$4 Solar Battery Charger

When I got into electrical circuits and solar power the first thing I wanted to do was build a little solar powered battery charger.  Only I had a heck of a time trying to find a simple and strait forward guide to doing this.  

So in this guide I'll give you a bit of info on solar power and battery charging, as well as show you how to make a solar battery charger for all of $4.

If you'd like some solar panels or solar kits I have quite a few on my gadget site, browndoggadgets.com or you can also buy them off ebay or various other websites.

Step 1: What You Need

To build a solar battery charger you need several things, as well as have several tools on hand.

Parts
A clear, water-proof container.  (Dollar Store tupperware with built in O-Ring)  
AA Battery Holder (Radio Shack, also fits AAAs if you're careful)
One or Two Solar Panels rated 4 Volts or above
Blocking Diode (Radio Shack, or buy 100 for $1 off eBay.)

Tools you need
Soldering Iron
Solder
Tape
Safety Goggles
Some wire

Time: 20-30 minutes

Difficulty: Easy

Comments

[JCG5]

On my battery clip I have a toggle switch. When I have the solar panel charging the batteries, do I leave the switch on or off?

[Fashiondez]

Is it okay if I want to hook my two 4.5v 80mAs in a series for just two batteries, to make it more powerful?

[GeoGyrl]

I am doing some research that requires about 300 AA batteries in a village with no electricity. I want to use the eneloop 2000 as it seems like they will last a long time and also recharge a lot of times. I decided to make solar chargers because $30 each for 30 chargers ON TOP of the battery costs is tough. I'll be right near the equator so sun isn't an issue. Been reading what I can, but what would you EXPERTS recommend to charge 4 AA batteries in about 8 hours? I'm guessing over 6V panel but what ma? I'm thinking of training women to make these little chargers for batteries and cellphones so trying to figure out how to solder without electricity. MAYBE can use butane if I can find a can in the city to recharge the cartridge. I leave in less than three weeks so any help would be appreciated.

[hendrosutono]

For soldering u can use any kine of metal with sharp tip... Heated with any kind of heat source, fire from wood chips would work, nor biogas, kerosene stove, alcohol burner and many more....

[Dbaby66]

They can also use butane, mine just uses lighter fluid

[Dbaby66]

You can find solder guns that use gas to burn. I have one. They are portable and use lighter fluid. You can find them on ebay pretty cheap.

[Billybob101]

You can get some that you actually put a lighter in. Thats a lot easier than trying to find lighter fluid

[Dbaby66]

I will be finding on of those because that is way easier. Thanks.

[GeoGyrl]

It was difficult to find small containers of butane over there in Uganda and I couldn't take them on the plane so I bought battery powered soldering irons that used 4 AA batteries and they worked just fine. They aren't using them a lot, just a couple of connections. I was able to get solar panels and battery holders cheap and used eneloop rechargable batteries and it went great. Thanks for your ideas!

[Fashiondez]

will two 4.5v solar cells be able to efficiently charger four AAs? Or is it best to stay with just 2 AAs for that?

[JoshuaZimmerman]

4 AAs equals 5V, so two of the 4.5V cells should work just fine.

[bigjohn1a]

Great project! I

'm now having a problem figuring out how to rig one of these up to charge 4AA's that are connected to 8 LED's and have a dusk to dawn circuit. I have 2 panels that are 4.5 volts +/-, 3.2 volt-20ma Amber LED's, but no charging circuit. I think it's best in the long run to have a full charging circuit for this project. Do you think I am correct? I also don't have a properly configured dusk to dawn circuit. Any idea's? Any help would be great. Thanks.

[GyroGearLoose47]

Well, Sir: I think teachers should get a lot more respect, hence the Sir or Maam as appropriate. Anyone who masters E=I X R,and all its permutations can cut a wider swath through life. My jack of many trades abilities place me well below those of the proffesionals. Still, I am happy with where I am at. I really do think that little electronics equation is way more practical than most others. E = MCsquard is way less important for practical applications. Master those five little symbols and everything about them, there is no telling how far one can go. Thanks for your effort.

