Monday, October 27, 2014

Small Solar Battery Charger

After a few attempts at different circuits, I finally got a "small" solar panel to charge my NiMH AA batteries (properly) today. I used a 4 volt, 1.5 watt solar panel from Radio Shack to charge 4 AA batteries to a total charge of 5.3 volts. 


Yep, that was 5.3 volts of charge from a 4 volt solar panel.

And, that was not even using direct sunlight for most of the day. But it did take most of the day, which is roughly the normal charge duration for a NiMH battery.

A couple of other home made circuits that I have tried did not work for one reason or another, but I'm happy to see this one work nicely for me. One required a 6 volt solar panel in order to charge 4 AA batteries, another used a buck boost, which drove up the voltage and made it looks like it charged. Only later did I discover that it had no "juice." 


There was no amperage applied to the battery (even though the battery appeared to be charged.  Turns out that I was missing the parts to drive up the amps, and my substitutes did not drive up the amperage properly. So....).

But, I had the parts for this one and tried it out. Worked nicely for me without the LM317 current regulator, but I did have the need to add diodes at the batteries. Basically, I charged the 4 AA batteries in parallel, add diodes at the batteries, and used the 4 volt solar panel as the voltage source.

Web page:

     3V to 9V DC Converters

I used this circuit of the three (edited to add in the transistor pin-outs). 


3V to 9V DC Converters, 4 transistor version


At any rate, there is no am regulator on this design (such as an LM317), but I found that the 1.5 watt solar panel only produced up to 0.09 amps during the process. So, no current regulator was used. Also, there is a need to prevent overcharging the AA batteries, and I hope to later add something that will tell the circuit to shut down (or switch over) when the targeted voltage is reached. I used voltage and amperage meters for monitoring this test.

I did try the 555 circuit on that web page with the 6 volt solar panel, but was not able to get it working for some reason.

I'd like to apply it to a 6 volt panel, just to see if a 6 volt panel would charge 6 AA NiMH batteries. Given that the circuit is designed for up to 4.5 volts, I may have the need to add on an LM7805 voltage regulator in the event that direct sunlight drives up the voltage on a 6 volt solar panel. 

Finally, it would probably be good to add the option to charge only one AA battery instead of all four. I am looking forward to testing to see how many AA batteries I can charge with a small 6 volt solar panel.

Update Sep 7, 2015:

I have yet another AA charging circuit posted here:


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