Lithium battery due to the relationship between materials, determines that the lithium battery can not be overcharged, overdischarge, overcurrent, while short circuit and charge and discharge at ultra-high temperatures will also cause a great risk, so lithium batteries are usually equipped with a protection board at the same time.
The design functions of the lithium battery protection board include the following:
1, overcharge detection voltage
2, overcharge release voltage
3, overdischarge detection voltage
4. Overdischarge release voltage
5, overcurrent 1 Detect voltage
6, overcurrent 2 Detection voltage
7, load short road detection voltage
8, the charger detects voltage
9, usually consume current when working
10. Overdischarge consumes current
Maintenance principle of lithium battery protection board
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. The current consumption of the protection circuit in this state is μA, usually less than 7μA.
2, overcharge protection
If the charger circuit is out of control, it will continue to charge at 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 chemical side reaction of the battery will be aggravated, which will lead to battery damage or safety problems.
When the lithium battery pack with a protection board is overcharged, the control IC detects that the battery voltage reaches 4.28V(determined by the control IC), 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, playing the role of overcharge protection.
3, short circuit protection
When the lithium battery is discharging, when the circuit current is large to a certain value (the value is determined by the control IC), the control IC is judged as a load short circuit, and its \"DO\" foot will quickly change from high voltage to zero voltage, so that V1 is turned off from on-off, cutting off the discharge circuit, and playing a short circuit protection role. The delay time of short-circuit protection is extremely short, usually less than 7 microseconds. Its working principle is similar to overcurrent protection, but the judgment method is different, and the protection delay time is also different.
4, overcurrent protection
During the discharge of a lithium battery, 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, the \"V-\" foot 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 to a certain value (the value is determined by the control IC), its \"DO\" foot will be converted 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, playing the role of overcurrent protection.
In the process of discharging a lithium battery, when the voltage drops to 2.5V, it indicates that the capacity has been completely emitted, and if the battery continues to discharge at this time, it will cause permanent damage to the battery. In the discharge of a lithium battery with a protection board, when the control IC detects that the battery voltage is lower than the set value (determined by the control IC), 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, and play an overdischarge protection role.
lithium battery protection plate
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The lithium battery protection board is the charge and discharge protection for the series lithium battery pack. When fully charged, it can ensure that the voltage difference between each single battery is less than the set value (generally ±20mV), so as to realize the uniform charging of each single battery pack, and effectively improve the charging effect under the series charging mode. At the same time, the state of overvoltage, undervoltage, overcurrent, short circuit and overtemperature of each single battery in the battery pack is detected to protect and extend the service life of the battery. Undervoltage protection enables each single battery to avoid damage due to overdischarge when discharging.
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The finished lithium battery consists of two main parts, lithium battery core and protection board, lithium battery core is mainly composed of positive plate, diaphragm, negative plate, electrolyte; The positive plate, diaphragm and negative plate are wound or stacked, packaged and injected with electrolyte, and the cell is made after encapsulation.
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Lithium battery protection board balance principle
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The commonly used balanced charging technology includes constant shunt resistance balanced charging, on-off shunt resistance balanced charging, average battery voltage balanced charging, switched capacitor balanced charging, buck converter balanced charging, inductance balanced charging and so on.
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When a group of lithium batteries are charged in series, each battery should be evenly charged, otherwise the performance and life of the whole battery will be affected during use. The existing single lithium battery protection chip does not contain balanced charge control function, and the balanced charge control function of multi-lithium battery protection chip requires external CPU; Through the serial communication with the protection chip (such as I2C bus), the complexity and design difficulty of the protection circuit are increased, the efficiency and reliability of the system are reduced, and the power consumption is increased.
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The charge overvoltage protection control signal of the single lithium battery protection chip in the control circuit of the lithium battery pack protection board system is isolated by the optocouple and output in parallel, which provides the gate voltage for the conduction of the charge switch device in the main circuit. If one or several lithium batteries first enter the overvoltage protection state during the charging process, the shunt discharge branch at both positive and negative ends of a single lithium battery is controlled by the overvoltage protection signal, and the corresponding single lithium battery connected in series in the charging circuit is disconnected from the charging circuit.