# Solar Battery Charger Circuit

Posted on Feb 7, 2014

This is the most simple and affordable solar battery charger that the hobbyist can make. It has a few drawbacks over other similar controls, but offers numerous advantages. It is intended for charging lead-acid batteries, but may also be used for charging any battery at a constant voltage. Voltage output is adjustable. This is also referred to dro

p-out voltage.  The input voltage must exceed the output voltage by about 2. 75V @ 1. 5A. Fortunately, when the battery discharged, the output voltage is lower so the solar panel voltage will also be lower. When fully charged, the battery voltage will be high, but the current is very low ”at this point, the drop-out voltage reduces to about 2V and the open circuit solar panel voltage also comes into play. The schottky rectifier was selected to reduce this head voltage requirement ”the voltage drop of the schottky is about 0. 5V @ 1. 5A or about half that of a typical silicon rectifier. In this solar battery charger project the power is limited by the thermal resistances of both the LM317T and the heat sink. To keep the junction temperature below the 125 °C Max, the power must be limited to about 10W. If a smaller or less effective heat sink is used, the maximum power dissipation must be de-rated. Fortunately, the LM317 has internal temperature limiting so that if it gets too hot, it shuts down thus protecting itself from damage. Max power comes into effect when charging a 12V battery @ 1. 5A: e. g. battery voltage = 12V, solar panel = 18V. P = (18V 12V) * 1. 5A = 9W. So thermally, it is carefully matched to the current rating. If a solar panel that is characterized for 12V is applied with a 6V battery, the maximum current must be reduced to about 0. 7A: e. g. battery voltage = 6V, solar panel voltage = 18V. P = (18V 6V) *...

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