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Picture of $4 Solar Battery Charger

When I got into electrical circuits and solar power, the first thing I wanted to do was build a little solar powered battery charger. Only I had a heck of a time trying to find a simple and straight forward guide to doing this.

So in this guide I'll give you a bit of info on solar power and battery charging, as well as show you how to make a solar battery charger for all of $4.

If you'd like some solar panels or solar kits I have quite a few on my gadget site, browndoggadgets.com or you can also buy them off ebay or various other websites.

 
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Step 1: What You Need

Picture of What You Need
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To build a solar battery charger you need several things, as well as have several tools on hand.

Parts
A clear, water-proof container. (Dollar Store tupperware with built in O-Ring)
AA Battery Holder (Radio Shack, also fits AAAs if you're careful)
One or Two Solar Panels rated 4 Volts or above
Blocking Diode
Tools you need
Soldering Iron
Solder
Tape
Safety Goggles
Some wire

Time: 20-30 minutes

Difficulty: Easy

Step 2: Things You Should Know

Solar Power is fun, and adding solar to your projects is even more fun. Plus these days it's darned cheap to do.

When making a battery charger there are things you should keep in mind.

First, know your batteries. NiMh batteries are the most common these days, and you can find them at any store. Your typical AA NiMh battery probably is 1.2 Volts and has anywhere between 2000- 3000 mah worth of charge in it. (Check your batteries, they probably have the capacity written on them. That or check the maker's webpage.)

Secondly you need to know your solar panels. For instance, the ones I'm using in this project put out a max of 4.5 volts and 80 ma of charge.

With only 4.5 volts coming in, I really shouldn't try charging up any more than two batteries (hooked up in a series giving me 2.4 volts). Also, because one of my solar panels only puts out 80 ma at a max, it's going to take a long time to charge up all 3000 mah hours my batteries hold. In this guide I hooked up two panels in parallel to give me around 160 mahs worth of power coming in. If I had a bigger case I could hook up another one or two to give me even more power.

You're probably asking yourself, "hey, why doesn't he hook up a whole lot of panels to throw down a massive amount of amps and fast charge those batteries!" Good point, but if I did that I'd kill the batteries. Your standard wall charger has brains that let it fast charge a battery without blowing it up. We're going about our charging using the "trickle" method. As a general rule of thumb, you don't want to throw more than 10% of the capacity of the battery (C/10) at the battery when charging. As our batteries are 3000 mah capacity, and we're throwing 160 mah of charge at it, we're ok. (AAA batteries hold between 800 -1800 mah, so we're probably ok for them as well as we're never going to actually get the full 160 ma from the cells.)

If you really want to charge up your batteries fast, you could try and hit the C/10 power supply. Though this being solar, it would still take a while.

So there you have it. Now you've got a basic idea of how to add solar power to your projects. Now go out and buy some Solar Panels and NiMh batteries.

Step 3: Panel Power

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I love these 4.5 volt panels. I use them for most all of my solar projects. They're small, light weight, strong, and throw out a lot of power.

Because my solar panels have little tabs on them, I'm going to need to do some extra soldering and taping that you might not need to do with your solar panels.

That being said, no matter what kind of solar panel setup you're using, you'll want to be wiring them up the same way.

(For instance you could use a smaller panel to charge up a single AA battery, or a bunch of panels in a series to charge up a whole bunch of batteries at once.)

Step 4: Blocking Diode

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The first thing I'm going to do is hook up my blocking diode. I just soldered it onto the positive wire coming off my battery holder, and then the other end of the diode onto one of the positive tabs on a solar panel. Also, at this time, solder the negative wire from the battery pack onto one of the negative tabs of the solar panels.

(If you're only using one solar panel, you're actually done with soldering.)

Why do we need this? Well, solar panels are great at creating power when it's sunny out. When it gets dark, they try and suck power back into themselves, which then destroys them. To stop this we use a blocking diode so that power only flows in one direction.

Also, see that black bar on the diode. Always make sure you know which way it's going. You want the black bar pointing in the direction you want power to flow.

Step 5: Strength in Numbers

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Ok, so if we only had one panel we'd be done now. Since we're using two panels hooked up in parallel we have a bit more soldering to do.

To hook them up in parallel we're going to use two more wires to connect both positive tabs and both negative tabs.

Cut two wires at about the right length for a bridge and solder.

Remember, we're hooking positive to positive, and negative to negative.

