Lm358 charging self-stop circuit diagram

Generally, ordinary nickel-metal hydride chargers use constant current charging, and there is no charging stop function, which often causes the battery to be in a state of transient charging, which will greatly shorten the service life of the nickel-hydrogen battery. The basic principle of the LM358 composing voltage comparator is that the negative input terminal of the op amp is connected to a reference voltage. When the positive input terminal reaches this voltage, the output terminal will be flipped, and the high level will be converted to a low level. The control circuit (relay or SCR) Stop charging the battery.

The principle of automatic charging disconnection: 1. The charging IC obtains the current charging current through the sampling resistor Rsns between the SNS and the VCC pin; 2. When the charging current is less than the set value of the constant current charging minimum current, the charging IC considers the battery It is full; 3. The cc pin of the charging IC sends a control signal to turn off the power switch tube (transistor or MOS tube) Q1, thereby cutting off the charging circuit. When the power is reconnected, the charging IC detects the battery voltage through the BAT pin to determine whether to charge. In fact, the lithium battery has a protection board, with over-current protection function, as long as the output voltage of the charger is not too high 4.2V, it will not cause damage to the battery in a short period of time.

For the working state of the switch, there are only two kinds of conduction and disconnection. We can replace the switch with a triode. As shown in Figure 2, add a "high level" to the base of the triode to keep it saturated when the battery is not fully charged. Turns on, and when the battery is fully charged, it applies a “low level” to the base of the transistor to turn it off. How to make the triode automatically switch between the on and off states according to the state of charge of the battery?

This requires an understanding of the charging characteristics of NiMH batteries. According to the charge-discharge characteristic curve of Ni-MH battery, when the nickel-hydrogen battery is discharged, the discharge termination voltage is 1.0V. When charging, the battery reaches 1.2V in a short time. When the Ni-MH battery is charged, its charging is terminated. The voltage is 1.5V. It can be seen that the voltage across the rechargeable battery is constantly changing as the charging process progresses. We can design a circuit to detect the voltage change across the rechargeable battery. When the battery voltage does not reach the charge termination voltage, the detection circuit always outputs a “high”. The level" signal is used to control the triode to make it saturate (equivalent to K-close). When the detection circuit detects that the battery voltage reaches the set end-of-charge voltage, the detection circuit always outputs a "low level" signal. Keep the transistor in the off state (equivalent to K disconnection).

Lm358 charging self-stop circuit

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