If the lead acid battery is not using for long time, self discharge takes place at the rate of 4 % per week at 27 degree. For example, a 125 Ah tubular battery self discharge at the rate of 5 Amps current per week if it is not in charge / discharge cycles. This add on circuit can be used to detect the self discharge of the 12 volt lead acid battery below the safe level of 11.6 volts. It is ideal to monitor the charge level in the battery, it the battery is not using for long period.
The Battery Self discharge Indicator circuit uses only a few components and its working is simple. A PNP transistor T1 act as a switch to light the LED, if the battery voltage drops below the safe level. The base bias of T1 is controlled by a Zener diode ZD. Its rating is 10 volt 1 W. The Zener diodes usually requires 1.6 volts excess than its rated value to enter into the “Avalanche state”.
Experiment of discharging a 6V 4.5Ah lead-acid battery
Relay contact of solid state switch
So, as long as the battery voltage is above 11.6 (10+1.6), Zener conducts and keeps the base of T1 high. Since T1 is a PNP transistor, it will not conduct till its base becomes negative. So LED remains dark. When the voltage in the battery reduces below 11 volts, Zener turns off and the base of T1 becomes negative. T1 then conducts and LED lights. So the battery can be charged again to keep it in top condition. Preset VR can be used to set the exact point at which LED turns on.