Step 6: Lots of Tape

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While all my soldering is done it's painfully obvious that my little solar panel tabs look like they're ready to break off. For good measure I always put some tape over the solder points on my solar panel, especially panels with little tabs on them.

Then just tape the panels into the lid of your enclosure. That was easy.

For my enclosure I used a little tupperware thing I got from a local Dollar Store. It has a freshness o-ring in it that keeps moisture in, which also means it'll keep moisture out. Handy for projects you want to leave outside for long periods of time.

Step 7: Enjoy

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That was easy. I've done this a couple of times before and at this point building one takes me under 20 minutes.

So the breakdown is this.

Cost: $4

$1 Tupperware
$2 Solar Panels
$1 Battery holder
$0.02 Blocking Diode

Time: 20 minutes.

You can use this EXACT same setup to power little light up projects. Throw in a few transistors and resistors and you can make a dark detecting circuit for all of $0.20 more.

If you're looking for solar panels or little solar kits I have several available on my website browndoggadgets.com.

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corbinearl1 year ago
where's the best place to find the solar panels? I'm sorry if someone asked this and I didn't see it when I looked

Hi,

Great idea, but your $4 build figure doesn't add up.

On your website you are selling the..1V 500mA Solar Cells for $8.99!!..so how can you build it for $4?

Sorry I dont mean to be negative, but this fact appears to have been "over looked"

I look forward to your comments.

Regards

What I did was got 2 little solor toys from the dollar store and hacked off the panels
Could a charge controller be placed somewhere between the solar panels and the batteries? And can the batteries be charged while they are also powering a small device?

In theory, you have the right idea, however as these solar panels product such a small amount of current.voltage, I am not sure if it would be worth it.

If you had a larger panel producing more power, say a 12v / 2.5W+ then you could connect this to acharge controller and car battery/inverter setup...

mrvhappy23 days ago

Hi,

Great idea, but your $4 build figure doesn't add up.

On your website you are selling the..1V 500mA Solar Cells for $8.99!!..so how can you build it for $4?

Sorry I dont mean to be negative, but this fact appears to have been "over looked"

I look forward to your comments.

Regards

SamW81 month ago

Works very well.

Thanks.<img src="http://s04.flagcounter.com/mini/kfoW/bg_FFFFFF/txt_DEDEDE/border_F7F7F7/flags_1.jpg" style="display:none">

Great idea, you mentioned the dark detecting circuit, how would you work that in? It would be perfect for some Halloween led "eyes" i built a few years back.
Thanks again
spatuladle2 months ago

how could you make it so that when the batteries are done charging an led lights up? Also what would happen if you let it charger to long?

I would like to know too.

vuralzone1 year ago

Nice job !!

But I have question, do we really need the blocking diode?

As far as I know, solar panels have almost the same physical structure with diodes and they don't allow back current.

Could you correct me please

Thanks

JaredD1 vuralzone4 months ago

Solar panels do allow back current.

iratozer6 months ago

What would be required to run a laptop computer?

JaredD1 iratozer4 months ago

You would need a much bigger solar panel (~15v and at least 60 watts) and good size car battery plus a 12v DC to AC 120v inverter, to plug in your AC laptop adapter.

naturefreak3 years ago
Will a small led work as a blocking diode ?

yes

SparkySolar8 months ago

so simple

rishichavda12 months ago

Very nice project, could have made it very smaller and thus more portable/convenient. Great make overall, hopefully make my own in the next few days/weeks.

kolotour1 year ago

Hey boys and girls, here's a source for some cheap solar battery chargers. Used solar powered yard lights. These ARE solar powered battery chargers already along with a circuit to turn the light on when it's dark. Folks toss these things when they stop working. Usually it's the battery that dies or corrosion that causes the failure. Many end up at your local 2nd hand store. Maybe 50 cents or less for each one. Insert a new battery and/or clean up the contacts and pull out the LED bulb you're done. They're inexpensive when new too. $2 - 4 bucks each.

conscious1 year ago
Hey, kids. Here are some electricity basics. mAh (milliAmpere-hour), a unit of charge and mA (milliAmpere), a unit of current (charge per unit time) are not the same thing. A 3000 mAh battery would run, in principle, for 30 hours if a constant current of 100 mA is drawn from it, for example.
Jobo171 year ago
I can't get over this charger, it's just "different" from the others, this is what got me wanting to make one, so whenever I pass by this when looking for a good design (which I'll probably just end up making my own), I just always click on it and look at it again and just see how simple it really is

