Since electricity is one of our basic needs and it doesn’t come for free. In fact it increases every year. Meanwhile sun power is free for all so why not use it? Here is a very simple way to charge your battery using solar power.
Why need a charging circuit?
Solar cells and batteries comes in different voltage and current capacity. You need to match these ratings for safety and reliability reasons. Connecting a higher voltage solar panel to a battery might over charge the battery and may cause it to explode. It is best to limit the voltage and current output of the solar panel to avoid this from happening.
About the Circuit
The Circuit is using a very simple linear regulator LM317. You can charge up to 2A using this circuit. D1 is used to make sure that the circuit will not become a load to the battery in the event of solar panel output drops below battery level when cloudy. Rs is used to limit the output current of the charger. In most cases, the output current should not be more than 1/10 of the capacity of the battery. The output voltage should be set to the charging voltage of the battery plus 0.7V to compensate for the diode drop of D1.
Circuit Issues
I did tried to build this circuit and use it for my solar powered home automation project(I will be posting this project soon). Good news is that this circuit works but the bad news is it doesn’t actually limit the current to what you have set. The current lowers as your battery voltage is near your target voltage. I have designed it for 14.4V/ 800mA output but I am reading only around 150mA of charge current at 12V which makes the charging slower. This is because the feedback resistor is still conducting even when the output voltage is below target. I am thinking to replace the R2 with a zener diode which will turn off when the output voltage is below the target voltage but I doubt it will work properly. With those limitations in mind, I have come up with a little complex circuit employing 2 LM317 regulator. One used to give constant voltage and the other will give constant current when the output is below the target.
Improved Version
The improved version uses 2 LM317, this will give you max charging current but I have to test it first to confirm my theory.:)