Lithium battery protection circuit

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.

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 turn off. To ensure that the battery is sufficient, please continue after the indicator light turns off. 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 no damage to 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 a printed circuit board diagram of the charging board (perspective view from the component side).