Introduction: Salt Water and Aluminum Foil Night Light

Picture of Salt Water and Aluminum Foil Night Light

A simple classroom (no soldering) project  to show aluminum air batteries in use. In this instructable I will show you how to create a simple 3 cell aluminum air fuel cell using household aluminum foil and saltwater. You can use this to drive a small light or possibly a motor. This device will function for extended periods as long as the fuell cell is periodically recharged with electrolyte and the aluminum foil is replaced when power output drops below acceptable levels.

To simplify construction I used one of those solar yard lights available from OSH, Home Depot or Lowe's for about $5. These contain a simple circuit to recharge a battery along with a Joule thief type circuit and use that to drive a super bright LED after dark. Sundown is detected by a photocell located on the face of the device.

This circuit has many drawbacks but it eliminates the need to build a circuit or run down to Radio Shack, although obviously for a science fair project one would presumably build a simple joule thief/recharger circuit.

The design can be scaled up with additional cells wired in series or in parallel to increase power output and the output power can be fed to any suitable low voltage circuit. Do a web search for "solar powered light circuit" and substitute the appropriate al/air fuel cell array  for the solar panel(s).

Now let's take a look at what we're going to need for this project.

Step 1: Bill of Materials

Picture of Bill of Materials

The basic construction of this device is extremely simple. We're going to use some non-conductive trays and make a sandwich of aluminum foil, paper towels and a carbon brush electrode which begins its life as an aquarium filter.

You're going to need some non-conductive tray containers. I used some carryout containers from a local rib joint but almost any tray may be used. Since there only two dinners I used two bases and one lid. The common glass 9X11 glass cooking trays would be perfect.

3 paper towels to act as the porous layer.

A box of carbon aquarium filters. I used the Fluval brand of carbon pad replacement filters available at PetSmart for about $4. These come 4 to a box, I used the "4 Plus" size. These are used as the carbon electrodes in our assembly and function in this role quite nicely.

Some salt to use as an electrolyte. Ordinary table salt is fine, you'll need about 5g to start with. That's approximately 1/2 TBSP. In a liter of water that should produce approximately a 5% solution. Feel free to vary electrolyte strength.

A solar yard light. (see picture). These are available at most hardware stores for about $4. This is a self contained solar powered light with a rechargeable battery. it works by using a joule thief type circuit to maximize energy stored in the battery, the difference being that this circuit uses a capacitor rather than a traditional toroid so it doesn't provide the power amplification normally associated with a joule thief. NOTE: In the joule thief circuit the toroid provides both power amplification and capacitance.

4 electrical leads with clips, these are available from Radio Shack in a package of 12. These are used to make all connections.

A scissors, ruler and ball point pen along with a small screwdriver for dissassembling the solar light.

Now let's get started...

Step 2: Assembling the Cells

Picture of Assembling the Cells

In this step we're going to cut out the aluminum electrode and construct the cells which will be used to provide power.

Each cell contains a sandwich composed of an aluminum foil electrode, a porous non-conductive layer (we use paper towels but almost any material may be used) and a carbon electrode.

As noted in the bill of materials almost any non-conductive shallow container may be used, so I measured the bottom of my pans and cut an electrode from the foil, leaving a significant lead approximately 1" wide and several inches long (see picture). This is folded over the end of the pan and is used to secure the lead cable for the cell.

In my case the pans measured 6"x10" across the bottom so I chose to make the foil electrode 5" wide and 9" long with a 1" lead to be trimmed after assembly . Then I cut out the porous layer from the paper towels and made them 5.5"x9.5" long. This insures that the foil will never come into direct contact the carbon brush and short out the power supply.

Even though the lid was slightly larger than the two bottom pieces I chose to keep the same dimensions in all cells. With the same surface area and electrolyte concentration each cell should produce approximately the same output.

I found it easiest to layout the electrode on the foil with a ball point pen and ruler then carefully tear the foil along the edge of the ruler. Alternatively scissors can be used to cut out the final shape. Make sure the power lead is long enough to reach easily over the edge of the pan and bend over the lip to firmly secure the foil. If using the ruler method always place the ruler so that it protects the electrode portion during the tear.

Once the foil has been cut out assemble the cells by placing the aluminum foil in the bottom anchored to one edge by the foil lead which is gently formed around the lip. Place an insulating layer of paper towel on top of the foil so that it is completely covered. Place one of the Fluval carbon filters on top of the paper towel and the cell assembly is complete.

