Solar Battery Charger
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.

This is a simple, solar charger circuit that uses a shunt regulator to prevent overcharging. SC1 is a 12 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 Up
Under bright sunlight, remove battery B1, and replace with a 40h watt 27 ohm load resistor. Adjust VR1 until the output voltage across the 27 ohm load resistor is 14.4 Volt. This is the maximum voltage the battery should reach during a charge. All excess power from the solar cell will then be dissipated via R4.

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