P.S. My Algebra teacher in 8th grade had a last name of Zimmerman, so I always think of him when I see yours
Jobo171 year ago
mpeterson19, I can't tell if you mean on/off switch or have the electricity go one way? If your talking about direction then use a diode inbetween the panel and the circuitry (so it goes panel, wire, diode, wire, circuit stuff). If you're talking about on/off switch for charging then you would put an on/off switch between circuit and output or between panel circuit (in order)
JCG51 year ago
Where did you get the solar panels from?
Jobo171 year ago
AndroidJack... it's the mA... the panel's mA shouldn't exceed 10% of the mA capacity of the battery...(theoretically)... the panel usual doesn't get what it says when it comes to mA so you have a little wiggle room.
Awesome build!

This really got me thinking, so I visited browndoggadgets.com for even more ideas. I have two 5V 100mA solar panels already that I want to use, but my question is what determines the rate at which the batteries charge? Is it the difference between the mA from the panels and the mA capacity of the batteries, is it something to do with the voltage, or is it a combination of both, and if so how exactly?
Great Instructable.

Just wondering how much the voltage of the solar cells matters? You went into depth about the amps, but only mentioned that your cells run at 4v.
Does the voltage need to be higher than the battery supply?

Also what happens if you do leave them to charge for months? I have some ideas for projects, but they would be sitting outside for a long time...
Yes, the charge voltage must exceed the total full-charge voltage of the batteries.

As long as you have your diode properly in place, keeping this setup out for a few weeks should be ok, but months may be pushing it, as the batteries are being charged and charged and charged without any usage save for leakage. In your case, this sounds unavoidable, so you may want to look into batteries that are built for this kind of duress, or even consider some type of device that switches off the solar panels when the batteries reach full capacity, and then on again when they become low.
JCG52 years ago
On my battery clip I have a toggle switch. When I have the solar panel charging the batteries, do I leave the switch on or off?
Depends on where the switch is located. If it is switching between the panels and the batteries, then leave it closed (on) so that the panels may charge the batteries. If it is after the batteries, then it really doesn't matter, unless there is a device plugged into the USB. In that case, close the switch to charge the device, and open it to prevent the device from charging. Are you using the switch INSTEAD of a diode? If that is the case, keep the switch closed only when you know the panels can harness sunlight.
Fashiondez2 years ago
Is it okay if I want to hook my two 4.5v 80mAs in a series for just two batteries, to make it more powerful?
GeoGyrl3 years ago
I am doing some research that requires about 300 AA batteries in a village with no electricity. I want to use the eneloop 2000 as it seems like they will last a long time and also recharge a lot of times. I decided to make solar chargers because $30 each for 30 chargers ON TOP of the battery costs is tough. I'll be right near the equator so sun isn't an issue. Been reading what I can, but what would you EXPERTS recommend to charge 4 AA batteries in about 8 hours? I'm guessing over 6V panel but what ma? I'm thinking of training women to make these little chargers for batteries and cellphones so trying to figure out how to solder without electricity. MAYBE can use butane if I can find a can in the city to recharge the cartridge. I leave in less than three weeks so any help would be appreciated.
For soldering u can use any kine of metal with sharp tip... Heated with any kind of heat source, fire from wood chips would work, nor biogas, kerosene stove, alcohol burner and many more....
Dbaby66 GeoGyrl3 years ago
You can find solder guns that use gas to burn. I have one. They are portable and use lighter fluid. You can find them on ebay pretty cheap.
You can get some that you actually put a lighter in. Thats a lot easier than trying to find lighter fluid
I will be finding on of those because that is way easier. Thanks.
GeoGyrl Dbaby663 years ago
It was difficult to find small containers of butane over there in Uganda and I couldn't take them on the plane so I bought battery powered soldering irons that used 4 AA batteries and they worked just fine. They aren't using them a lot, just a couple of connections. I was able to get solar panels and battery holders cheap and used eneloop rechargable batteries and it went great. Thanks for your ideas!
Dbaby66 GeoGyrl3 years ago
They can also use butane, mine just uses lighter fluid
Fashiondez3 years ago
will two 4.5v solar cells be able to efficiently charger four AAs? Or is it best to stay with just 2 AAs for that?
JoshuaZimmerman (author)  Fashiondez3 years ago
4 AAs equals 5V, so two of the 4.5V cells should work just fine.
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