9V 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.

Did you know that you can buy 9V rechargeable batteries? You can, but they're rare in stores. I recently bought some off ebay to use on projects and loved them. Only downside is that I don't have a 9V battery charger and all the ones I found on ebay are quite expensive.

Luckily I'm resourceful and decided to build my own. The entire project only cost me $10 and didn't take that long.

Cost: $10
Time: 1/2 Hour
Difficulty: Easy

Step 1: What You Need

Supplies:
Solar Cells - Read the next page for lots of details
1N914 Diode
9V Battery Holder
Wire
Enclosure - Optional

Tools:
Soldering Iron
Tape
Cutting Tool

All the supplies for this project can be found on my website, http://www.browndoggadgets.com. I even have a 9V kit of everything you need. 90% of all sales go towards more fun projects. The other 10% goes to doggy toys.

Step 2: What Kind of Solar Cells?

We have to be careful when picking solar cells, much more when designing trickle charging circuits for AA batteries.

When trickle charging NiMh Rechargeable batteries, the highest amount of power you can throw at them is 10% of their capacity. For instance most AA NiMh batteries have between 2,000-3,000 mah of charge, which means we can throw between 200-300 (depending on the battery) of current at them.

Now my 9V NiMh batteries have only 300 mah of charge, meaning the max I can throw at them is 30 ma. As most cells you find online are at least 50 ma of current we're going to have to find a better solution.

Enter in some cheap solar garden lights. I found these at a local store for $3 each, on sale for $2. Their cells put out 3.5V @ 20 ma. To make our 9V charger we're going to combine 4 of them together to get 14V @ 20 ma.

You can probably find similar cells elsewhere on the internet, but where would the fun be in that?

Step 3: Gutting the Solar Light

Most solar garden lights have very similar setups to the ones I have, probably because they all come from the same factory in China.

First remove the base of the body.

Then remove screws.

Inside you'll find all kinds of goodies. Remove the battery and the circuit, then snip the wires as close to top as you can.

Bonus: Save the circuit and the battery. You can always use the light detecting circuit for other projects. If nothing else you have a free battery and LED.

Step 4: Remove the Solar Cell

This part is tricky. The best solution I found was to use big tin snips to cut away the plastic until the solar cell was free. Be careful as the cell is cased in glass which does chip.

You also need to be careful because you don't want to rip out the two soldering points from the cell. Trying to get new solder on can be a big pain.

Step 5: Solder Them Together

Once you have your four cells, you need to solder.

We'll be connecting these in a series, going positive to negative all the way around. This way our voltage goes up while our current stays the same.

I placed mine in a little square.

Then put some tape down to hold them together.

Using small bits of wire I connected the cells together.

If you have a multimeter you can always give them a test when you're done.

Step 6: Add Battery Holder and Diode

Lastly we need to add the diode and battery connector.

The reason we use a blocking diode is to prevent the solar panels from trying to suck power back out of the batteries at night. Without the blacking diode we'd end up with damaged cells and dead batteries.

First wrap the positive (red) wire from the battery connecter to the black stripped side of the diode. It's important that you connect the battery connector to the BLACK STRIP side of the diode otherwise you'll end up blocking power coming from he solar cells.

Then solder.

Now all you have to do is solder the other end of the diode to the positive point on the solar cell and the black wire from the battery connector to the negative point on the solar cell.

For good measure, tape things up. You can never use enough tape.

Now you've got a mini solar panel that puts out around 14V @ 20 ma.

Step 7: Housing

You can use anything to house this charger in. Shoot, you could just leave it like it is if you're going to be keeping it inside.

I like using some plastic food containers from my local $1 store. The ones I find have a rubber o-ring in them to keep moisture out.

That being said I wouldn't want to leave these out in a rainstorm.

And yes, I did just use a bunch of tape to stick the cells on. Be fancy and use caulk if you wish, I'm going cheap with tape.

