Lithium-ion battery integrated protection circuit principle

During the use of lithium-ion batteries, overcharge, over-discharge and over-current are important factors that affect the service life and performance of lithium-ion batteries. The lithium-ion battery integrated protection circuit effectively monitors and prevents damage to the lithium-ion battery through each protection unit circuit. Lithium-ion battery charge and discharge protection circuit. The lithium-ion battery protection circuit consists of two field effect transistors and a control integrated circuit plus some resistors and capacitors.

(1)Normal state

Under normal conditions, both the CO and DO terminals of N1 output high voltage in the circuit, the FETs are in a conductive state, and the lithium-ion battery can be charged and discharged freely. Since the conductive resistance of a field effect transistor is very small, usually less than 30 meters, its conductive resistance has almost no impact on circuit performance. In this state, the current consumption of the protection circuit is microamperes, which is generally less than 7a.

(2) Overcharge state

The required charging method for lithium-ion batteries is constant current and constant voltage charging. The initial stage of charging is constant current charging. As the charging process proceeds, the charging voltage gradually increases to 4.2v (depending on the cathode material, some batteries require a constant voltage of 4.1v). During the charging process of the lithium-ion battery, if the charger circuit loses control and exceeds 4.2v, the lithium-ion battery will continue to be charged at a constant current, and the voltage of the lithium-ion battery will continue to increase. When the lithium-ion battery voltage is higher than 4.3v, the chemical side effects of the lithium-ion battery will be exacerbated, causing battery damage or safety issues.

In the lithium-ion battery protection circuit, when the control integrated circuit tests the lithium-ion battery voltage is 4.28V (the value is determined by the control IC, different integrated circuits have different values). The company will eventually convert from high potential to zero potential, causing VT2 conduction to turn off, cutting off the charging circuit, and the lithium-ion battery charger cannot charge, which has a charge protection effect. At this time, due to VT2's own body diode VD2, the lithium-ion battery can discharge 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 about 1 to prevent interference from misjudgment of protection.

(3)Excessive discharge state

During the discharge process, the voltage of the lithium-ion battery will gradually decrease with the discharge process. When the lithium-ion battery voltage drops to 2.5v, its capacity is fully exposed. If the lithium-ion battery continues to discharge, it will cause permanent damage to the lithium-ion battery. During the discharging process of the lithium-ion battery, when the control integrated circuit tests the lithium-ion battery voltage below 2.3V, the value is determined by the control integrated circuit (different integrated circuits have different values). It will convert from high potential to zero potential, causing VT1 to conduct to off, cutting off the discharge circuit, so that the lithium-ion battery can no longer be loaded and discharged, and the discharge protection effect is achieved. At this time, due to the existence of VT1 body diode VD1, the charger can charge the lithium-ion battery through the diode. Since the voltage of the lithium-ion battery cannot be further reduced in the over-discharge protection state, the current consumption of the protection circuit is required to be minimum. At this time, the control integrated circuit will enter a low power consumption state, and the power consumption of the entire protection circuit will be less than 0.11a. There is also a delay between the control integrated circuit detecting that the lithium-ion battery voltage is lower than 2.3v and sending the offVT1 signal. The delay time is determined by C3 and is generally set to about 100ms to prevent interference from misjudgment of protection.

(4) Overcurrent status

Due to the chemical nature of lithium-ion batteries, manufacturers limit their discharge rate to 2 degrees Celsius. When the discharge rate of a lithium-ion battery exceeds 2C, it will cause permanent damage or safety issues to the lithium-ion battery.When a lithium-ion battery is normally discharged into a load, the discharge current will cause a voltage drop across the series FET due to the FET's conduction resistance.