Lithium battery charging circuit principle

　　Lithium battery charging circuit principle

　　1. Lithium batteries and nickel-cadmium and nickel-metal hydride rechargeable batteries

　　The negative electrode of a lithium-ion battery is graphite crystal, and the positive electrode is usually lithium dioxide. During charging, lithium ions move from the positive electrode to the negative electrode and are embedded in the graphite layer. During discharge, lithium ions detach from the surface of the negative electrode in the graphite crystal and move toward the positive electrode. Therefore, during the charging and discharging process of the battery, lithium always appears in the form of lithium ions, not in the form of metallic lithium. Therefore, this kind of battery is called a lithium-ion battery, or lithium battery for short.

　　Lithium batteries have the advantages of small size, large capacity, light weight, no pollution, high single cell voltage, low self-discharge rate, and high number of battery cycles, but they are more expensive. Nickel-cadmium batteries are being gradually phased out due to their low capacity, serious self-discharge, and environmental pollution. Nickel metal hydride batteries have a high cost performance and do not pollute the environment, but the cell voltage is only 1.2V, so their range of use is limited.

　　2. Characteristics of lithium batteries

　　1. It has higher weight-to-energy ratio and volume-to-energy ratio;

　　2. High voltage, the voltage of a single lithium battery is 3.6V, which is equal to the series voltage of three nickel-cadmium or nickel-metal hydride rechargeable batteries;

　　3. The battery has low self-discharge and can be stored for a long time, which is the most prominent advantage of this battery;

　　4. No memory effect. Lithium batteries do not have the so-called memory effect of nickel-cadmium batteries, so lithium batteries do not need to be discharged before charging;

　　5. Long life. Under normal working conditions, the number of charge/discharge cycles of lithium batteries is much greater than 500 times;

　　6. Can be charged quickly. Lithium batteries can usually be charged with a current of 0.5 to 1 times the capacity, shortening the charging time to 1 to 2 hours;

　　7. Can be used in parallel at will;

　　8. Since the battery does not contain heavy metal elements such as cadmium, lead, and mercury, it does not pollute the environment and is the most advanced green battery in the world;

　　9. High cost. Compared with other rechargeable batteries, lithium batteries are more expensive.

　　3. Internal structure of lithium battery

　　Lithium batteries usually come in two shapes: cylindrical and rectangular.

　　The internal structure of the battery is a spiral winding structure, with a very fine and highly permeable polyethylene film isolation material separated between the positive and negative electrodes. The positive electrode includes a lithium ion collector composed of lithium and cobalt dioxide and a current collector composed of an aluminum film. The negative electrode consists of a lithium ion collector composed of sheet carbon material and a current collector composed of copper film. The battery is filled with organic electrolyte solution. In addition, a safety valve and PTC element are installed to protect the battery from damage in abnormal conditions and output short circuits.

　　The voltage of a single lithium battery is 3.6V, and the capacity cannot be infinite. Therefore, single lithium batteries are often connected in series or parallel to meet the requirements of different occasions.

　　4. Charging and discharging requirements of lithium batteries

　　1. Charging of lithium batteries: According to the structural characteristics of lithium batteries, the maximum charge termination voltage should be 4.2V. It cannot be overcharged, otherwise the battery will be scrapped due to too much lithium ions in the positive electrode being taken away. Its charging and discharging requirements are relatively high, and a dedicated constant current and constant voltage charger can be used for charging. Usually, constant current charging reaches 4.2V/section and then it switches to constant voltage charging. When the constant voltage charging current drops to less than 100mA, charging should be stopped.

　　Charging current (mA) = 0.1~1.5 times battery capacity (for example, for a 1350mAh battery, the charging current can be controlled between 135~2025mA). The conventional charging current can be selected to be around 0.5 times the battery capacity, and the charging time is about 2 to 3 hours.

　　2. Discharge of lithium battery: Due to the internal structure of the lithium battery, all lithium ions cannot move to the positive electrode during discharge. A part of the lithium ions must be retained at the negative electrode to ensure that the lithium ions can be smoothly embedded in the channel during the next charge. Otherwise, battery life will be shortened accordingly. In order to ensure that some lithium ions remain in the graphite layer after discharge, the minimum discharge termination voltage must be strictly limited, which means that the lithium battery cannot be over-discharged. The discharge termination voltage is usually 3.0V/section, and the minimum cannot be lower than 2.5V/section. The length of battery discharge time is related to battery capacity and discharge current. Battery discharge time (hours) = battery capacity/discharge current. Lithium battery discharge current (mA) should not exceed 3 times the battery capacity. (For example, for a 1000mAH battery, the discharge current should be strictly controlled within 3A) Otherwise, the battery will be damaged.

