What are the protective circuits and working principles of lithium batteries?

The circuit has the functions of overcharge protection, overdischarge protection, overcurrent protection, and short circuit protection. Its working principles are analyzed as follows:

1 Normal

In the normal state of the circuit, N1's "CO" and "DO" feet output Gaodianya, both MOSFETs are in a state of conduction, and the battery is free to charge and discharge, because the MOSFETs have a very small conduction impedance, usually less than 30 mm, Therefore, its conduction resistance has little effect on the performance of the circuit. The current consumed by the protection circuit in this state is μA, usually less than 7μA.

2 overcharge protection

Lithium-ion batteries are a type of rechargeable battery. The required charging method is the constant current/constant pressure. In the initial stage of charging, the constant current is charged. With the charging process, the voltage will increase to 4.2 V(depending on the cathode material, some batteries require constant pressure). The value is 4.1 V), Switching to constant voltage charging until the current becomes smaller and smaller. When the battery is charged, if the charger circuit is out of control, the battery voltage will continue to charge after the battery voltage exceeds 4.2 V. At this time, the battery voltage will continue to rise. When the battery voltage is charged to more than 4.3 V, the chemical side of the battery will The reaction will intensify, Will cause battery damage or safety problems.

In a battery with a protective circuit, when the control IC detects a battery voltage of 4.28 V(this value is determined by the control IC and different ICs have different values), its "CO" foot will be converted from Gaodianya to zero voltage., The V2 is turned off from the guide to the off, thus cutting off the charging loop, so that the charger can no longer charge the battery and play a role in overcharging protection. At this time, due to the presence of V2's own body diode VD2, the battery can discharge the external load through the diode.

There is also a delay time between the control IC detects that the battery voltage exceeds 4.28 V and the off V2 signal. The length of the delay time is determined by C3 and is usually set to about 1 second to avoid misjudgment due to interference.

3 Over-discharge protection

During the external load discharge process, the voltage of the battery will gradually decrease with the discharge process. When the battery voltage drops to 2.5 V, its capacity has been completely discharged. If the battery continues to discharge the load at this time, it will cause the battery. Permanent damage.

During battery discharge, when the control IC detects that the battery voltage is less than 2.3 V(this value is determined by the control IC and different ICs to have different values), its "DO" foot will be converted from Gaodianya to zero voltage. The V1 is switched from conduction to shut down, thus cutting off the discharge loop, so that the battery can no longer discharge the load and play an over-discharge protection role. At this time, due to the existence of V1's own body diode VD1, the charger can charge the battery through this diode.

Since the battery voltage can no longer be reduced under over-discharge protection, the current consumed by the protection circuit is extremely small. At this time, the control IC will enter a low-power state, and the entire protection circuit will consume less than 0.1 μA.

There is also a delay time between the control IC detects that the battery voltage is less than 2.3 V and the off V1 signal. The length of the delay time is determined by C3 and is usually set to about 100 milliseconds to avoid interference. Misjudgment.

4 overcurrent protection

Due to the chemical characteristics of lithium batteries, battery manufacturers have stipulated that the discharge current must not exceed 2C(C = battery capacity/hour). When the battery exceeds 2C current discharge, it will cause 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 of the MOSFET due to the conduction impedance of the MOSFET. The voltage value U = I * RDS * 2, RDS is a single MOSFET conduction impedance, The "V-'' foot on the control IC detects the voltage value. If the load causes an anomaly for some reason, the loop current increases, when the loop current is large enough to make U" 0.1 V(this value is determined by the control IC. When different ICs have different values, their "DO" feet will be converted from Gaodianya to zero voltage, so that V1 will be switched on and off, thus cutting off the discharge loop, so that the current in the loop is zero, playing an overcurrent protection effect.

There is also a delay time between the control IC detects that the overcurrent occurs and the 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 size of the overcurrent detection value depends not only on the control value of the control IC, but also on the conduction impedance of the MOSFET. When the MOSFET conduction impedance is larger, the protection value of the overcurrent for the same control IC is smaller.

Short circuit protection

When the battery discharges the load, if the loop current is large enough to make U "0.9 V(this value is determined by the control IC and different ICs to have different values), the control IC is judged to be a load short circuit. Its "DO" foot will quickly change from Gaodianya to zero voltage, so that V1 will switch off from conduction to off, thus cutting off the discharge loop and playing a short-circuit protection role. The delay time for short-circuit protection is extremely short, usually less than 7 microseconds. Its working principle is similar to that of overcurrent protection, but the method of judgment is different, and the protection delay time is also different.

The above details the working principle of a single-segment lithium-ion battery protection circuit. The protection principle of Multi-segment series lithium-ion batteries is similar. It is not repeated here. The control IC used in the above circuit is the R5421 series of Ricoh Corporation of Japan. In the actual battery protection circuit, there are many other types of control ICs, such as Japan Seiko's S-8241 series, Japan's MITSUMI's MM3061 series, Taiwan's Fujing's FS312 and FS313 series, Taiwan's analog technology AAT8632 series and so on, Its working principle is similar, but there are differences in specific parameters. In order to save peripheral circuits, some control ICs have achieved filter capacitors and delay capacitors inside the chip, and their peripheral circuits can be very few, such as Japan Seiko's S-8241 series. In addition to controlling the IC, there is an important component in the circuit, MOSFET, which acts as a switch in the circuit. Since it is directly linked between the battery and the external load, its conduction impedance has an effect on the performance of the battery. When the selected MOSFET is good, its conduction impedance is small, the internal resistance of the battery pack is small, the carrying capacity is also strong, and the electrical energy consumed during discharge is also small.

With the development of technology, the volume of portable equipment is getting smaller and smaller, and with this trend, the requirements for the protection circuit volume of lithium-ion batteries are getting smaller and smaller. In these two years, there have been products that integrate the control IC and MOSFET into a protection IC, such as the DA7112 series of DIALOG. Some manufacturers even encapsulate the entire protection circuit into a small size IC, such as MITSUMI's products.

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