In view of the existence of some unstable factors in the power supply circuit, the circuit designed to prevent such unstable factors from affecting the effect of the circuit is called a protection circuit.
In lithium-ion batteries, lithium battery protection circuit plays an important role and affect the use of batteries. There are many types of lithium battery protection circuits, and this article has sorted out some common lithium battery protection circuits.
1. Working principle of the lithium battery protection circuit
Overcharge, overdischarge and short circuit are strictly prohibited in the use of lithium ion batteries, especially lifepo4 battery. They will cause shortened battery life or accidents such as fire and explosion.
Therefore, rechargeable lithium batteries will be connected to a charge and discharge protection circuit board to protect the safety of the battery cell. The protection function of lithium batteries is usually completed by the lithium battery protection circuit board and PTC.
● Protect board
The circuit board in the lithium battery protection circuit is composed of electronic components, and the operating temperature range accurately monitors the voltage of the battery cell and the current of the charge and discharge circuit at all times, and controls the on-off of the current circuit in time.
The main function of PTC is to protect the battery in a high temperature environment to prevent malignant accidents such as combustion and explosion. PTC is positive temperature coefficient, which means positive temperature coefficient resistance.
This element can play the role of overcurrent protection, that is, to prevent high temperature discharge of the battery and unsafe large current charges and discharges. PTC devices are made of polymer materials and are manufactured by a strict process. When it fails, the component can automatically return to the original state to ensure the normal operation of the circuit.
2. Charging and discharging requirements for lithium batteries
● Charging of lithium batteries
The maximum charge termination voltage of a single lithium battery is 4.2V, which cannot be overcharged, otherwise the battery will be scrapped due to the loss of too many lithium ions in the cathode.
When charging lithium batteries, special constant current and constant voltage chargers should be used, and the voltage at both ends of the lithium battery should be charged to 4.2V before being transferred to constant voltage charging mode. When the constant voltage charging current drops to 100mA, charging should be stopped.
● Discharge of lithium batteries
Due to the internal structure of lithium batteries, lithium ions cannot all move to the positive electrode during discharge, and a portion of lithium ions must be retained in the negative electrode to ensure that lithium ions can be smoothly embedded in the channel when the next charge is made.
Otherwise, the battery life will be shortened. In order to ensure that some lithium ions remain, it is necessary to strictly limit the minimum discharge termination voltage. Lithium batteries cannot be overcharged.
3. Composition of lithium battery protection circuit
The protection circuit is usually composed of control ICs, MOs switch tubes, blow fuses, resistors, capacitors and other components. Under normal circumstances, the control IC output signal controls the MOs switch tube to turn on, and when the cell voltage or loop current exceeds the specified value, it immediately controls the MOS tube to turn off to protect the safety of the cell.
The control IC has a built-in high-precision voltage detection circuit and a multi-stage current detection circuit. Among them, the voltage detection circuit is to detect the charging voltage, and once it reaches its set threshold, it immediately enters the overcharge protection state.
The second is to detect the discharge voltage, and once it reaches its set threshold, it immediately enters the over-discharge protection state. In addition, some lithium battery protection circuits are also equipped with NTC and ID signal forming components.
NTC is a negative temperature coefficient resistor, which mainly plays the role of over-temperature protection. If the temperature exceeds a certain value, the system enters a protected state and stops charging and discharging. ID is the meaning of identification, and its information identification components are divided into two types: one is memory, the other is identification resistance, both of which can play a role in product traceability and application restrictions.
4. Features of lithium battery protection circuit
The normal output voltage of a single lithium battery is about 3.7V, which can be directly used as a power supply for mobile phones, MP3/MP4 and some small screen tablets. For electrical appliances that require higher voltages.
Multiple lithium batteries can be connected in series to obtain the required voltage, such as a tablet computer that requires 11.1V power supply, and the battery assembly is three lithium batteries connected in series.
The protection circuits of single cell lithium batteries and multi cell series lithium batteries are different, and the following examples are analyzed. The lithium battery protection circuit is designed to protect the battery itself, which can be protected by normal use and maintenance in daily life. If there is a failure during use, you can go to battery stores nearby to learn about or replace it.
5. Single cell lithium battery protection circuit
The specific composition scheme of the single cell lithium battery charge and discharge protection circuit is more, but the working principle is not much different, and the following is an example of a circuit that is used more in mobile phones.
● Overdischarge protection
When the cell is discharged through an external load, the voltage across the cell will slowly decrease. After entering the overcharge protection state, the cell voltage will rise.
● Overcharge protection
When charging, when the battery is charged normally through the charger, the voltage at both ends of the cell will gradually increase as the charging time increases. When the protection circuit is connected to the discharge load, the positive direction of the internal diode is the same as the current direction of the discharge circuit, so the load can still be discharged.
● Overcurrent protection
Due to internal resistance, a voltage drop occurs when current flows through. When the voltage rises to the IC protection threshold, the discharge circuit is disconnected.
● Over-temperature protection
The port on the protection board is an over-temperature protection terminal that is connected to the CPU of the electrical appliance. The common over-temperature protection circuit is simpler, and the resistance is installed close to the cell.
When the electrical appliance is in a high-power working state for a long time (such as a mobile phone in a long-term call state), the temperature of the battery cell will rise. When the threshold is set, the CPU immediately issues a shutdown command to stop the battery from supplying it and maintains only a small standby current to protect the battery.
6. Multi cell lithium battery protection circuit
The lithium battery charge and discharge control chip can provide overcharge, overdischarge and overcurrent protection for 3 or 4 lithium battery pack. The chip samples the voltage of each cell in the battery pack and compares it to the exact reference voltage inside. When any battery is in an overvoltage or undervoltage state, the chip controls it accordingly to prevent further charging or discharging.
● Connection of the battery pack
Pay attention to the order of the battery pack and IC connection. The bottom end of the battery pack is connected to the AN4 end, and the top end is connected to the VDD end, and every battery hookup should in the corresponding order.
When discharging, the diode through which the lithium battery pack passes supplies power to the load. When the load requires a large current, the voltage drop across the current sense resistor is also large.
● Undervoltage protection
When either battery is detected to be overcharged (below the undervoltage threshold), the output is high while shutting down to sleep. Only when detected by the chip will it exit hibernation.
When connected to the charger, the output is low, the charging switch is on, and the battery pack is charged. During charging, if in sleep, the charging current charges the battery pack through the diode. Turn on when the voltage of each cell is higher than the undervoltage value.
● Overcurrent protection
To accommodate large capacitive loads, a secondary overcurrent protection mode is used for each threshold voltage. This secondary overcurrent protection can not only provide a fast response to short circuits, but also allow the battery pack to withstand a certain amount of inrush current to prevent unnecessary overcurrent protection actions due to the large capacity of the filter capacitor.
● Overvoltage protection
If the charging voltage of a battery exceeds the charging threshold, it enters an overvoltage protection state. In addition, if the battery pack connection is broken, it will also enter an overvoltage protection state.
7. ConclusionOvercharging and overdischarging lithium batteries will affect battery life. When designing, the lithium battery protection circuit should pay attention to the charging voltage and charging current of the lithium battery. Then select the appropriate charging chip. Pay attention to prevent overcharge, overdischarge, short circuit protection and other problems with lithium batteries. At the same time, the design is completed and goes through a lot of testing.