What is the design principle of the 3.7v lithium battery charging circuit?

　　According to the structural characteristics of the lithium battery, the highest end-of-charge voltage should be 4.2V, and it cannot be overcharged. Otherwise, too much lithium ions from the positive electrode will be taken away and the battery will be scrapped. The charging and discharging requirements are relatively high, and a dedicated constant current and constant voltage charger can be used for charging.

　　1. Charging of lithium battery:

　　According to the structural characteristics of lithium batteries, the maximum end-of-charge voltage should be 4.2V, and it cannot be overcharged. Otherwise, too much lithium ions in the positive electrode will be taken away and the battery will be scrapped. The charging and discharging requirements are relatively high, and a dedicated constant current and constant voltage charger can be used for charging. Normally, the constant current charge to 4.2V/cell will switch to constant voltage charge. When the constant voltage charge current drops to less than 100mA, the charge should be stopped.

　　Charging current (mA)=0.1～1.5 times

　　battery capacity (such as 1350mAh battery, its charging current can be controlled between 135-2025mA). The normal charging current can be selected at about 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, and some lithium ions must be retained in the negative electrode to ensure that lithium ions can be inserted into the channel smoothly during the next charge. Otherwise, the battery life will be shortened accordingly. In order to ensure that some lithium ions remain in the graphite layer after discharge, it is necessary to strictly limit the minimum discharge termination voltage, which means that the lithium battery cannot be over-discharged. The final discharge voltage is usually 3.0V/cell, and the minimum should not be lower than 2.5V/cell. 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. (Such as 1000mAH battery, the discharge current should be strictly controlled within 3A) Otherwise, the battery will be damaged.

　　3, lithium battery protection circuit

　　It consists of two field effect transistors and a dedicated protection integrated block S-8232. The overcharge control tube FET2 and the overdischarge control tube FET1 are connected in series in the circuit, and the battery voltage is monitored and controlled by the protection IC. When the battery voltage rises to 4.2V , The overcharge protection tube FET1 is turned off and charging is stopped. In order to prevent malfunction, a delay capacitor is generally added to the external circuit. When the battery is in a discharging state, when the battery voltage drops to 2.55V, the over-discharge control tube FET1 is cut off, and the power supply to the load is stopped. Over current 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 the field effect tube.

　　4. Charging circuit:

　　Principle: Use a constant voltage to charge the battery to ensure that it will not be overcharged. The input DC voltage is 3 volts higher than the charged battery voltage. R1, Q1, W1, and TL431 form a precision adjustable voltage regulator 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 rises, the charging current will gradually decrease. After the battery is fully charged, the voltage drop on R4 will decrease, so that Q3 will be turned off and the LED will go out. To ensure that the battery is sufficient, please continue after the indicator light goes out. Charge for 1-2 hours. Please install suitable radiators for Q2 and Q3 when using.