How to Make a Solar-Powered Battery Charger

The storms that blew through the Midwest and Mid-Atlantic on June 29 left us, along with millions of other people and all of the businesses in my town, without power for several days. I was incredibly grateful for a handcrank/solar-powered radio that I’ve had for years. It keep me abreast of the news, and provided some musical entertainment. It also inspired me to figure out how to make some solar appliances for future power outages. I heard stories on the radio of  people buying up batteries, and it occurred to me that I would prefer not to have to rely on them.

So, I headed to a library a couple of towns away (the closest one that was open) and did some research. I decided to start with a solar-powered battery charger since it was an easy first project. It required knowledge of how to use a soldering iron, but I had learned this as a child, having put together many Radio Shack kits with my father. The charger would be a low-cost-of-entry project, and if it turned out okay, I would go on to bigger and better things!

In addition to a 25-watt soldering iron ($8.99) and some solder ($5.49), which I can use for numerous future projects, I found that all I needed was the following items:

1) A solar panel. You need to have a total of 9V to have enough energy to charge your batteries. I purchased a 1W 9V ($16.99), but some articles I read used anywhere from 2V to 4.5V and connected them. You can even salvage some from inexpensive solar garden lights for less than that.

2) A rechargeable battery holder. I chose an enclosed one that can hold four AA’s ($2.29), but they come in a variety of configurations for different types of batteries.

3) A few things I read said that I needed to have a “blocking diode” to make sure that once the batteries are charged, and the light source is taken away (i.e., the sun goes down), that the power doesn’t flow back from the batteries into the solar cell and damage it. However, at one of the Radio Shacks I visited (I ended up going to a total of 3), the guy helping me said it wasn’t necessary, and since I didn’t know the exact number/name of the diode that I needed, I decided to forgo it and do some more reading.

Apparently, there is some debate over the necessity of blocking diodes for such a small project. However, I came across an article by someone who had actually measured the amount of energy flowing in and out, and he concluded it was necessary even for a project of this size. So, after some more reading, I decided to purchase a 2-pack of 1N4001 micro 1A diodes ($1.29).

Once I had gathered all of my materials, the actual putting together of the battery charger was very simple.

1) I soldered the negative (black) wire from the solar panel to the negative (black) wire from the battery holder.

2) Then I soldered one end of the diode to the positive (red) wire from the solar panel, and the other end of the diode to the positive (red) wire of the battery holder. I then trimmed the excess wire.

NOTE: It is important that you attach the diode in the proper direction to make sure that the energy is flowing TO the batteries and is blocked from returning to the solar panel, so it is important to read the diagram on the diode packaging to see which way it should be facing. Diodes have colored (or in my case grayed) bands indicating which end is which.

3) This part is not necessary, but since you have to leave your charger in the sunlight for several hours, it is nice to have some sort of weather protection. So I taped the solar panel to the inside of the lid of a takeout container. You can use anything with a clear top.

4) Once it was assembled, I just inserted four AA NiMh rechargeable batteries and let it sit for a total of 15 hours in the sunlight (I read that it takes anywhere from 10 to 15 hours for the batteries to fully charge).

Once the charging period was over, I transferred the batteries into this little flashlight. It worked like a charm! It was very satisfying to know that I had harnessed the sun’s power for this simple task, and it gave me the confidence to move onto bigger things.

My next project will be a solar USB charger so that I won’t have to worry about running out of juice in my phone during the next extended power outage!

[UPDATE: I came across some new instructions for a solar-powered battery charger in which the author recommended the use of a 1N914 Diode. I don't know enough about the differences between the 1N914 and 1N4001 to say which is more appropriate. If there are any experts out there who can shed some light on this, I would greatly appreciate it!].

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68 thoughts on “How to Make a Solar-Powered Battery Charger

  1. Great stuff! I love the do it yourself ethic – that builds knowledge and intellectual curiosity, and you get a lot more satisfaction out of the final product.
    I used to love those “250-in-1″ electronic kits when I was a kid, and my life was literally changed when my grandfather and I built a shortwave radio together. I really need to look into solar-powered shortwave; that would allow communication should cell service or all electricity be disrupted.

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    • Thanks, Rob! That’s so cool that you had such a great experience creating a shortwave with your grandfather. It’s amazing how many people I’ve spoken to have fond memories of putting together those electronic kits as children. I had no idea how popular those were. Let me know if you end up building a solar shortwave.

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  2. Very cool! Now that we’re living in Arizona my husband and I are trying to figure out ways to harness the sun’s power for more things that run on electric. I think my husband is going to want to read this as a start for his solar pool heater project. I’m thinking of making my own solar powered lights now. Thanks for the inspiration!

