The protection function of lithium-ion battery is usually completed by the protection circuit board and current devices such as PTC. The protection board is composed of electronic circuits, which can accurately monitor the voltage of the battery cell and the charging and discharging circuit under the environment of -40℃ to +85℃. It can control the on-off of the current loop in time; PTC prevents the battery from being damaged badly in a high temperature environment.
Ordinary 18650 lithium ion battery protection board usually includes control IC, MOS switch, resistor, capacitor and auxiliary devices FUSE, PTC, NTC, ID, memory, etc. Among them, the control IC controls the MOS switch to turn on under all normal conditions to make the cell and the external circuit conduct. When the cell voltage or loop current exceeds the specified value, it immediately controls the MOS switch to turn off to protect the cell's Safety.
When the lithium-ion battery protection board is normal, Vdd is high, Vss and VM are low, DO and CO are high. When any parameter of Vdd, Vss, VM is changed, the value of DO or CO The level will change.
1. Normal state
In the normal state, the CO and DO pins of N1 both output high voltage, and both MOSFETs are in the on state, and the battery can be charged and discharged freely. Because the on-resistance of the MOSFET is very small, usually less than 30 milliohms, Therefore, 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. Overcharge protection
The charging method required for lithium-ion batteries is constant current/constant voltage. In the early 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 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 (this value is determined by the control IC, different ICs have different values), the CO pin will change from high voltage to zero voltage, making V2 It turns from on to off, which cuts off the charging circuit, so that the charger can no longer charge the battery for 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 battery discharges 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 discharge the load, it will cause battery damage. Permanent damage. 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 high voltage to zero voltage, making V1 lead 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 the body diode VD1 of V1, the charger can charge the battery through this diode. Since the battery voltage cannot be lowered 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 battery voltage is lower than 2.3V and when the V1 signal is turned off. The delay time is determined by C3 and is usually set to about 100 milliseconds to prevent errors caused by interference. judgment.
4. Short circuit protection
When the battery is discharging the load, if the loop current is so large that U>0.9V (this value is determined by the control IC, different ICs have different values), the control IC will judge that the load is short-circuited, and its DO pin will quickly change from The high voltage turns into zero voltage, which turns V1 from on to off, thereby cutting off the discharge circuit for short-circuit protection. The delay time of short circuit protection is extremely short, usually less than 7 microseconds. Its working principle is similar to that of over-current protection, but the judgment method is different, and the protection delay time is also different. In addition to the control IC, there is also an important component in the circuit, which is the MOSFET, which acts as a switch in the circuit. Because it is directly connected in series between the battery and the external load, its on-resistance has a significant effect on the performance of the battery. Influence, when the selected MOSFET is better, its on-resistance is small, the internal resistance of the battery pack is small, the load capacity is also strong, and it consumes less electric energy during discharge.
5. Overcurrent protection
Due to the chemical characteristics of lithium-ion batteries, the battery manufacturer stipulates that the maximum discharge current cannot exceed 2C (C=battery capacity/hour). When the battery discharges with a current exceeding 2C, it will cause permanent damage to the battery or safety problems. When the battery discharges the load normally, when the discharge current passes through the two MOSFETs connected in series, a voltage will appear at both ends of the MOSFET due to the on-resistance of the MOSFET. The voltage value U=I*RDS*2, RDS is a single MOSFET conduction resistance, the V-pin on the control IC detects this voltage value. If the load is abnormal for some reason, the loop current will increase. When the loop current is large enough to make U>0.1V (this value is determined by the control IC It is decided that when different ICs have different values), its DO pin will change from high voltage to zero voltage, turning V1 from on to off, thereby cutting off the discharge circuit, making the current in the circuit zero, and acting as an overcurrent Protection purpose. There is also a delay time between when the control IC detects the occurrence of overcurrent and when it sends the turn-off V1 signal. The length of this delay time is determined by C3, usually about 13 milliseconds, to prevent misjudgment due to 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-resistance of the MOSFET. When the on-resistance of the MOSFET is greater, the overcurrent protection of the same control IC The smaller the value.