Introduction: $4 Solar Battery Charger

About: I used to teach middle school science, but now I run my own online educational science website. I spend my days designing new projects for students and Makers to put together.

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, or you can also buy them off ebay or various other websites.

Step 1: What You Need

To build a solar battery charger you need several things, as well as have several tools on hand.

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
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

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

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

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

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

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