Battery Charger Regulator Circuit Diagram – 8V Design

Description

Many commercially available automobile battery chargers are unsuitable for extended use due to the risk of overcharging and subsequent battery deterioration. This supplementary circuit is incorporated in series with the battery being charged and is powered directly by the battery itself. In essence, the circuit employs a high-current MOSFET to regulate the charging current and it deactivates automatically once the battery voltage reaches a designated limit. Power for the circuit originates from the battery, supplying 8V via a 3-terminal regulator, REG1.

LED1 signifies that the battery is connected and that electrical power is available. The 555 timer IC is configured as an astable oscillator operating at approximately 100kHz. This component provides a drive signal to a diode rectifier arrangement (D1 and D2), generating sufficient gate voltage for MOSFET Q3 to switch on with remarkably low on-resistance—typically 14 milliohms. Upon activation of the MOSFET, current flows from the charger’s positive terminal, enabling the charging process to continue. The battery’s voltage is continuously monitored by a 10kO potentiometer, VR1.

555 Timer IC

The 555 timer IC is a versatile integrated circuit often utilized in timing and oscillator applications. In this circuit, it functions as an astable oscillator, producing a continuously oscillating waveform. Its primary function is to generate a precise timing signal, which is crucial for controlling the charging process. The IC’s internal circuitry generates this signal, providing a stable frequency of approximately 100kHz, delivering a consistent drive to the diode rectifier. This component is a self-contained chip delivering the required waveform for the circuit's operation and the high frequency used here is to provide optimum gate drive to the MOSFET.

Circuit diagram:

When the voltage at the wiper terminal of zener diode ZD1 surpasses its breakdown voltage, transistor Q1 activates, pulling pin 4 (reset) low to deactivate the 555 and eliminate gate drive to the MOSFET. This process occurs in a progressive manner, causing the charging cycle to rapidly repeat itself as the battery charges. Eventually, a point is reached where the battery nears its fully charged state, and the cycle slows significantly. Transistor Q2 and LED2 function as an indicator of the charging status. During charging, LED2 remains consistently illuminated, representing the active charging process. When the battery is fully charged, LED2 briefly extinguishes (charging) and then returns to the on state (not charging) for a prolonged period.