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$4 Solar Battery Charger

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Step 2: Things You Should Know

Solar Power is fun, and adding solar to your projects is even more fun.  Plus these days it's darned cheap to do.

When making a battery charger there are things you should keep in mind.

First, know your batteries.  NiMh batteries are the most common these days, and you can find them at any store.  Your typical AA NiMh battery probably is 1.2 Volts and has anywhere between 2000- 3000 mah worth of charge in it. (Check you batteries, they probably have the capacity written on them.  That or check the maker's webpage.)

Secondly you need to know your solar panels.  For instance the ones I'm using in this project put out a max of 4.5 volts and 80 ma of charge.

With only 4.5 volts coming in I really shouldn't try charging up any more than two batteries (hooked up in a series giving me 2.4 volts).  Also, because one of my solar panels only puts out 80 ma at a max it's going to take a long time to charge up all 3000 mah hours my batteries hold.  In this guide I hooked up two panels in parallel to give me around 160 mahs worth of power coming in.  If I had a bigger case I could hook up another one or two to give me even more power.

Your'e probably asking yourself, "hey, why doesn't he hook up a whole lot of panels to thrown down a massive amount of amps and fast charge those batteries!"  Good point, but if I did that I'd kill the batteries.  Your standard wall charger has brains that let it fast charge a battery without blowing it up.  We're going about our charging using the "trickle" method.  As a general rule of thumb you don't want to throw more than 10% of the capacity of the battery (C/10) at the battery when charging.  As our batteries are 3000 mah capacity, and we're throwing 160 mah of charge at it, we're ok.  (AAA batteries hold between 800 -1800 mah, so we're probabl ok for them as well as we're never going to actually get the full 160 ma from the cells.)

If you really want to charge up your batteries fast you could try and hit the C/10 power supply.  Though this being solar it would still take awhile.

So there you have it.  Now you've got a basic idea of how to add solar power to your projects.  Now go out and buy some Solar Panels and NiMh batteries.
 
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kz1o3 years ago
It's worth mentioning that the "mah" unit you are mentioning is for the battery, and not for the charger. It's like the difference between "kilowatt hours", and "kilowatt". The first one might be how much energy a light bulb uses in a month, and the second one is just how much the light bulb uses. Some people compare this to a hose filling up a bucket with water. The size of the hose is how fast the water can go into it, and the size of the bucket is how much water it holds. Think of how many hose-seconds it takes to fill up a bucket.

Your charger circuit here will have the desired effect, and for the batteries you mention, the issues with milliamps should not be a concern. The kind of series (blocking) diode is not a big deal either, since the forward drop will not matter. You may use any garden variety silicon diode, and worst case is that the diode will fail (without damaging anything else) and you are out a penny. Just be sure to pick a diode that will allow the max current from the solar cell, in your case, 80 mA, which is 0.08 Amps.

Good luck, and always, safety first. It's a great habit to have.
DualPhase kz1o2 years ago
"Hose-seconds", lol. great example. Will ANY diode work, zener included? Are there any benefits of one over another?
JoshuaZimmerman (author)  kz1o3 years ago
Oh goodness. I can't believe I made that mistake. D'oh!

I really shouldn't be writing instructables at 1 a.m. anymore.

Thanks!
I disagree! Write them whenever you feel the motivation.... those creative juices flow at the most insane times of day or night, that is for sure.

The biggest tragedy would be for someone to have a great idea and not share it.
cloud_bam2 years ago
Ok so I have a question. I ask because I am a total amateur in electronics. So you mention that you don't want to charge the batteries at more that 1/10th of the capacity and that is something I have seen in a few places. But what about voltage?

Say we have two AA batteries in series Then wired in parallel to another set.

That's about 2.4-2.8v at like...4000Mah.
What if we used a cell that is 6v @330Mah? The Mah is cool but what about voltage? If anybody could help I would be really grateful.
I get lots of AAA batteries and the highest I have seen for NiMH was 1000 mAh, not 1800. Usually they are 700-900 or unlabelled. Also, the highest AA I have seen is 2800 mAh, usually 2000-2500 or unlabelled. For unlabelled, assume the lower end of that scale. For NiCad, AAs are 600 to 1000 mAh and AAA are 150 to 300 mAh.

Also, you shouldn't charge NiMH above 30 C or 86 F so if it is getting really hot, like in a car in the sunlight, you will have a lower capacity from that charge.

kz1o3 years ago
Oh yeah, one more thing. Be sure that the Tupperware lid that you select will pass plenty of sunlight in the part of the spectrum that the solar cell uses. You can test this yourself: hook up a milliamp meter across the solar cell, with the meter set to the highest scale (like 100 mA). Hold it up to the light, and without changing anything else, watch what happens to the current output when you put the tupperware in front of the solar cell. You will probably see the current drop by about half. This, logically, will cause your batteries to charge half as fast. So if you can find the best cover for this purpose you will do well.

Again, good luck, and keep experimenting!

kz1o
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