Detailed explanation of 18650 li ion rechargeable battery overcharge, over discharge, short circuit protection circuit

　　The circuit is mainly composed of 18650 li ion rechargeable battery protection dedicated integrated circuit DW01, charge and discharge control MOSFET1 (containing two N-channel MOSFETs), etc. The single 18650 li ion rechargeable battery is connected between B+ and B-, and the battery pack outputs voltage from P+ and P-. When charging, the charger output voltage is connected between P+ and P-, and the current flows from P+ to B+ and B- of the single battery, and then passes through the charging control MOSFET to P-.

　　During the charging process, when the voltage of the single battery exceeds 4.35V, the OC pin of the dedicated integrated circuit DW01 outputs a signal to turn off the charging control MOSFET, and the 18650 li ion rechargeable battery stops charging immediately, thereby preventing the 18650 li ion rechargeable battery from being damaged due to overcharging.

　　During the discharge process, when the voltage of the single cell drops to 2.30V, the OD pin of DW01 outputs a signal to turn off the discharge control MOSFET, and the 18650 li ion rechargeable battery stops discharging immediately, thereby preventing the 18650 li ion rechargeable battery from being damaged due to over-discharge. The CS pin of DW01 is the current detection pin. When the output is short-circuited, the on-state voltage drop of the charge and discharge control MOSFET increases sharply, and the CS pin voltage increases rapidly. The DW01 outputs a signal to turn off the charge and discharge control MOSFET rapidly, thereby completing over-current or short-circuit protection.

　　What are the advantages of secondary lithium batteries?

　　1. High energy density

　　2. High operating voltage

　　3. No memory effect

　　4. Long cycle life

　　5. No pollution

　　6. Light weight

　　7. Low self-discharge

　　What are the advantages of lithium polymer batteries?

　　1. No battery leakage problem, the battery does not contain liquid electrolyte, and uses colloidal solids;

　　2. Can be made into thin batteries: with a capacity of 3.6V400mAh, its thickness can be as thin as 0.5mm;

　　3. The battery can be designed into a variety of shapes;

　　4. The battery can be bent and deformed: the polymer battery can be bent up to about 900;

　　5. Can be made into a single high voltage: liquid electrolyte batteries can only obtain high voltage by connecting several batteries in series. Because polymer batteries have no liquid themselves, they can be made into a multi-layer combination in a single battery to achieve high voltage;

　　6. The capacity will be twice that of lithium-ion batteries of the same size.

　　IEC stipulates that the standard cycle life test of lithium batteries is:

　　After the battery is discharged to 3.0V/piece at 0.2C;

　　1.1C constant current and constant voltage charge to 4.2V, cut-off current 20mA, leave for 1 hour, and then discharge to 3.0V at 0.2C (one cycle);

　　After repeated cycles 500 times, the capacity should be more than 60% of the initial capacity;

　　National standards stipulate that the standard charge retention test of lithium batteries is (IEC has no relevant standards);

　　After the battery is discharged to 3.0/piece at 0.2C at 25 degrees Celsius, it is charged to 4.2V at 1C constant current and constant voltage, with a cut-off current of 10mA, and stored at a temperature of 20+_5 for 28 days, and then discharged to 2.75V at 0.2C to calculate the discharge capacity.

　　What is the self-discharge of secondary batteries? What is the self-discharge rate of different types of batteries?

　　Self-discharge is also called charge retention capacity, which refers to the ability of the battery to retain the stored electricity under certain environmental conditions in an open circuit state. Generally speaking, self-discharge is mainly affected by manufacturing process, materials, and storage conditions. Self-discharge is one of the main parameters for measuring battery performance.

　　Generally speaking, the lower the battery storage temperature, the lower the self-discharge rate. However, it should also be noted that too low or too high a temperature may cause the battery to be damaged and unusable. BYD conventional batteries require a storage temperature range of -20~45. After the battery is fully charged and left open for a period of time, a certain degree of self-discharge is normal. IEC standards stipulate that after nickel-cadmium and nickel-metal hydride batteries are fully charged, they are left open for 28 days at a temperature of 20 degrees and a humidity of 65%. The 0.2C discharge time is greater than 3 hours and 3 hours and 15 minutes respectively. Compared with other rechargeable battery systems, the self-discharge rate of solar cells containing liquid electrolytes is significantly lower, about 10%/month at 25.

