Instructables
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

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?

 
Remove these adsRemove these ads by Signing Up
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.

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.

ArduinoGuido5 months ago

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.

djsupajoe6 months 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!

cinnonym6 months ago

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

ZaneEricB8 months 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.
Schmidty161 year ago
Dang i wish u were my 8th grade science teacher
JeenJogi2 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.
rosebudfarm2 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?
NickFiya2 years ago
Does this work with only 3 of the solar cells?
JoshuaZimmerman (author)  NickFiya2 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.