When the battery voltage rises to 4.2V, the overcharge protection transistor FET1 stops charging. To prevent malfunction, a delay capacitor is usually added to the external circuit. When the battery is discharging, and the battery voltage drops to 2.55V, the over-discharge control transistor FET1 fails, and the load power supply stops. Overcurrent protection refers to the operation of FET1 cutting off and stopping the discharge to the load when there is a large current flowing through the load, in order to protect the battery and the MOSFET. Overcurrent detection uses the on-resistance of the MOSFET as the detection resistor to monitor its voltage drop, and stops the discharge when the voltage drop exceeds a set value. A delay circuit is usually added to the circuit to distinguish between surge current and short-circuit current. This circuit is comprehensive and reliable, but it is highly specialized, and the dedicated chips are not easy to purchase, making it difficult for laymen to replicate.
Simple charging circuit:
Many vendors sell lithium-ion batteries without a charging board. Their superior performance and low price, along with their use in DIY projects and for repairing and replacing lithium-ion batteries, make them popular among electronics enthusiasts. Interested readers can refer to Figure 2 to build a charging board. The idea is to charge the battery at a constant voltage to ensure it doesn't overcharge. The input DC voltage is 3 volts higher than the rechargeable battery voltage. R1, Q1, W1, and TL431 form a fine-adjustable voltage regulator circuit; Q2, W2, and R2 form an adjustable constant current circuit; and Q3, R3, R4, R5, and the LED serve as a charging indicator circuit. As the rechargeable battery voltage increases, the charging current gradually decreases. Once the battery is fully charged, the voltage across R4 drops, causing Q3 to stop charging and the LED to turn off. To ensure the battery is fully charged, continue charging for 1-2 hours after the indicator light goes out. When using this circuit, please install a heatsink suitable for Q2 and Q3. The advantages of this circuit are: simple construction, readily available components, safe charging, intuitive display, and no damage to the battery. Replacing W1 allows for the series charging of multiple lithium-ion batteries, while replacing W2 allows for adjustment of a wider range of charging currents. The drawback is the lack of an over-discharge control circuit.
