Instructables
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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
IMG_6120.jpg

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?

Picture of 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?

AmitD229 days ago

How do you regulate over charging?

AmitD229 days ago

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

rreeves3 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?
johnp147 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
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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.
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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.