Circuit : Andy Collinson
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A solar battery charger that uses a shunt regulator to prevent overcharging. Circuit uses a 12V solar battery, but can be adapted for other voltages.
NotesThis is a simple, solar charger circuit that uses a shunt regulator to prevent overcharging. SC1 is a 15 Volt solar panel with a rating of 40 Watt. RS is not in the circuit but represents the solar cells internal resistance. The solar cell, powers a simple op-amp voltage regulator. R2 and D2, hold the non-inverting input of the op-amp at 5.1 Volt. The voltage on the inverting input is set by VR1 and R1. Under charging conditions, the output of the op-amp is low and all voltage, reaches battery B1. Diode D1 prevents the battery from powering the circuit when there is no output from the solar cell.
When there is an excess of sunlight, the op-amp output will swing high, switching on the BDW193 Darlington transistor, and excess energy is dissipated into the load, R4. This is a 2.2 ohm resistor and should be rated at the solar cells full power, which is 40 Watts in this circuit.
Setting UpUnder bright sunlight, remove battery B1, and replace with a 40 Watt 27 ohm load resistor. Adjust VR1 until the output voltage across the 27 ohm load resistor is 14.4 Volt. Diode D1, is a Schottky diode and features a very low forward voltage drop, typically 500mV at 3A forward current for the SB320 to SB340 range. This limits the maximum voltage available to charge the battery to 14.5V but you do not want to overcharge the battery so adjust VR1 for a maximum voltage of 14.4V or less, according to the battery manufacturers recommendations.
During charging any excess power from the solar cell will then be dissipated via R4.