How the battery circuit works
The circuit has the functions of over-charge protection, over-discharge protection, over-current protection and short circuit protection, and its working principle is analyzed as follows:
1. Normal state
In the normal state of the circuit N1 “CO” and” DO” foot are output high voltage, the two MOSFETs are in the on-state, the battery can be freely charged and discharged, because the on-impedance of the MOSFET is very small, usually less than 30 milohm, so its on-resistance has little effect on the performance of the circuit. 7| The current consumed by the protection circuit in this state is μA, usually less than 7μA.
2, overcharge protection
Lithium-ion batteries require constant current/constant voltage charging, in the early charging, for constant current charging, with the charging process, the voltage will rise to 4.2V(according to the positive material is different, some batteries require a constant voltage value of 4.1V), into constant voltage charging, until the current becomes smaller and smaller. When the battery is being charged, if the charger circuit is out of control, it will continue to charge at constant current after the battery voltage exceeds 4.2V, and the battery voltage will continue to rise. When the battery voltage is charged to more than 4.3V, the chemical side reaction of the battery will be aggravated, which will lead to battery damage or safety problems.
In a battery with a protection circuit, when the control IC detects that the battery voltage reaches 4.28V (the value is determined by the control IC, and different ics have different values), its “CO” foot will change from high voltage to zero voltage, so that V2 will be turned off from on, thus cutting off the charging circuit, so that the charger can no longer charge the battery, and play an overcharge protection role. At this time, due to the presence of the body diode VD2 of V2, the battery can discharge the external load through the diode.
Between the control IC detects that the battery voltage exceeds 4.28V and sends out the shutdown V2 signal, there is a delay time, the length of the delay time is determined by C3, usually set to about 1 second, to avoid misjudgment caused by interference.
3, over discharge protection
During the discharge of the battery to the external load, its voltage will gradually decrease with the discharge process. When the battery voltage drops to 2.5V, its capacity has been completely emitted. At this time, if the battery continues to discharge the load, it will cause permanent damage to the battery.
In the process of battery discharge, when the control IC detects that the battery voltage is lower than 2.3V (the value is determined by the control IC, different ics have different values), its “DO” foot will be transformed from high voltage to zero voltage, so that V1 is turned off from on, thus cutting off the discharge circuit, so that the battery can no longer discharge the load, play the role of overdischarge protection. At this time, due to the existence of V1’s own body diode VD1, the charger can charge the battery through the diode.
Since the battery voltage can no longer be reduced in the overdischarge protection state, the current consumption of the protection circuit is required to be very small, and the control IC will enter the low-power state, and the power consumption of the entire protection circuit will be less than 0.1μA.
Between the control IC detects that the battery voltage is lower than 2.3V and sends out the turn-off V1 signal, there is also a delay time, the length of the delay time is determined by C3, usually set to about 100 milliseconds, to avoid misjudgment caused by interference.
4, overcurrent protection
Due to the chemical characteristics of lithium-ion batteries, battery manufacturers stipulate that the maximum discharge current cannot exceed 2C (C= battery capacity/hour), when the battery exceeds 2C current discharge, it will lead to permanent damage to the battery or safety problems.
During the normal discharge of the battery to the load, when the discharge current passes through two MOSFETs in series, a voltage will be generated at both ends due to the on-impedance of the MOSFET, the voltage value U=I*RDS*2, RDS is the on-impedance of a single MOSFET, and the “V-” pin on the control IC detects the voltage value. If the load is abnormal for some reason, the loop current increases, when the loop current is large enough to make U>0.1V (the value is determined by the control IC, different ics have different values), its “DO” foot will be transformed from high voltage to zero voltage, so that V1 is turned off from on, thus cutting off the discharge loop, so that the current in the loop is zero, and play an overcurrent protection role.
Between the control IC detects that the overcurrent occurs and sends out the turn-off V1 signal, there is also a delay time, the length of the delay time is determined by C3, usually about 13 milliseconds, to avoid misjudgment caused by interference.
In the above control process, it can be seen that the overcurrent detection value depends not only on the control value of the control IC, but also on the on-impedance of the MOSFET. When the on-impedance of the MOSFET is larger, the overcurrent protection value is smaller for the same control IC.