Solar Charger Monitor Circuit

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.

Solar Charger Monitor Circuit Diagram


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    if i will use two NPN transistor instead of TWO PNP then what will happen??? plz tell me…

    • Carter

      Rashi, with two NPN transistors this circuit will not work. This is because T2 will not have current going into the base from the Emitter. This circuit is based on the fact that the current is going out of the T2 base when things are going well due to the fact that the voltage is higher on the emitter than the base, because of the voltage drop across the diodes.

  • piczim

    I have several batteries being charged by a solar panel through a regulator then connected in parallel to feed various pieces of equipment. I would like to put a circuit in between the batteries and the epuipment so that the equipment run from the particular battery feeds only that piece of equipment The only common connection to the batterieswillbe the solar panels through the regulator charging the battery.

    Many thanks


  • charles hossa

    This circuit looks interesting I want to make one but I want to add a GREEN LED to indicate that the battery is full charged so where can i place it and what changes can i make in order add this LED?

  • John Highet

    Hi, The circuit looks interesting and I want to make one but modified to use with a 5W panel charging a 12V NiMh battery pack. Can you suggest what modifications would be needed please? Does this circuit still need a blocking diode? last question – what voltage drop would occur across the circuit- i.e. how efficient is the circuit?

  • sohail

    how much its expensive?

  • azizul

    if i want charge battery 12v 8ah. which components must be replace??

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