In a later step we will wire these cells in series and charge them using a saltwater electrolyte.

For now let's set these aside and turn out attention to solar powered yard light and its mysterious inner workings....

Step 3: The Power and Light Assembly

Picture of The Power and Light Assembly

In this step we're going to extract the key components for light and power from the solar yard light. For demonstration purposes we are simply going to disconnect the solar panel and substitute the 3 cell aluminum air fuel cell as power the supply.

Be very, very gentle and do not tug on the wires. I am not going to attempt to explain the circuit here I will simply explain its function.

The light assembly contains a solar panel and a light detecting diode located on the face. When the sun is up the solar panel provides power which is used to recharge a battery located inside the head of the device. When the sun goes down the photodetector allows power to pass to the light. The light is actually driven primarily by the battery which is recharged by the solar panel

First take apart the light assembly by gently twisting off the base and then the lens. We will use the lens later to house the final light assembly. In the base where the light is located there are 3 small screws, remove these and gently pull the light assembly open ( see picture ).

I found corrosion inside the assembly ( see picture ) but when I cleaned off the leads and replaced the battery it seemed to work okay.

There are 4 leads coming off of the face, two lead to the small photodetector and two other lead to the solar cell. Generally the power leads will be red and black. Clip these leads close to the solar panel which we will not be using. Strip a short length off the end, be careful these are fine wires and its easy to pull out several strands along with the covering. Carefully cut the covering just enough so it slides easily off the wire.

On the other side you will see a small circuit board which is held in place by two plastic posts. GENTLY pop the circuit board off the posts being carefully not to pull on the wires leading to the light. The light is held in its socket by a small blob of thermoplastic glue. This can be carefully scraped off and will come off cleanly allowing the light to be easily removed from it's holder BY PUSHING DOWN ON THE TOP, NOT PULLING THE WIRES AT THE BOTTOM. If you have a hot glue gun you can also heat it up and soften the glue, allowing the light to be removed.

This technique may also allow you to easily remove the photodiode from the face. Then a battery holder from radio shack could be used to hold the batteries and a more finished assembly created.

Now we've extracted a usable charger and light circuit let's take a look at the final assembly...

Step 4: Final Assembly

Picture of Final Assembly

Now we're ready to bring the pieces together. This is an exploded view that shows how the various components are connected, in the next step we will use a more compact arrangement to operate the device in actual sunlight.

The cells are wired in series, the yellow wires show the interconnect while the red and black leads represent the power taps. For more information wiring in series see my related instructable:
How to Wire Batteries in Series or in Parallel

The leads are clipped either directly to the carbon electrode or attached to the foil lead and secured to the lip of the cell.

The power taps are attached to the appropriate red/black leads on the power assembly ( see picture ).

At this stage the system is ready to be relocated to a sunny spot (for sundown detection) and charged with electrolyte to begin energy generation.

Step 5: Charging and Operating the Assembly

Picture of Charging and Operating the Assembly

As you can see in the picture I used a stacking arrangement when I placed the assembly outdoors. This allows the photo-detector to operate and properly trigger the light in response to sundown/sunup.

The power and light assembly needs to be balanced so that the face is exposed to the sunlight and the light positioned so that it is viewable. The light, circuit board and power leads can be CAREFULLY inserted back into the lens to provide a weather resistant housing. You can see this in the intro picture if you look closely, the light and circuit board are both inside the original lens housing. With some thought and a little soldering of the leads the whole assembly should fit completely back into the lens housing which can then be reattached to its original base and the power leads going out through the original hole in the base.

Once the cells are in place they are charged with a mild saline solution. I used a 5% solution ( 5g of salt in 1L of tap water) and got satisfactory results. I charged each cell with approximately 300mL of electrolyte and began to measure performance of the circuit. As you can see I rapidly got 1.3V but this dropped to about 800mV after a few hours.

The device will operate without maintenance for about a week or so. With maintenance it will operate as long as there is fuel (aluminum) and oxygen. Overall I found this assembly more reliable than the solar lights which generally lose power by about 2 or 3 A.M while the al air run reliably from sundown to sunup.

I found it best to circulate the electrolyte a couple of times a day and replace every two or three days. I replaced the foil once a week and got satisfactory results.

Enjoy and good luck.