　　Currently, all lithium battery packs sold on the market are equipped with matching charge and discharge protection boards. Just control the external charge and discharge current.

　　5. Lithium battery protection circuit

　　The charge and discharge protection circuit of two lithium batteries is shown in Figure 1. It consists of two field effect transistors and a special protection integrated block S--8232. The overcharge control tube FET2 and the overdischarge control tube FET1 are connected in series in the circuit. The protection IC monitors the battery voltage and controls it. When the battery voltage rises to 4.2V , the overcharge protection tube FET1 cuts off and stops charging. To prevent malfunction, a delay capacitor is generally added to the external circuit. When the battery is in a discharge state and the battery voltage drops to 2.55V, the over-discharge control tube FET1 is turned off and stops supplying power to the load. Overcurrent protection is to control FET1 to cut off and stop discharging to the load when a large current flows through the load. The purpose is to protect the battery and field effect transistor. Overcurrent detection uses the on-resistance of the field effect transistor as the detection resistor, monitors its voltage drop, and stops discharging when the voltage drop exceeds the set value. A delay circuit is generally added to the circuit to distinguish surge current and short-circuit current. This circuit has complete functions and reliable performance, but it is highly professional, and the dedicated integrated block is not easy to purchase and is difficult for amateurs to imitate.

　　6. Simple charging circuit

　　There are now many merchants selling single-cell lithium batteries without charging boards. Its performance is superior and its price is low. It can be used for the repair and replacement of homemade products and lithium battery packs, so it is very popular among electronic enthusiasts. Interested readers can refer to Figure 2 to make a charging board. The principle is: use a constant voltage to charge the battery to ensure it will not be overcharged. The input DC voltage is 3 volts higher than the voltage of the battery being charged. R1, Q1, W1, and TL431 form a precision adjustable voltage stabilizing circuit, Q2, W2, and R2 form an adjustable constant current circuit, and Q3, R3, R4, R5, and LED are charging indication circuits. As the voltage of the charged battery increases, the charging current will gradually decrease. When the battery is fully charged, the voltage drop on R4 will decrease, causing Q3 to cut off and the LED to go out. To ensure that the battery is sufficient, please continue after the indicator light goes out. Charge for 1-2 hours. Please install appropriate radiators for Q2 and Q3 when using them. The advantages of this circuit are: simple production, easy purchase of components, safe charging, intuitive display, and will not damage the battery. By changing W1, multiple series-connected lithium batteries can be charged, and by changing W2, the charging current can be adjusted in a wide range. The disadvantage is: no over-discharge control circuit. Figure 3 is the printed circuit board diagram of the charging board (perspective view from the component side).

　　7. Application examples of single-cell lithium batteries

　　1. Use as battery pack repair and replacement products

　　There are many battery packs: such as those used in laptop computers. After repairs, it is found that when the battery pack is damaged, only individual batteries are faulty. You can choose a suitable single-cell lithium battery for replacement.

　　2. Make a bright miniature flashlight

　　The author once used a single 3.6V1.6AH lithium battery and a white ultra-high brightness light-emitting tube to make a miniature flashlight, which is easy to use, compact and beautiful. And because of the large battery capacity, it can be used for an average of half an hour every night, and it has been used for more than two months without charging. The circuit is shown in Figure 4.

　　3. Replace 3V power supply

　　Since the voltage of a single lithium battery is 3.6V. Therefore, only one lithium battery can replace two ordinary batteries to power small household appliances such as radios, walkmans, cameras, etc. It is not only light in weight, but also can be used continuously for a long time.

　　8. Storage of lithium batteries

　　Lithium batteries need to be fully charged before storage. It can be stored for more than half a year at 20°C, which shows that lithium batteries are suitable for storage at low temperatures. Someone once suggested that rechargeable batteries should be stored in the refrigerator compartment, which is indeed a good idea.

　　9. Precautions for use

　　Lithium batteries must not be disassembled, drilled, punctured, sawed, pressurized, or heated, otherwise serious consequences may occur. Lithium batteries without a charging protection board cannot be short-circuited and cannot be played by children. Keep away from flammable items and chemicals. Scrapped lithium batteries must be disposed of properly.

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