This add on circuit can be attached to the solar charger to see whether the battery is charging or not. It lights a Red LED to indicate that the battery is not accepting charge. It gives a warning indication if there is any loose connection with the charger and battery.
The Solar charger monitor circuit uses two PNP transistors T1 and T2 to give a warning indication if there is any loose connection with the charger and battery. If the connection is intact and current is flowing into the battery, diodes D2 and D3 forward bias and drops around 1.2 volts. This forward voltage drop across the diodes causes T2 to conducts. The collector current from T2 keeps the base of T1 high and it remains off. Red LED connected to the emitter of T1 remains off indicating that current is flowing to the battery and the connections are intact. When there is any break in the connecting cables or any loose contacts in the terminals, no more current passes and D2 and D3 reverse biases. This turns off T2 and T1 conducts .LED lights indicating that battery is not getting charging current.