　　What is the internal resistance of a battery and how to measure it?

　　The internal resistance of a battery refers to the resistance encountered by the current flowing through the battery when the battery is working. It is generally divided into AC internal resistance and DC internal resistance. Since the internal resistance of a rechargeable battery is very small, when measuring the DC internal resistance, the electrode capacity is polarized, and polarization internal resistance is generated, so the actual value cannot be measured. However, measuring its AC internal resistance can eliminate the influence of polarization internal resistance and obtain the actual internal value.

　　The AC internal resistance test method is: using the characteristic that the battery is equivalent to an active resistor, giving the battery a constant current of 1000HZ and 50mA, sampling its voltage, rectifying and filtering, and then accurately measuring its resistance.

　　What is the internal pressure of a battery? What is the normal internal pressure of a battery?

　　The internal pressure of a battery is the pressure caused by the gas generated during the charging and discharging process. It is mainly affected by the manufacturing process, structure and other application process factors of the battery material. Generally, the internal pressure of the battery is maintained at a normal level. In the case of overcharging or over-discharging, the internal pressure of the battery may increase.

　　If the speed of the recombination reaction is lower than the speed of the decomposition reaction, the generated gas will not be consumed in time, which will cause the internal pressure of the battery to increase.

　　What is the internal pressure test?

　　The internal pressure test of lithium batteries is: (UL standard)

　　Simulate the battery at an altitude of 15240m (low pressure 11.6kPa) to check whether the battery leaks or bulges.

　　Specific steps: Charge the battery 1C with constant current and constant voltage to 4.2V, with a cut-off current of 10mA, and then store it in a low-pressure box with an air pressure of 11.6Kpa and a temperature of (20+_3) for 6 hours. The battery will not explode, catch fire, crack, or leak.

　　What is the effect of ambient temperature on battery performance?

　　Among all environmental factors, temperature has the greatest impact on the charge and discharge performance of the battery. The electrochemical reaction at the electrode/electrolyte interface is related to the ambient temperature. The electrode/electrolyte interface is regarded as the heart of the battery. If the temperature drops, the reaction rate of the electrode also decreases. If the battery voltage remains constant and the discharge current decreases, the power output of the battery will also decrease. If the temperature rises, the opposite happens, that is, the battery output power will increase. The temperature also affects the transfer speed of the electrolyte. If the temperature rises, the transfer speed will increase. If the temperature drops, the transfer will slow down, and the battery charging and discharging performance will also be affected. But if the temperature is too high, exceeding 45, it will damage the chemical balance in the battery and cause side reactions.

　　What are the control methods for overcharging?

　　In order to prevent the battery from overcharging, the end of charging needs to be controlled. When the battery is full, there will be some special information that can be used to determine whether the charging has reached the end. There are generally six ways to prevent batteries from being overcharged:

　　1. Peak voltage control: Determine the end of charging by detecting the peak voltage of the battery;

　　2. dT/dt control: Determine the end of charging by detecting the peak temperature change rate of the battery;

　　3. T control: The difference between the temperature and the ambient temperature will reach the maximum when the battery is fully charged;

　　4. -V control: When the battery is fully charged and reaches a peak voltage, the voltage will drop to a certain value;

　　5. Timing control: Control the end of charging by setting a certain charging time, generally set the time required to charge 130% of the nominal capacity to control;

　　6. TCO control: Considering the safety and performance of the battery, high-temperature charging (high-temperature battery outside) should be avoided, so charging should be stopped when the battery temperature rises by 60.

　　What is overcharging and what impact does it have on battery performance?

　　Overcharging refers to the behavior of continuing to charge the battery after it is fully charged through a certain charging process.

