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Solar Charger Circuit

The Beginning

I’ve been interested in renewable energy for a while now. This past Spring during my trip to Kuala Lumpur, Malaysia, I picked up 2 solar panels that could provide 10 watts of total power under optimal conditions. Oh, and they were only $30USD. Now I have an excuse to start generating my own power!

My goal was to be able to charge a 12V battery that would then be able charge 5V USB devices. There we a few things that I had to keep in mind.

  • How do I regulate the voltage and current that would be charging the 12V battery?
  • How will I know when the 12V battery is charged?
  • What if I’m charging my USB devices and my battery is starting to run low?

The solution ended up being a combination of using a microcontroller to control and monitor the power lines throughout the circuit. I decided to use a PIC12F1840 since I didn’t need a lot of processing power; just a few GPIO and an ADC to monitor the 12V battery voltage.

Revision one had a mistake on the layout which was quickly fixed in revision two. I was also able to shrink the size of the board substantially to 1.7inĀ². Was much more difficult to solder, but I think it turned out well!

Solar Charger – Rev 2
PCB Renderings of Rev 2
The finished product! Too bad the silk screen text didn’t render correctly…

Conclusion

After building this and running it, I noticed that the Low Drop-Out (LDO) regulator that I used to drop 12V to 5V was not the most ideal choice. LDO’s drop the voltage not by fancy power transformation, but by releasing that access energy as good ol’ heat. Not the most ideal when trying to make an energy efficient charger. I think that in revision 3 of this I’ll be switching the regulators to buck converters due to how efficient they are and the relatively large voltage conversions that are taking place.

All in all, this did indeed charge my 5V USB devices and recharge the 12V battery!

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