Under normal circumstances, the control IC output signal controls the MOs switch tube to conduct, so that the cell and the external circuit are connected. When the cell voltage or loop current exceeds the specified value, it immediately controls the MOS tube to turn off to protect the cell. Safety. 1. Normal state In the normal state, the CO and DO pins of N1 in the circuit both output high voltage, and both MOSFETs are in the on state, and the lithium battery can be charged and discharged freely. Because the on-resistance of the MOSFET is very small, usually Less than 30 milliohms, so its on-resistance has little effect on the performance of the circuit. In this state, the current consumption of the protection circuit is μA, usually less than 7 μA. 2. The charging method required for overcharge protection lithium battery is constant current/constant voltage. In the initial stage of charging, it is constant current charging. With the charging process, the voltage will rise to 4.2V (depending on the positive electrode material, some batteries require constant voltage The value is 4.1V), and switch to constant voltage charging until the current becomes smaller and smaller. When the battery is being charged, if the charger circuit loses control, the battery voltage will continue to be charged with constant current after the battery voltage exceeds 4.2V. At this time, the battery voltage will continue to rise. When the battery voltage is charged to more than 4.3V, the battery’s chemistry Side reactions will intensify, causing 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 change from high voltage to zero voltage, turning V2 from on to off, thereby cutting off the charging circuit and charging The charger can no longer charge the lithium battery, which plays a role of overcharge protection. At this time, due to the existence of the body diode VD2 of V2, the battery can discharge the external load through the diode. There is a delay time between when the control IC detects that the battery voltage exceeds 4.28V and when the V2 signal is turned off. The length of the delay time is determined by C3 and is usually set to about 1 second to prevent errors caused by interference. judgment. 3. Over-discharge protection When the lithium battery is discharged to an external load, its voltage will gradually decrease with the discharge process. When the battery voltage drops to 2.5V, its capacity has been completely discharged. At this time, if the battery continues to load Discharging will cause permanent damage to the battery. During the battery discharge process, when the control IC detects that the battery voltage is lower than 2.3V (this value is determined by the control IC, different ICs have different values), its DO pin will change from a high voltage to a zero voltage, so that V1 is switched from conduction. Turning on is turned off, which cuts off the discharge circuit, so that the battery can no longer discharge the load, playing the role of over-discharge protection. At this time, due to the existence of the body diode VD1 of V1, the charger can charge the battery through this diode. Because the battery voltage can't be lowered in the over-discharge protection state, the current consumption of the protection circuit is required to be extremely small. At this time, the control IC will enter a low power consumption state, and the power consumption of the entire protection circuit will be less than 0.1μA. There is also a delay time between when the control IC detects that the voltage of the lithium battery is lower than 2.3V and when the V1 signal is turned off. The length of the delay time is determined by C3 and is usually set to about 100 milliseconds to prevent interference. Misjudgment. 4. Overcurrent protection Due to the chemical characteristics of lithium batteries, the lithium battery processing manufacturer stipulates that the maximum discharge current cannot exceed 2C (Cu003dbattery capacity/hour). When the battery discharges with a current exceeding 2C, it will cause permanent damage to the battery. Or there is a security problem. Disclaimer: Some pictures and content of the articles published on this site are from the Internet. If there is any infringement, please contact to delete.