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. It can accurately monitor the voltage of the battery cell and the current of the charging and discharging circuit under the environment of -40℃～+85℃, and control it in time. The on-off of the current loop; the important purpose of PTC is to protect 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 level, usually less than 7μA.
2. Overcharge protection
The charging method required for lithium batteries 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 a constant voltage value of 4.1V) , Switch to constant voltage charging until the current gets 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 be aggravated, which will cause damage to the battery 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 is used for overcharge protection. At this time, due to the presence of the body diode VD2 of the 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
In the process of discharging a lithium battery to an external load, its voltage will gradually decrease along 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 discharge the load, it will cause the battery The permanent damage.
In the process of battery discharge, 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 high voltage to 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 presence of V1's own body diode VD1, the charger can charge the battery through this diode.
Since the battery voltage cannot be reduced in the overdischarge 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.
To learn more about Li-ion batteries, please refer to https://www.junleepower.com/