[naturefreak]

Will a small led work as a blocking diode ?

[kz1o]

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]

"Hose-seconds", lol. great example. Will ANY diode work, zener included? Are there any benefits of one over another?

[ac-dc]

Actually the diode type used does matter. For example if you are charging 2 x NiMH cells in series you will reach a peak charge voltage of approximately 2.9V as 1.45V/cell, BUT we have to consider the sum nominal cell voltage over most of its recharge cycle, 2.4V as 1.2V/cell.

Now, if you have a silicone diode with about 0.6V forward drop, you have to produce 3V. If you have a schottky diode with 0.3V drop you only need to produce 2.7V.

3/2.7 = 1.11, an 11% efficiency increase. More importantly, during periods of less sunlight your cells may not even be able to get the voltage up to 3V at a meaningful charge current so that extra 0.3V margin could allow for charging for more hours per day too.

[kz1o]

You are right. My comment was made based on the statement made in the original post:

"One or Two Solar Panels rated 4 Volts or above"

If the 4-volt requirement is no longer true, then yes, it does matter.

[JoshuaZimmerman]

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!

[kz1o]

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.

[Vinsu]

Good instructable, JoshuaZimmerman! Nothing to do with this one but I guess that I'm one of those people that doesn't have guts to comment if something is annoying me. Don't know if it's a bad thing not to comment everything that comes into my mind because most of those disrespectful things are not constructive at all. I find it that myself should not say anything if it isn't "nice" or constructive. "How many usb-resistor-led-things do we need?"-comments are the ones that are the most hard to not to write when I feel the motivation... Flew! Glad got that out of my system. :)

[techno guy]

Why hook them up in parallel? Wouldnt it be better in sieries?

[JoshuaZimmerman]

Hooking the up in a Series would double the voltage. 4.5 + 4.5 = 9 volt, but keeping the current the same at 80 am.

Hooking up in Parallel will double the current, 80 ma + 80 ma = 180 ma, but keep the voltage at 4.5 volts.

It works the same for any power source. Try and imagine the solar panels as big barrels of water. In a series the two barrels of water are hooked up one on top of the other. They have twice as much water pressure source, but can only squeeze 80 ma of water out at a time through the one pipe.

In parallel you have the who barrels side by side, each pumping out 80 ma of water, doubling the amount of water coming out, but only one barrels worth of pressure.

[NiKiToS]

This is the best explanation I have ever heard,.. truly genius,
just a quick question though, I have 5 solar cells, 2V 50mA each, I want 6 volts and 100mA I worked out how to do 2 in parallel and two in series, but I'm not sure where to stick the fifth one to increase the voltage.

[bladefun]

you would put it in parallel with the series of the other 4 solar cells

[JoshuaZimmerman]

To be honest, I'm not sure either. I'm pretty sure the most you're going to be able to do is 4V @ 100 ma. That extra one will limit the other two sets.

A solar panel is only as good as the least powerful cell. Which is why a single leaf falling on a solar panel can really mess up your output.

[tomx63]

Great instructable, Joshua!

[cloud_bam]

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.

[tomx63]

hey cloud_bam, the charge voltage must exceed the total full chare voltage of the batteries. the charge current is the most important factor though. keeping the charge current close to 10% of the batteries mAh rating. your example of using a 6v/330mA solar cell for the 4000mAh batteries should do a decent job for a trickle charge.

[rumplesnitz]

My application would be for charging a deep cycle marine battery to run a trolling motor on a 12/14 ft john boat while I crappie fish in smaller creeks and ponds. Lotsa sunshine, not much demand, lotsa time to enable a trickle charge to get the job done. But I don't have the G-2 to adjust the components for the bigger battery.

I picture the battery being inside the tub to minimize weathering over time, with the battery cables to the motor running out the side of the tub... whatcha thank about that?

