Battery Over Discharge Indicator

Most of the battery voltage indicators always remain on even if the load is off. The LED indicator and the circuit consume current which reduces the battery charge and the charger should be continuously switched on to keep the charge of the battery. Here is an ideal solution to prevent this. The indicator turns on only if the load is running. It gives a Red LED indication when the battery voltage reduces below 4.7 volts. So that the load can be switched off to prevent deep discharge of the battery.

The circuit uses the OpAmp CA3140 as a voltage comparator and SCR 2P4M as switch. The inverting input of IC1 gets half supply voltage(3V) from the potential divider R2-R3 and its non inverting input gets a higher voltage through R1. Capacitor C1 maintains stable voltage level at the non inverting input of IC1. Power supply to the IC is obtained through the SCR 2P4M when the load switches on.

Output voltage of IC1 is used to drive the deep discharge indicator LED.As long as the output voltage from IC1 is above 5 volts, Zener conducts and keep the PNP transistor off. So that LED remains off. When the battery voltage reduces below 5 volts, output voltage of IC1 also reduces to 5 volts or less. This makes the Zener out of conduction and T1 turns on and LED lights to indicate low battery level.

Battery over discharge indicator circuit

Experiment of discharging a 6V 4.5Ah lead-acid battery

Relay contact of solid state switch


Normally IC1 will be off since SCR is not conducting. The gate of SCR (point A) should be connected after the load switch so that SCR fires only when the load turns on. Before connecting the circuit to the battery, adjust the breakdown point of Zener using a variable power supply. Give 5 volts and slowly adjust VR till LED turns on. This is the breakdown point of Zener. It will be around 5 volts. If Zener is still conducting, reduce its value to 4.1 volts. CA3140 is a low power BiMOS Op Amp and its output will be nearly full supply voltage. Once the SCR triggers, it latches itself and remains conducting even if the gate voltage is removed. SCR can be switched off by removing power through S1.


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