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

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

1. Lithium battery charging:

According to the structural characteristics of the lithium battery, the maximum charge termination voltage should be 4.2V, and it should not be overcharged. Otherwise, the lithium ion of the positive electrode will take too much, and the battery will scrapped. The charging and discharging requirements are relatively high, and a dedicated constant current and constant voltage charger can used for charging. Normally, the constant current charged to 4.2V/section and then switched to constant voltage charging. When the constant voltage charging current drops to within 100mA, the charging should stopped.

Charging current (mA) = 0.1 to 1.5 times

Battery capacity (such as 1350mAh battery, its charging current can be controlled between 135 ~ 2025mA). The normal charging current can selected to be 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, lithium ions cannot all move to the positive electrode during discharge, and a part of lithium ions must retained at the negative electrode to ensure that lithium ions can be smoothly inserted into the channel during the next charging. Otherwise, the battery life will shortened accordingly. In order to ensure that some lithium ions remain in the graphite layer after discharge, it is necessary to limit the minimum voltage at the end of discharge, that is, 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 related to the 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, 1000mAH battery, the discharge current should strictly controlled within 3A) otherwise, the battery will damaged.

3, lithium battery protection circuit

It consists of two FETs and a dedicated protection integrated block S--8232. The overcharge control transistor FET2 and the over discharge control transistor FET1 connected in series to 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 turned off and the charging stopped. In order to prevent malfunction, a delay capacitor generally added to the external circuit. When the battery is in a discharged state and the battery voltage drops to 2.55 V, the over discharge control transistor FET1 turned off, and the supply of power to the load stopped. Overcurrent protection is to control the FET1 to turn off and stop discharging to the load when a large current flow through the load, in order to protect the battery and the FET.

4. Charging circuit:

Principle: Charge battery with a constant voltage to ensure that it does not overcharge. The input DC voltage is higher than charged battery voltage by 3 volts. 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 indicating circuits. As the voltage of the rechargeable 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 turned off and the LED will extinguished. To ensure that the battery is enough, please continue after the indicator is off. Charge for 1-2 hours. When using, please put appropriate radiator on Q2 and Q3.

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