36v 7.5ah lithium ion battery pack.Analyze the working principle of the protection circuit in the battery

　　In addition to the chemical composition or electrode parameters of the battery, there are several certain parameters for lithium-ion batteries. If exceeded, the battery will enter an out-of-control state. In the diagrams explaining these parameters (refer to the Lithium Ion Parameters Chart), any point outside the corresponding threshold curve is a runaway condition. As the battery voltage increases, the temperature threshold decreases. On the other hand, anything that causes the battery voltage to exceed its design value will cause the battery to overheat. 1. Normal state: In the normal state, the "CO" and "DO" pins of N1 in the circuit both output high voltage, and both MOSFETs are in the on state. The battery can be charged and discharged freely, because the on-resistance of the MOSFET is very small. Small, usually less than 30 milliohms, so its on-resistance has little impact on the performance of the circuit. 7|The current consumption of the protection circuit in this state is μA level, usually less than 7μA. 2. The charging method required for overcharge protection lithium-ion batteries is constant current/constant voltage. In the early stage of charging, constant current charging is used. As the charging process progresses, the voltage will rise to 4.2V (depending on the positive electrode material, some batteries require constant current (voltage value is 4.1V), switch to constant voltage charging until the current becomes smaller and smaller. During the charging process of the battery, if the charger circuit loses control, the battery voltage will continue to be charged with constant current after it exceeds 4.2V. At this time, the battery voltage will continue to rise. When the battery voltage is charged to exceed 4.3V, the battery chemistry will Side reactions will be aggravated, 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. V2 is turned from on to off, thus cutting off the charging circuit and preventing the charger from charging the battery, thus serving as overcharge protection. At this time, due to the existence of the body diode VD2 of V2, the battery can discharge the external load through this diode. There is still a delay time between when the control IC detects that the battery voltage exceeds 4.28V and sends the shutdown V2 signal. The length of this delay time is determined by C3 and is usually set to about 1 second to avoid errors caused by interference. judge. 3. Over-discharge protection When the battery discharges 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 , will cause permanent damage to the battery. During the battery discharge process, 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 change from high voltage to zero voltage, causing V1 From on to off, the discharge circuit is cut off, so that the battery can no longer discharge the load, which plays the role of over-discharge protection. At this time, due to the existence of the body diode VD1 of V1, the charger can charge the battery through this diode. Since the battery voltage cannot be reduced in 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 consumption state, and the power consumption of the entire protection circuit will be less than 0.1μA. There is also a delay time between the control IC detecting that the battery voltage is lower than 2.3V and sending the shutdown V1 signal. The length of this delay time is determined by C3 and is usually set to about 100 milliseconds to avoid errors caused by interference. judge. 4. Over-current protection Due to the chemical characteristics of lithium-ion batteries, the battery manufacturer stipulates that the maximum discharge current cannot exceed 2C (C=battery capacity/hour). When the battery is discharged with a current exceeding 2C, it will cause permanent damage to the battery. or security issues arise. During the normal discharge process of the battery to the load, when the discharge current passes through two MOSFETs connected in series, a voltage will be generated at both ends due to the on-resistance of the MOSFETs. The voltage value U=I*RDS*2, RDS is a single MOSFET conduction resistance, the "V-" pin 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 enough to make U>0.1V (this value is determined by When the control IC determines (different ICs have different values), its "DO" pin will change from high voltage to zero voltage, causing V1 to turn 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 the control IC detecting the occurrence of overcurrent and sending the shutdown V1 signal. The length of this delay time is determined by C3, usually about 13 milliseconds, to avoid misjudgment caused by interference. From the above control process, it can be seen that the overcurrent detection value not only depends on the control value of the control IC, but also depends on the on-resistance of the MOSFET. When the on-resistance of the MOSFET is larger, for the same control IC, the over-current protection The smaller the value. 5. When the short-circuit protection battery discharges the load, if the loop current is large enough to make U>0.9V (this value is determined by the control IC, different ICs have different values), the control IC will judge that the load is short-circuited, and its " The DO" pin will quickly change from high voltage to zero voltage, causing V1 to turn from on to off, thus cutting off the discharge circuit and playing a role in short-circuit protection. The delay time of short circuit protection is very 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. The above describes in detail the working principle of the single-cell lithium-ion battery protection circuit. The protection principle of multi-cell series lithium-ion batteries is similar and will not be repeated here. The control IC used in the above circuit is the R5421 series of Japan's Ricoh Company. In actual battery protection circuits, there are many other types of control ICs, such as Japan's Seiko S-8241 series, Japan's MITSUMI's MM3061 series, Taiwan's Fujing's FS312 and FS313 series, Taiwan's Analog Technology's AAT8632 series, etc. The working principles are similar, but there are differences in specific parameters. In order to save peripheral circuits, some control ICs have filter capacitors and delay capacitors built into the chip, and their peripheral circuits can be very few, such as the S-8241 series of Japan's Seiko. In addition to the control IC, there is another important component in the circuit, which is the MOSFET. It plays the role of a switch in the circuit. Since it is directly connected in series between the battery and the external load, its on-resistance has a significant impact on the performance of the battery. Influence, 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 strong, and it consumes less power during discharge. With the development of science and technology, the size of portable devices is getting smaller and smaller, and with this trend, the requirements for the size of the protection circuit of lithium-ion batteries are also getting smaller and smaller.

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