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  3. i am working on a slimair project in the uk to and as everyone that has ever been to england the weather is a bit ummm shitty (lot of wind) so i done a bit of messing around with your idea and add a fan connected to a alternater so that i get the extra power so if the sun is not enough to charge my battery and now that i have worked out all the kinks in it i am looking at starting up a business in it

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  4. Well said, but we all need to realize that adding Solar in their home is an purchase which will increase the future value of their residence if / when they decide to sell. With the environment the way it is going we cannot overlook any product or service that gives totally free power at no cost to both the client and more significantly the world!

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  5. the 1N4001 is the better choice. The 1N914 is a signal diode and will handle only 150mA of current, the 1N4001 will handle 1 full amp, (1000mAs).

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      • The solar panel you describe is unlikely to get above the maximum current rating of the 1N914. The 1N914 has a lower forward voltage drop, which means it will dissipate less heat and make the charging more efficient. Germanium diodes and Schottky diodes have lower forward voltage drops than silicon diodes. A power diode can drop as much as 1.5V at high currents.

        BTW here’s one for when you have a 12V panel lying about:

        http://www.den-uijl.nl/electronics/solar_COTS.html

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      • Thank you for your comment, Oscar. I am no electrical engineer, and was going by what I had read in several articles, plus the advice of some folks I spoke to int he store where I purchased the diode.

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    • As I mentioned in the article, Jay, I came across an article by someone who had actually measured the amount of energy flowing in and out a small charger like this one, and he concluded it was necessary even for a project of this size. So I would suggest you use a diode.

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      • Thank you for your comment, Dennis. I agree. I read conflicting information on this point, but decided to use the diode, which I think makes the most sense.

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    • Hi Bobby, This was my first solar project, so I’m still a novice. I would suggest Googling the topic, as I am sure that somebody has written instructions on how to make a phone charger!

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    • I am working on a scaled up version of this. I am using a solar panel with a charge control to keep a deep cycle (marine) 12 V battery charged. Connected to that is a power inverter providing me with AC power. This allows me to plug in phone chargers, power tool battery chargers and pretty much anything I want that falls under the wattage of the power inverter. The charge controller is not required if you get the right diodes, but it helps to maintain a long life on the battery and therefore a long life on the solar generator.

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      • That sounds great, Joe! Thanks for sharing and for your advice on the diodes. If you are posting pics/info on your charger, please feel free to leave a link here. I would love to see a photo of it.

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  6. i understand the idea of the enclosure and keeping the weather out but would like to know just how hot that little greenhouse gets while charging..the batteries themselves get hot just from the charge but the sun beating down must raise the inside temp to something close to 80 or 90 degrees..maybe adding something like mylar or tinfoil to the inside of the lid to reflect heat away from it might be good for longer lasting abilities for the battery and the device itself..

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    • Hi Tobor, Thank you very much for your comment and suggestions. Yes, in direct sun it can get quite hot (although I haven’t actually measured the temperature, which I would like to do at some point). I will try the reflective material on the inside of the lid at some point.

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  7. Hi I am looking to build something significantly larger if when u have a chance we could discuss notes it would be great leave me a message on my email.

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    • Hi Ian, Thank you for your confidence in my “expertise,” but I’m afraid that was my first attempt at building a solar-powered device and I have not built anything larger. So, I don’t feel qualified to give you advice on such things. If you do a search for “solar” at Make (http://makezine.com), or check out Build It Solar (http://www.builditsolar.com), you should be able to find lots of helpful information. Best of luck!

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    • Hi James, I’m guessing you can add more panels so that you can add additional holders. However, I don’t know what you would need to do, exactly, to make sure you had sufficient energy to charge your batteries.

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  8. Hey can you make these on video and put on YouTube? I understand things better with verbal teaching but great job
    I really want to learn how to do it :-)

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  9. This was so well done. Thank you. Another question please…we are hoping to make a battery for a solar cooker to use in case of rain or no sun. Do you think this could be used in some way?

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  10. I used two 5V 200mAH solar panels that I got from Fry’s Electronics for 13 bucks apiece to make a simple solar tracker using a hacked Futaba servo and some erector set gears.
    I mounted a thin board that holds a dozen 75 cent Walmart solar garden lights around the perimeter with the tracker panels in the center (with a small 6″ vertical board between them) onto an axle suspended 8 1/2″ above a wooden frame, so that it’s set up like a see-saw. I remove the clear plastic “lamp” piece on each light and plug the bases into 1-7/8″ holes in the board.
    I hang this tracker from the southern side of my garage in the morning at the sun and in the evening it’s pointed directly West as the sun goes down. I pop out my light bases and twist each lens back on. Place them throughout the house… In the kids rooms at night. I’ve been doing this all Summer, and it’s always worked great… Especially living in Phoenix ;)

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  11. Splendid I did mine too…….coool cos my country has epileptic power supplu and I cant just be feeding my generator with fuel moreso when I need just little power.

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  12. I found this very helpful! I have this school project and I needed to be able to charge batteries with a solar panel. I was planning to do it, but then I noticed it was due tomorrow. So this was, again, very helpful and because of this, I just may be able to get it turned in!

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