[Barbwire620]

You can buy a solar 12 volt battery maintainer from Home Depot for around 16 dollars, made by Sunforceproducts.com. I bought one to do just that. You have to waterproof it for outdoor use, but they tell you how to do that too. I take an extra cheap battery with me and have it charging while I use the other one. That way I have one charged at all times. The only drawback is that it has to be facing the sun directly before it is in charging mode. If you are handy, you can make a mount that will swivel so you can keep it adjusted. Hope this is an easy solution for you. It would also be a good source for some other solar panel needs. I saw an ad in
TSC book where a man actually used some of them to change his tractor over to an electric solar power engine and he said he had more power than the original engine. I want an idea of how to make an electric engine for my lawn mower that I could run that way.

[JoshuaZimmerman]

You'd need a much much bigger setup. To power your marine battery you'd need a solar setup that could charge at 12 volts. A system like that can be easily found and setup for under $100.

But directly powering your motor would be tough as I doubt the solar panel could put out the amps you need to drive the motor.

Also, because you're dealing with a much much bigger system you would need a power controller. It's a little box that makes sure the solar panel isn't providing too much, or too little, power. Those are rather cheap off eBay.

Again, you're looking at between $50-100 worth of setup for such a system. Not bad really.

[rumplesnitz]

This is great info. Of course I'm not trying to power the motor with the charger - just to trickle-charge the battery as I sit out on the pond so I know I have power to get back to camp at the end of the day or at least a little more battery than I would have without the trickle. I putter out onto the pond initially relying on the battery to be charged and run the motor. Then I sit in the sun without the motor running, maybe use the battery to pump a little leakage water out of the boat once in a while, maybe putter back over to a different spot or back against the wind after drifting away from a good spot. But mostly just sitting and fishing. Of course space is at a premium on a small boat like mine, so the battery sitting in the tub the charging equipment is mounted to seems like a great concept. Would I have to worry about 'the ripple effect' shortening the battery life?

[ac-dc]

You do realize the panel area you need will be several square feet to make any significant difference in a few hours out in the boat? You can find something like that for $100, but in a small boat you will need a mast + roof type arrangement to hold it overhead, adding to the cost. It could help keep you from getting a sunburn though... ;)

[nerys]

but not with much capacity. to get enough solar capacity to apply a reasoanble "charge" to your trolling battery your looking at $240 to $300

sometimes you can get close to $200 on sale for example

http://www.harborfreight.com/45-watt-solar-panel-kit-90599.html

thats 45 watts. not much power but would put a measurable charge back into your battery. assuming a 50 amp hour deep cycle you need 650watts for a full charge so this panel "could" in ideal conditions recharge it fully in about 14 hours of GOOD FULL broad day light sunlight.

so if you were fishing for say 5 hours you could replace 1/3 of your battery capacity.

use TWO of those panels and you have something more usable. you go go till your battery dies in the morning fish for 8 hours and have a fully charged battery to get home with IF you had good broad daylight sunlight AND you tilted the panel TO the sun in the morning and toward evening to catch the most light.

[Sounds4cc]

I've seen the solar panels at HF. They are of the older technology (usually black in color) and take a lot of space for the given power. The newer technology ones are more compact for the same wattage. You might have to dish out more $ though. But for your boat, you could mount them above your motor using PVC or aluminum brackets. I would recommend 2 x 10-20 watt panels AND a voltage regulator. I've been near a battery that overcharged, the explosion was not much fun. Also you could mount one directly to the lid of your battery box. Look for some salvage solar panels at auctions or industrial sales to keep the price down and get bigger power. Have fun!

[ac-dc]

You can get 60W panels for a bit under $150, you just have to shop around instead of at the major consumer products type websites. For example,
http://www.ecrater.com/search.php?cid=0&keywords=60W+solar+panel&x=0&y=0

[nerys]

wow definately keeping that bookmark !! but note those are short term sales and do not include controllers etc..

but still wow $108 for 60watts. Very very tempted. if I had $108 in the bank I would already have ordered it!

[rumplesnitz]

I'll go check that out. Is that a pre-assembled kit or part of something I'd put together?

[syed_ahmed9003]

great instructable,