　　Because the negative electrode capacity is higher than the positive electrode capacity in the design, the gas generated by the positive electrode passes through the separator paper and combines with the cadmium generated by the negative electrode. Therefore, under normal circumstances, the internal pressure of the battery will not increase significantly, but if the charging current is too large, or the charging time is too long, the generated oxygen will not be consumed in time, which may cause the internal pressure to rise, the battery to deform, leak, and other undesirable phenomena. At the same time, its electrical performance will also be significantly reduced.

　　What is overdischarge and what impact does it have on battery performance?

　　After the battery has discharged the internal stored electricity and the voltage reaches a certain value, continuous discharge will cause overdischarge. The discharge cut-off voltage is generally determined according to the discharge current. 0.2C-2C discharge is generally set at 1.0V/piece, and 3C or above, such as 5C or 10C, is set at 0.8V/piece. Battery overdischarge may bring catastrophic consequences to the battery, especially high current overdischarge, or repeated overdischarge has a greater impact on the battery. Generally speaking, overdischarge will increase the internal pressure of the battery, damage the reversibility of the positive and negative active materials, and even if charged, it can only be partially recovered, and the capacity will also be significantly attenuated.

　　What problems will occur if batteries of different capacities are used together?

　　If batteries of different capacities or new and old batteries are mixed together, leakage, zero voltage and other phenomena may occur. This is because during the charging process, the capacity difference causes some batteries to be overcharged and some batteries to be undercharged. During discharge, some batteries with high capacity are not fully discharged, while those with low capacity are over-discharged. In this vicious cycle, the battery is damaged and leaks or has low (zero) voltage.

　　What is battery explosion and how to prevent battery explosion?

　　Any part of the solid matter in the battery is instantly discharged and pushed to a distance of more than 25cm from the battery, which is called explosion. To determine whether the battery has exploded or not, the following conditions are used for testing. Cover the test battery with a net, with the battery in the middle and 25cm away from any side of the net. The density of the net is 6-7 pieces/cm, and the net wire uses soft aluminum wire with a diameter of 0.25mm. If no solid part passes through the net in the test, it proves that the battery has not exploded.

　　Problems with 18650 li ion rechargeable battery series connection

　　Because the battery has to go through many processes from coating to finished products during the production process. Even if the voltage, resistance and capacity of each group of power supplies are consistent through strict testing procedures, there will be differences of one kind or another after a period of use. Just like twins born to a mother, they may look exactly the same when they are just born, and it is difficult for the mother to tell them apart. However, as the two children continue to grow, there will be differences in one way or another. The same is true for lithium-powered batteries.

　　After differences occur after a period of use, the method of overall voltage control is difficult to apply to lithium-powered batteries. For example, a 36V battery stack must be connected in series with 10 batteries. The overall charging control voltage is 42V, and the discharge control voltage is 26V. Using the overall voltage control method, there may be no problems in the initial use stage because the battery consistency is very good. After a period of use, the internal resistance and voltage of the battery fluctuate, forming an inconsistent state (inconsistency is absolute, consistency is relative). At this time, the overall voltage control still cannot achieve its purpose.

　　For example, when 10 batteries are discharged, the voltage of two batteries is 2.8V, the voltage of four batteries is 3.2V, and the voltage of four batteries is only 3.4V. The overall voltage is now 32V. We let it continue to discharge and work until it reaches 26V. In this way, the two 2.8V batteries are lower than 2.6V and are in an over-discharge state. Lithium batteries are useless if they are over-discharged several times. On the contrary, charging with the method of controlling charging with the overall voltage will also result in overcharging. For example, the voltage state of the above 10 batteries is used for charging at that time.

　　When the overall voltage reaches 42V, the two 2.8V batteries are in a "hungry" state, and the rapid absorption of electricity will exceed 4.2V. Batteries that are overcharged and exceed 4.2V will not only fail due to excessive voltage, but may even cause danger. This is the characteristic of lithium power batteries.

