Build Your 12V Lead Acid Battery Meter with LEDs
Description
Within this circuit, a quartet of voltage comparators – specifically, the LM339 – is implemented as a basic charge indicator for a 12-volt lead-acid battery. A 5-volt reference signal is supplied to each of the positive (+) terminals of the four comparators. Simultaneously, the negative (-) terminals are connected via a voltage-dividing network to sequential points. The light-emitting diodes (LEDs) activate when the voltage at the negative (-) input surpasses the reference voltage. Precise adjustments to the 2K potentiometer are performed to ensure that all four LEDs illuminate when the battery voltage reaches 12.7 volts, signifying a full charge with no load applied to the battery. Conversely, at 11.7 volts, the LEDs should be extinguished, signifying a depleted battery state. Each LED approximately corresponds to a 25% variation in the charge level, or 300 millivolts, allowing for a visual indication of 3 LEDs representing 75%, 2 LEDs signifying 50%, and so forth. The exact voltages are subject to variations resulting from environmental factors, such as temperature, and differences in battery types, including wet cell or gel cell configurations.
LM339 Voltage Comparator
The LM339 is a versatile operational amplifier (op-amp) that functions as a voltage comparator. It is commonly used in circuits where a decision needs to be made based on whether one voltage is greater than or less than another. In this application, the LM339 compares the battery voltage with a fixed reference voltage (5V), triggering the LEDs based on the comparison result. Its high input impedance and slew rate make it suitable for accurate voltage comparisons. It is a single-supply op-amp, simplifying the circuit design. The LM339 offers excellent performance characteristics, including low offset voltage and good linearity, which are crucial for accurate charge level determination. It operates over a wide temperature range, ensuring reliable performance under various conditions. The device is typically used in charge level indication circuits, voltage level detection, and signal switching applications.
Circuit diagram
