Principle Analysis of Lithium Ion Battery Protection Circuit in Automobile Emergency Start Power Supply


In today's cars, seat heating, air conditioning, navigation, infotainment, driving safety and other systems have been designed to improve comfort and driving experience. From these systems, it is easy to understand the electronic systems that power various functions in the car. benefit. Now we can hardly imagine a scene just over 100 years ago, when there was no electronic component in a gasoline-powered car. In the turn of the century, the car began to have a crank handle, the headlights began to be illuminated with acetylene gas, and the bells could be used to send a message to the pedestrian. Today's cars are at the junction of electronic systems, minimizing the adoption of mechanical systems and becoming the largest and most expensive "digital tool" in people's lives.


As more and more mechanical systems are replaced by electronic systems, power consumption becomes more and more important. Regardless of whether the owners are not using the car, there is a possibility that the car battery cannot be started due to insufficient battery power. Therefore, the merchants have developed an electronic product based on the car's power - the car emergency start power.

The protection function of the lithium-ion battery is usually completed by the protection circuit board and the current device such as the PTC. The protection board is composed of electronic circuits, and the voltage of the battery core and the charging and discharging circuit are accurately monitored at -40 ° C to 85 ° C. Current, timely control of the current circuit on and off; PTC in the high temperature environment to prevent battery damage. Ordinary lithium-ion battery protection boards usually include control ICs, MOS switches, resistors, capacitors, and auxiliary devices FUSE, PTC, NTC, ID, memory, and the like. The control IC controls the MOS switch to be turned on under all normal conditions, so that the cell and the external circuit are turned on, and when the cell voltage or the loop current exceeds a prescribed value, it immediately controls the MOS switch to turn off, and protects the cell. Safety.mobile power plant



As shown in the figure, the protection loop consists of two MOSFETs (V1, V2) and a control IC (N1) plus some RC components. The control IC is responsible for monitoring the battery voltage and the loop current, and controlling the gates of the two MOSFETs. The MOSFET functions as a switch in the circuit to control the conduction and shutdown of the charging circuit and the discharging circuit, respectively, and C3 is a delay capacitor. With overcharge protection, over discharge protection, over current protection and short circuit protection.


First, the normal state


In the normal state, the "CO" and "DO" pins of N1 output high voltage in the circuit, both MOSFETs are in conduction state, and the battery can be freely charged and discharged. Since the on-resistance of the MOSFET is small, it is usually smaller. 30 milliohms, 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 level, usually less than 7 μA.


Second, overcharge protection


Lithium-ion batteries require a constant current/constant voltage. In the initial stage of charging, they are charged at a constant current. As the charging process, the voltage rises to 4.2V. (Depending on the cathode material, some batteries require a constant voltage of 4.1V.) ), switch to constant voltage charging until the current is getting smaller and smaller. When the battery is being charged, if the charger circuit loses control, the battery voltage will continue to be constant current when it exceeds 4.2V. At this time, the battery voltage will continue to rise. When the battery voltage is charged to over 4.3V, the battery chemistry Side effects will increase, 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 and different ICs have different values), the "CO" pin will be converted from a high voltage to a zero voltage. Turning V2 from on to off, thus cutting off the charging circuit, so that the charger can no longer charge the battery, which acts as an overcharge protection. At this time, due to the presence of the body diode VD2 that is provided by the 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, usually set to about 1 second to avoid the error caused by the interference. judgment.


Third, over discharge protection


When the battery is discharged to the 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 the battery. Permanent damage. During battery discharge, 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 be converted from high voltage to zero voltage, making V1 Turning from conduction to shutdown, the discharge circuit is cut off, so that the battery can no longer discharge the load, which acts as an over-discharge protection. At this time, due to the presence of the V1 body diode VD1, the charger can charge the battery through the diode. Since the battery voltage can no longer be lowered under the over-discharge 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 state, and the entire protection circuit consumes less than 0.1 μA. There is also a delay time between when the control IC detects that the battery voltage is lower than 2.3V and the signal that turns off the V1. The length of the delay time is determined by C3, usually set to about 100 milliseconds to avoid errors caused by interference. judgment.


Fourth, over current protection


Due to the chemical characteristics of lithium-ion batteries, battery manufacturers stipulate that their discharge current should not exceed 2C (C=battery capacity/hour). When the battery exceeds 2C current discharge, it will cause permanent damage or safety problems. During normal discharge of the battery, the discharge current is passed through two MOSFETs in series. Due to the on-resistance of the MOSFET, a voltage is generated across the MOSFET. The voltage value U=I*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 increases, and when the loop current is so large that U>0.1V (this value is When the control IC determines that different ICs have different values, the "DO" pin will be converted from a high voltage to a zero voltage, causing V1 to turn from on to off, thereby cutting off the discharge loop and causing zero current in the loop. It acts as an overcurrent protection. There is also a delay between when the control IC detects an overcurrent and when the V1 signal is turned off. The length of the delay is determined by C3, usually about 13 milliseconds, to avoid misjudgment caused by interference. In the above control process, 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 MOSFET on-resistance is larger, the over-current protection is applied to the same control IC. The smaller the value.


Five, short circuit protection


When the battery is discharging to the load, if the loop current is so large that U>0.9V (this value is determined by the control IC and different ICs have different values), the control IC determines that the load is short-circuited, and its “DO” pin will It quickly turns from a high voltage to a zero voltage, turning V1 from on to off, thereby cutting off the discharge loop and providing short-circuit protection. The short-circuit protection has a very short delay time, usually less than 7 microseconds. Its working principle is similar to overcurrent protection, except that the judgment method is different, and the protection delay time is also different.


The working principle of the single-cell lithium-ion battery protection circuit is described in detail above. The protection principle of the multi-cell series lithium-ion battery is similar, and will not be described here. In addition to the control IC, there is another important component in the circuit, the MOSFET, which acts as a switch in the circuit. Since it is directly connected in series between the battery and the external load, its on-resistance has a performance on the battery. The effect is that when the selected MOSFET is better, its on-resistance is small, the internal resistance of the battery pack is small, the load carrying capacity is also strong, and the power consumed during discharge is also small.


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