　　The rated voltage of lithium-ion batteries is 3.6V (some products are 3.7V). The stop charging voltage when fully charged is related to the battery anode material: 4.2V for graphite anode material; 4.1V for coke anode material. The internal resistance of different anode materials is also different. The internal resistance of coke anode is slightly larger, and its discharge curve is also slightly different, as shown in Figure 1. They are generally called 4.1V lithium-ion batteries and 4.2V lithium-ion batteries. Most of the ones used now are 4.2V, and the stop discharge voltage of lithium-ion batteries is 2.5V~2.75V (the battery factory gives the operating voltage range or the stop discharge voltage, and the parameters are slightly different). Continuous discharge below the stop discharge voltage is called over-discharge, which is harmful to the battery.

　　Portable electronic products use batteries as power sources. With the rapid development of portable products, the use of various batteries has increased greatly, and many new batteries have been announced. In addition to the high-performance alkaline batteries, rechargeable nickel-cadmium batteries, and nickel-hydrogen batteries that everyone is familiar with, there are also lithium batteries developed in recent years. This article mainly introduces the basic knowledge about lithium batteries. This includes its characteristics, main parameters, the meaning of types, the scope of application, and precautions for use.

　　Lithium is a metal element with the chemical symbol Li (its English name is lithium). It is a silvery white, very soft, chemically active metal, and the lightest metal. In addition to being used in the atomic energy industry, it can be used to make special alloys, special glass (fluorescent screen glass used on TVs) and lithium batteries. In lithium batteries, it is used as the anode of the battery.

　　Lithium batteries are also divided into two categories: non-rechargeable and rechargeable. Non-rechargeable batteries are called disposable batteries, which can only convert chemical energy into electrical energy once, and cannot restore electrical energy back to chemical energy (or the restoration performance is extremely poor). Rechargeable batteries are called secondary batteries (also called storage batteries). They can convert electrical energy into chemical energy for storage, and when used, they can convert chemical energy into electrical energy. It is reversible, such as electrical energy and chemical energy. The main characteristics of lithium batteries.

　　Smart portable electronic products require small size and light weight, but the size and weight of batteries are often the largest and heaviest compared to other electronic components. For example, the "big brother" of the past was quite "bulky and heavy", while today's mobile phones are so light. Among them, the improvement of batteries has played an important role: in the past it was nickel-cadmium batteries, and now it is lithium-ion batteries.

　　The biggest feature of lithium batteries is high specific energy. What is specific energy? Specific energy refers to the energy per unit weight or unit volume. Specific energy is expressed in Wh/kg or Wh/L. Wh is the unit of energy, W is watt, h is hour; kg is kilogram (unit of weight), L is liter (unit of volume). Here is an example to illustrate: the rated voltage of a No. 5 nickel-cadmium battery is 1.2V, and its capacity is 800mAh, so its energy is 0.96Wh (1.2V×0.8Ah). The rated voltage of a No. 5 lithium-manganese dioxide battery of the same size is 3V, and its capacity is 1200mAh, so its energy is 3.6Wh. The volume of these two batteries is the same, so the specific energy of the lithium-manganese dioxide battery is 375 times that of the nickel-cadmium battery!

　　A No. 5 nickel-cadmium battery weighs about 23g, while a No. 5 lithium-manganese dioxide battery weighs about 18g. One lithium-manganese dioxide battery is 3V, while two nickel-cadmium batteries are only 2.4V. Therefore, when using lithium batteries, the number of batteries is small (making portable electronic products smaller and lighter), and the battery has a long working life.

　　In addition, lithium batteries have the advantages of stable discharge voltage, wide operating temperature range, low self-discharge rate, long storage life, no memory effect and no pollution.

　　The disadvantage of lithium batteries is that they are expensive, so they are not widely used at present. They are mainly used in handheld computers, PDAs, communication equipment, cameras, satellites, special, special, instruments, etc. With the development of technologyWith the development, improvement of technology and increase in output, the price of lithium batteries will continue to decline, and their use will become more common.

　　Non-rechargeable lithium batteries

　　There are many types of non-rechargeable lithium batteries. The commonly used ones are lithium-manganese dioxide batteries, lithium-thionyl chloride batteries and lithium and other compound batteries. This article only introduces the first two most commonly used ones.

