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

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.

Comments

[Sherriff]

can i use any other diode?

[Matthew Yang]

You can use either the diode mentioned (rectifier type), or use a Scottky diode (cheaper than rectifier type).

[jonathanh93]

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

[Matthew Yang]

Still 20ma if you connect in series, 40 ma if you connect in parrelell.

[neons8]

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

[Matthew Yang]

1. Connect solar panel to a battery pack of two Nimh cells in parrelell with diode as BrownDogGadgets mentioned.
2. Connect LDR to one of the battery pack's terminals.
3. Connect LED to LDR and other battery pack terminal.
4. If it doesn't work, try reversing LED leads.

[ChrisG439]

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?

[Matthew Yang]

It won't overcharge if you trickle charge it like BrownDogGadgets said.

[sandy666]

Can u say the use and its name

[Matthew Yang]

Blck bar points where you want the current to go.

[Matthew Yang]

diode

[Matthew Yang]

Please don't hack the nice garden lights. I'd rather leave the lights at the store for other people to enjoy if I weren't using them as garden lights. You can get a few solar cells on amazon.

[jordan_chung18]

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

[neons8]

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

[neons8]

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

[AMFerg]

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.

[sblauvelt1]

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.

[seeyato3]

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

[LuisM96]

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

[mshawaii]

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

thanks and God bless