6. Comparator IC CA3140 along with medium power PNP transistor BD140 act a switch to prevent over charging of battery. IC1 is designed as a comparator to compare the input voltage and the terminal voltage of battery. The Non inverting input( pin3) of the comparator gets voltage from regulator IC through R5. The inverting input (pin2) is connected to the battery terminal. So in the charging mode, the output of comparator will be high because the inverting input remains low due to the flow of current into the battery. Diode D2 remains reverse biased. Since the base of T1 gets positive bias through R5, VR1 and R6, the transistor will be off. When the battery attains full charge, its terminal voltage rises to 13.5 volts, no more current passes into the battery ( if it is good). This increases the voltage at pin2 of the comparator and its output turns low. Diode D2 then forward biases and T1 conducts. This drains the output voltage of the regulator IC to ground. Red LED indicates the full charge status. VR1 can be used to adjust the biasing of T1. Set it to keep Red LED off in the normal charging mode.
7. A Cow bar protection circuit is also added to protect battery from excess voltage (Suitable in Solar charger) . SCR 2P4M and Zener diode ZD act like a Cow bar circuit. When the charging voltage or the battery voltage is excess than 15 volts, Zener conducts and triggers SCR. Excess voltage will be drained through SCR. Yellow LED indicates the activation of protection circuit.