Working of the circuit is simple. When the output from the solar panel is 12 volts or more, zener diode ZD1 conducts and provides 11 volts to the inverting terminal of IC1. Since its non-inverting input gets a higher voltage at this time, the output of the comparator turns high and the same is indicated by glowing green LED1. Transistor T1 then conducts and relay RL1 energizes. Thus the battery gets charging current from the solar panel through the normally-open (N/O) and common contacts of relay RL1.
Things which you can learn:
1. Solar energy harvesting
2. Solar charging
3. application of relays
4. step-down transformer
5. Optimum battery charging
Build a simple hybrid charger for your home/office so that it can automatically change the source for charging the battery, thus making sure that the battery charging is not affected by the clouds.
The efficiency of a solar charging system depends on the weather conditions. Usually, the solar panel gets four to five hours of bright sunlight in a day. If the weather is cloudy or rainy, it affects the charging process and the battery does not attain full charge. This simple hybrid solar charger can solve the problem as it can charge the battery using both solar power as well as AC mains supply. When the output from the solar panel is above 12 volts, the battery charges using the solar power. When the output drops below 12 volts, the battery charges through AC mains supply.