$4 Solar Battery Charger




Posted in TechnologyElectronics

Introduction: $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.

Step 1: What You Need

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

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 browndoggadgets.com.



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    As long as you dont overload the LED depends on the power of the panels.

    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

    1 reply

    effectively you would require two seperate circuits that overlap at the battery.. One to recharge the battery and the other using the battery as a source.


    Nice build you made! I want to make my own now aswell. I have a question, what will happen if there is no battery hooked up? Will this destroy the cells?

    1 reply

    It shouldn't as the circuit wiill remain open with-out batteries.

    Have 2 solar panel each 5v at 1.2A. Want to charge 3v batteries. Possible?

    I have been struggling with this forever. All I want to do is use my 5v panel to charge my 1.5v NiHM triple A batteries. Seems like it kills every battery, they start out low at 1.0V and then after I use the panel with + to + - to -, with a diode on the negative lead.

    What am I doing wrong?

    5 replies

    First of all, the blocking diode usually goes on the positive terminal of the solar panel going towards the batteries. Secondly, you are using multiple batteries correct? I completely disagree with this article, if you have a 4.5v panel you need to have 3 batteries connected in series to charge safely since each battery is 1.35v charged. With only two batteries on a 4.5v panel you are putting too much voltage on the batteries 4.5v on 2.6v max of batteries, and risk damaging them, especially long term. Over voltage destroys batteries. This is why most solar lights have 1.5v, or if they use two batteries 3v, panels.

    Sorry I missed that you have a 5v panel. In this case you may want to use either 4 batteries (which would never get fully charged but would come pretty close at 1.25v) or else consider using a resistor with at least 2 batteries. Remember R=V/I from physics? If you need a refresher see this page here: http://www.gtsparkplugs.com/Dropping_Resistor_Calc.html You do need to be careful though since the more voltage you are dropping, the higher the wattage resistor you need (in general, most resistors are 1/4 of a watt. For one battery ideally shoot for a voltage of 1.5, 3v for 2 batteries, 4.5 for 3 6 for 4, and so on.

    Ok, also if I stepped the volts from 4v5. to 3.6 v to charge 2 batts or something like that, could I get more mA current, charging the battery faster?

    Ok, I have a 4.5V Panel at 200mA, about 1W, and I only had 2 NiMH triple A's connected for charging.

    I realized I did the diode on the wrong side, but was confused because I guess the Positive side of the battery actually has the most Negatively charged particles (sigh..lol) so everything I read prior about current flow direction was misleading to the project.

    Soo, I set the diode on the pos terminal of the panel facing toward the battery pack (I guess about 2.5-2.7v) and It didn't die, but didn't seem to charge after a couple hours in the sun.

    Is there a way to directly check if the current is flowing into the batterie's + terminal?

    I would first make sure that you have the diode the right way round. Diodes do obviously only go in one direction and have a set polarity. The line on the diode always indicates the negative (out) end.

    The easiest way to check is to put a multimeter, set to the ma current range and then connect it in series between the diode and the battery pack with the red positive probe on the diode out side and the black negative probe going to the battery pack. The meter will read a positive reading in mA if everything is working correctly, which is how much power is flowing into the batteries. If you don't have a multimeter I would highly suggest getting one as it's really an essential tool and worth it just for checking batteries and basic repair/automotive uses; For basic dc, low voltage, use, they can be found quite cheaply at around $7 (I can recommend some if you don't have one). You can also short circuit the solar panel through a multimeter with just the diode and nothing else attached to it to get an idea of it's performance. Solar panels are one of the few things it's fine to short circuit. Unfortunately apart from a multimeter the only thing I can suggest is connecting an LED temporarily in series before the battery packs just to see if the panel is producing power (though if you leave the LED in it will burn out without a properly sized resistor).

    The other thing to keep in mind is that solar panels in general, although especially these smaller ones, are very sensitive to the angle of the sun and the output can go from maybe 100mA or possibly less laying flat on the ground to 200mA angled into the sun. This site has lots of detail on solar panel angles (and the effect on output) and was very helpful to me: http://www.solarpaneltilt.com/

    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

    3 replies


    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.


    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?