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Picture of 9V Solar Battery Charger

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

 
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Step 1: What You Need

Picture of What You Need
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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?

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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

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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

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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

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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

Picture of 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

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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

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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.

AmitD21 month ago

How do you regulate over charging?

AmitD21 month ago

This is a great idea! However I tried to put it together and it doesn't work? can someone help me? :(

rreeves4 months ago

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?

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?
johnp148 months ago

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

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
bdailey22 years ago
How do you avoid over charging the battery?
JoshuaZimmerman (author)  bdailey22 years ago
The method we're using is only a "trickle" charge, which means we don't have to worry about over charging. We're not throwing enough current at it to do any damage. They'll just stay topped off.

Correct.

ty.buchanan.7510 months ago

You can only trickle charge 9volt rechargeables. Unlike AA and AAA that can take higher power. The battery internals just explode out of the casing. The lower the trickle charge the better - if you want a charged solid 9 volt battery left to use.

I found several 9V/AA/AAA battery chargers on eBay for $4 shipped! What kind were you looking for? Perhaps I'm missing something.

9Volt chargers you buy on eBay are built to run on electricity in Hong Kong. It is not identical to the power in Australia. Chargers tend to overcharge - to much power. Hong Kong 9volt rechargables tend to be very low power. The result is when you try to charge them on the Australian power grid they "explode'. There is no sound when they do, but the "guts" of the battery opens out like a concertina. You cannot force it back in. Buy chargers and batteries sold in Australia. They are expensive. You are waisting your money buying 9volt on eBay. I know and have learnt my lesson.

djsupajoe1 year ago

hey wassup?... I was just wondering... I've been thinking of using one of those waterproof LED strips in back yard to light up fence at night in summer...i've powered them off 9v battery before... I was thinking about daisy chaining a couple old solar lawn stake light panels to make it solar...do you think this would work??? 12v would be better i suppose because they run 12v.... any idea??? THanks!

cinnonym1 year ago

What will happen if i leave it on charge permanently n00b; please help!!!!

ZaneEricB1 year ago

I know it been two....three years since you did this but thank you! I am in the process of linking all my smoke alarms ( that seem to want/need/pine/whine for new batteries at 3 am) with this linkage.

No more 3 am wake up calls, no more replacement batteries...and its essentially free!!

Thank YOU!!

giltech1 year ago
i need to charge 4 9v battery can this charge tem
Another great instructable as always. I love all ur stuff. Made the Altoid flashlight & the AA solar charger to go with it. Thanks for doing such a great job. They r easy to follow.
Schmidty162 years ago
Dang i wish u were my 8th grade science teacher
JeenJogi3 years ago
Commercial Solar Panels are Expensive!
Fact: It will take you more than 10 years to pay back
Solution: Using Surplus Solar Cells You can get pay back in 1-2 years
There is an Engineer from Chicago his name is John Sommer
He explain it All in his diy solar panels Blog Search for him using Google
Type "top diy solar panels" Open the first Result.
Note: Ignore the adds at the top.
rosebudfarm3 years ago
Awesome design! Those solar lamps are dump fodder for sure and easy to come by.

I am looking for a way to trickle charge 9v irrigation timers. Would this set-up be adequate and could I just piggyback the leads from the timer battery to the charger?
NickFiya3 years ago
Does this work with only 3 of the solar cells?
JoshuaZimmerman (author)  NickFiya3 years ago
Well 3.5V x 3 = 11.5, which is not going to be enough to charge up your 9V. You'd need at LEAST 12V, but more like 16 to play it safe.

Joshua
Thanks bud
jim_20003 years ago
How much current do you get out of these cells?
JoshuaZimmerman (author)  jim_20003 years ago
Something like 20mA. Usually less.

Thats what we need though. These 9V batteries are rated at only 280mA, and we don't want to throw more than 10% of their capacity at them at once or they'll die.
JoshuaZimmerman (author) 3 years ago
If you're having issues finding solar cells, you may just wish to buy three 6V cells. I do have some small ones on my website that work wonders.