Repair method of lithium battery protection board
1. The normal state of the battery protection board
In the normal state of the lithium battery pack protection board, the 'CO' and 'DO' pins of N1 in the circuit both output high voltages. All MOSFETs are in the on state, and the battery can be charged and discharged freely. Since the on-resistance of the MOSFET is very small, usually less than 30 milliohms, 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 initial stage of charging, it is constant current charging. As 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), its 'CO' pin will change from high voltage to zero voltage. Turn V2 from on to off, thereby cutting off the charging circuit, so that the charger can no longer charge the 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 avoid errors caused by interference. judgment.
3. 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 When the value), the control IC judges that the load is short-circuited, and its 'DO' pin will quickly change from high voltage to zero voltage, so that V1 is turned from on to off, thereby cutting off the discharge circuit and playing a role of 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. The influence is that when the selected MOSFET is better, its on-resistance is very small, the internal resistance of the battery pack is small, the load capacity is also strong, and it consumes less electric energy when discharging.
4. Overcurrent protection
Due to the chemical characteristics of lithium-ion batteries, the battery manufacturer stipulates that the maximum discharge current cannot exceed 2C (Cu003dbattery capacity/hour). When the current exceeds 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, due to the on-resistance of the MOSFET, a voltage will be generated at both ends of the MOSFET. The voltage value Uu003dI*RDS*2, RDS is a single MOSFET on-resistance, the 'V-' pin on the control IC detects the 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 (the value is determined by When the control IC decides that different ICs have different values), its 'DO' pin will change from high voltage to zero voltage, and turn V1 from on to off, thereby cutting off the discharge loop and making the current in the loop zero. Plays the role of over-current protection. 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 the delay time is determined by C3, usually about 13 milliseconds, to avoid 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 larger, the overcurrent protection of the same control IC The smaller the value.
5. Over-discharge protection
In the process of discharging the battery 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 When the battery is completely discharged, if the battery continues to discharge to the load, it will cause permanent damage to the battery.
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