All rechargeable battery has their specific level of charging and discharging, they are likely to get damage if the battery voltage exceeds that level. Here is a simple circuit battery voltage monitor used to indicate the state of battery by monitor them.

Circuit description of battery voltage monitor

The circuit of battery voltage monitor is fabricated and designed around op-amp IC LM709 configured as comparator. Where bi-color LED is used as indicator and indicates three voltage level state of a 12V battery. Resistor R₁ with potentiometer VR₁ is used as potential driver of voltage monitor circuit.

When voltage level rise above 13.5 volts, the output from IC₁ goes low as a result LED begins to emit RED light. Similarly, when the voltage fall below a preset level (10Volts) the output goes high and the LED start to emit GREEN light. Resistors R₃ and R₄ is used as current limiter of LED.

NOTE: Adjust VR₁ such that LED begins to emit GREEN light when 10V DC is connected.

PARTS LIST

Resistors (all ¼-watt, ± 5% Carbon)

R₁ = 1 KΩ

R₂ = 18 KΩ

R₃, R₄ = 680 Ω

VR₁ = 10 KΩ (Potentiometer)

Semiconductors

IC₁ = LM709

D₁ = 1N4003

Miscellaneous

B₁ = 12V Battery

LED = Bi-color LED