　　1. Lithium-manganese dioxide battery (LiMnO2)

　　Lithium-manganese dioxide battery is a disposable battery with lithium as anode, manganese dioxide as cathode, and organic electrolyte. The main characteristics of this battery are high battery voltage, rated voltage of 3V (twice that of ordinary alkaline batteries); stop discharge voltage of 2V; large specific energy (see the example above); stable and reliable discharge voltage; good storage performance (storage time of more than 3 years), low self-discharge rate (annual self-discharge rate ≤2%); operating temperature range -20℃～+60℃.

　　This battery can be made into different shapes to meet different requirements. It has rectangular, cylindrical and button-shaped (button-shaped). Cylindrical ones also have different diameter and height standards. Here are the main parameters of the 1# (standard code D), 2# (standard code C) and 5# (standard code AA) batteries that everyone is familiar with.

　　CR stands for cylindrical lithium-manganese dioxide battery; in the five-digit number, the first two represent the diameter of the battery, and the last three represent the height with one decimal. For example, CR14505 has a diameter of 14mm and a height of 50.5mm (this type is universal).

　　It should be pointed out here that the parameters of the same type of batteries produced by different factories may be somewhat different. In addition, the standard discharge current value is smaller, the actual discharge current can be greater than the standard discharge current, and the allowable discharge current for continuous discharge and pulse discharge is also different, and the battery factory provides relevant data. For example, the CR14505 produced by Lixing Power Supply Company gives a maximum continuous discharge current of 1000mA and a maximum pulse discharge current of up to 2500mA.

　　Most lithium batteries used in cameras are lithium-manganese dioxide batteries.

　　Button (button) batteries are smaller in size, with a diameter of 12.5 to 24.5 mm and a height of 1.6 to 5.0 mm.

　　CR is a cylindrical lithium-manganese dioxide battery. The first two of the last four digits are the diameter of the battery, and the last two are the height with a decimal point. For example, the diameter of CR1220 is 12.5 mm (excluding the number after the decimal point) and its height is 2.0 mm. This type of representation is universal.

　　This button battery is commonly used in clocks, calculators, electronic notebooks, cameras, hearing aids, electronic game consoles, IC cards, backup power supplies, etc.

　　2. Lithium-thionyl chloride battery (LiSOCl2)

　　Lithium-thionyl chloride battery is the one with the highest specific energy, which can now reach 500Wh/kg or 1000Wh/L. Its rated voltage is 3.6V, and it has an extremely flat 3.4V discharge characteristic when discharged with a medium current (it can be discharged flatly within 90% of the capacity range, with little change). The battery can operate within the range of -40℃～+85℃, but the capacity at -40℃ is about 50% of the capacity at room temperature. The self-discharge rate is low (annual self-discharge rate ≤1%) and the storage life is more than 10 years.

　　Compare the specific energy of 1# (standard code D) nickel-cadmium battery and 1# lithium-thionyl chloride battery: the rated voltage of 1# nickel-cadmium battery is 1.2V and the capacity is 5000mAh; the rated voltage of 1# lithium-thionyl chloride is 3.6V and the capacity is 10000mAh, then the specific energy of the latter is 6 times greater than that of the former!

　　Precautions for use

　　The above two lithium batteries are disposable batteries and cannot be charged (dangerous when charging!); the positive and negative poles of the battery cannot be short-circuited; they cannot be discharged with excessive current (discharge exceeding the maximum discharge current); when the battery is used to the stop discharge voltage, it should be removed from the electronic product in time; the used battery cannot be squeezed, burned or disassembled; it cannot be used beyond the specified temperature range.

　　Because the voltage of lithium batteries is higher than that of ordinary batteries or nickel-cadmium batteries, do not make mistakes when using them to avoid damaging the circuit. By understanding the CR and ER in the type, you can know its type and rated voltage. When buying a new battery, be sure to buy it according to the original type, otherwise it will affect the performance of electronic products.

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