Step 8: Enjoy

We'll you're done. Never again will a smoke alarm in your home, or guitar amp in your basement be without a 9V battery.

I hope you've found this instructable helpful. If you're in need of parts for this project, or for a premade charger, I have all the necessary parts on my website http://www.browndoggadgets.com. At a user request, I put together a 9V charger kit at a bit of a discount. 100% of all sales go right back to other projects, which means more instructables.

It also means I can keep annoying my 4th grade science club with silly inventions and gadgets. Children are so easily amused.

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    39 Discussions

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    sandy666

    12 months ago

    Can u say the use and its name

    FQASIPIGMFIPZ16.SMALL.jpg
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    ChrisG439

    1 year ago

    any chance of overcharging with this setup? I'm planning on opening a 1/2 inch, 9v solenoid water valve with a motion sensor at night to scare deer. Will any fewer panels work if charging all day in the Texas sun and battery only owed at night?

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    neons8jordan_chung18

    Reply 2 years ago

    Make sure you have a diode going in the right direction. Just a suggestion

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    neons8neons8

    Reply 2 years ago

    I see somebody already cashed of that sorry. But you might check to see if your solar panels are putting out the right voltage also

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    AMFergjordan_chung18

    Reply 2 years ago

    Did you ever figure it out? I realize it is a little late now. Did you put the diode in the correct orientation? If you flipped it the diode would just act as a stop.

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    neons8

    2 years ago

    I made it
    How would I connect a LED light to it were it would come on at night and recharge in the day?
    Thank you
    neons8

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    sblauvelt1

    2 years ago

    Thanks for the information about trickle charging NiMh batteries! I didn't know about the 10% rule and blowing up batteries scares the heck out of me.

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    jonathanh93

    2 years ago

    Can you increase the ma to 40ma to charge two 9v batteries or will it still be 20ma to charge them?

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    seeyato3

    2 years ago

    How do you protect from over charging since power consumption is very minimum for smoke alarm and panel is charging all the time

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    LuisM96

    3 years ago

    excuse me?, if i have a battery of 9 volts why i have to charge it with solar power in 14 volts?, or a 1.2 rechargable battery putting juice in it with

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    Sherriff

    3 years ago

    can i use any other diode?

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    mshawaii

    3 years ago

    Good morning! can i have a copy of this diagram.

    thanks and God bless

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    rreeves

    3 years ago on Introduction

    This is a great Instructable! I would like to power a set of 20-50 LED lights using this. Therefore I think a 9v would be best. However I am worried about the type of solar panel being used. If I used AA batteries, would I be right in thinking a slow trickle charge would be ok as you said you can throw more power at them?

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    pip.cartwright.3

    3 years ago

    Brilliant! Now i know how to stop having to find batteries for my trombone headphone amp! I'm a bit of a dimbo with electricity. Am i right that it's a14v charger for a 9v battery? No rush of over volting the battery?

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    johnp14

    4 years ago on Introduction

    I want to jam a doorbell into this circuit. It's got a lighted doorbell button. I can't figure out how to connect the pieces together. I've attached a crude drawing of my best guess.

    doorbell_base.jpg
    1 reply
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    TheIronHobojohnp14

    Reply 3 years ago on Introduction

    This looks correct! Though its hard to give specific help without more information on the lighted switch itself. Pressing the switch should momentarily allow current to pass through the switch and the bell. But if you want the switch to be lighted at all times, then it would need its own supply of current when the switch is not being pressed. My guess is that your lighted switch has more than two contacts on it(perhaps 4?). Two to power the LED inside of it and the two that let the current pass through when pressed. If so your schematic would look something like this(picture attached). The value of the led inside and what power it requires would also need your consideration. I'm already off topic with the instructable above so I'll let you do the research as to what that your lighted switches power requirements might be and how you would fulfill them.

    FJGDI5BI2LM3OPZ.LARGE.jpg
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    bdailey2

    6 years ago on Step 8

    How do you avoid over charging the battery?