In general, standard nickel-metal hydride (NiMH) chargers operate using a constant current charging method and typically lack an automatic shut-off function. This can lead to the battery being in a transient charging state, which significantly reduces the lifespan of the NiMH battery. To address this issue, circuits like those based on the LM358 operational amplifier can be used to create a voltage comparator that detects when the battery is fully charged.
The basic principle of the LM358-based voltage comparator involves connecting the negative input terminal of the op-amp to a reference voltage. When the voltage at the positive input terminal reaches this reference value, the output switches from high to low. This signal can then be used to control a relay or an SCR, effectively stopping the charging process once the battery is full.
The automatic charging cut-off mechanism works as follows: First, the charging IC measures the charging current through a sampling resistor connected between the SNS and VCC pins. When the current drops below a predefined minimum threshold, the IC determines that the battery is fully charged. At this point, the CC pin sends a signal to turn off the power switch (such as a transistor or MOSFET), thereby interrupting the charging circuit. Once power is reconnected, the IC checks the battery voltage via the BAT pin to decide whether to resume charging.
It’s worth noting that lithium-ion batteries usually come with a built-in protection board that offers overcurrent protection. As long as the charger’s output voltage stays below 4.2V, the battery should not be damaged even if it's overcharged for a short time.
Regarding the LM358-based self-shutoff circuit, the operation involves two states: conduction and disconnection. These can be replicated using a transistor. As shown in the diagram, applying a "high" signal to the base of the transistor keeps it in saturation (turned on) when the battery is not fully charged. When the battery is fully charged, a "low" signal is applied to the base, turning the transistor off.
To enable the transistor to automatically switch between these states based on the battery's charge level, we need to understand the charging characteristics of NiMH batteries. According to their charge-discharge curve, the discharge termination voltage is around 1.0V, while the charging termination voltage is approximately 1.5V. During charging, the voltage across the battery increases rapidly, reaching about 1.2V quickly.
By designing a circuit that monitors this voltage change, we can trigger the switching mechanism. If the battery voltage hasn’t reached the set termination level, the detection circuit outputs a "high" signal, keeping the transistor in the on state. Once the voltage reaches the threshold, the circuit outputs a "low" signal, turning the transistor off and cutting off the charging current.
This type of circuit is widely used in simple NiMH battery chargers to prevent overcharging and extend battery life. The LM358-based design is both cost-effective and reliable, making it a popular choice for DIY projects and basic charging applications.
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