Ibizibbik (author)2016-02-19

Should the cell be fully submerged or just up to (but not touching) the electrode clips? I'm thinking of making a compact version of this and am trying to make it last as long as possible without maintenance (like changing out the electrolyte)

science-tech (author)Ibizibbik2016-02-22

wow that is great

RedstoneM (author)2015-09-24

Can i recharge it? I did it with carbon pellets and vinegar, when i wanted to recharge it it gave off a bad smell (12v7ah battery, a lot of amps)

daxxvondrachen (author)2015-01-27

I and my wife live near a national park......We like to hike often....guns are not allowed in national parks...says nothing about stun guns.....Question How to increase amperage on stun gun to kill a wolf, cougar or black bear I ask because I was tracked by wolves and my wife was trail biking and stalked by a cougar don't want that situation ever again so I either carry illegally or at least if it gets into a fight the stun gun will kill the problem

ketrovin (author)daxxvondrachen2015-08-05

the solution is to go hike elsewhere, not to kill animals.

gaieb (author)daxxvondrachen2015-06-05

Forget the stun gun, to get it powerful enough to kill would be very hard and you would have to place it where the current would pass through the heart. I would suggest a shark stick,(divers use them) that is a short tube with a 12 gauge shotgun shell that is activated by pressing the shaft forward against the shark, or in your case the wolf, cougar or bear. I don't think it would be consider a "gun" if not then a good sharp knife with a six inch or longer blade, preferably 8 inches or so, any longer and it might be too hard to use in close quarters.

techno man (author)2014-10-03

how can i increase the output

Output is a function of surface area of aluminum, amount of available oxygen and strength of the electrolyte. The easiest way is to just add more cells. Keep in mind that your max theoretical output is 1.2V per cell. To increase the voltage use more cells wired in parallel..

Using a dilute lye solution will increase the output amps but shorten the life, using more aluminum will also increase the output amps.

Thank you

deathwisher (author)2013-10-02

ADD H2O2 OR KNO3 TO THE ELECTROLYTE!!!!!!this would keep the output stable

That will short out the circuit since you can't keep the H202 separated and everything will happen on the surface of the aluminum. For that you need a two cell aparatus similar to the one in my MFC project.

kclo42112 (author)2010-01-07

nice experiment a simple galvonic cell , I used to use a lemon to get the same effect,any acid or in this case a ionic solution,and to disimilar metals will create electricity, but this one can be done at any diner table with scraps,basicly

Its not a galvanic cell, there is only one metal ( Aluminum ) which reacts with a saline solution. There is an excellent explanation of the chemistry here:

Absolutely. I have on many occasions made a cell out of piece of foil, a carryout salt packet and a pencil lead....

Now you got me thinking about that old electronics kit from radio shack I had waayyy back when.... I think theres some graphite rods from some old dry cell batteries in that box,I could teach this trick while camping with my nephews

There's an instructable that shows how to make biochar in an altoids tin. You could make the biochar on the camping trip and use soda can strips.

Run some water through wood ash to get a mild sodium hydroxide solution and I think you've pretty much MacGyver'ed yourself out....I believe with a little experimentation in advance you can drive an LED directly from such a device but you'll probably need multiple cells.

And let's face it, showing them how to make light from a campfire and a soda can pretty much rocks....

serp1000 (author)egbertfitzwilly2011-07-18

brilliant instructable :) im going to go and try that now. Im studying engineering and technology and think fuel cells are extremely interesting and could become the future, expecially hydroged based or methanol based fuel cells.

By the way, if you've got a camp fire you dont really need the light do you... :D
but i still think its a great idea and would do the very same thing on a camping trip :)

JORDANBOOKE (author)2011-05-08

what is another option for a carbon electrode. i cannot find the fluval carbon pads anywhere. were your carbon pads filled with small carbon "rocks"?

Thnder (author)JORDANBOOKE2011-06-12

As I understand it, you just need two different metals to cause the galvanic reaction. Copper can fill this requirement so you could try copper flashing. You might be able to find this at your local Hardware store.

egbertfitzwilly (author)Thnder2011-06-16

Its not a galvanic cell, its an aluminum air battery which requires a non reactive electrode.

Thnder (author)egbertfitzwilly2011-06-21

I am no expert, what makes it non-reactive?

egbertfitzwilly (author)Thnder2011-06-28

It is electrically conductive and does not react with the salt or the aluminum. This insures that the voltage output is being produced by the aluminum oxidation process. While using copper will work it clouds the measurement of the result since there is both an aluminum-air as well as a galvanic reaction happening.

Its more of a carbon brush so its not filled with anything. However almost any solid carbon object should work, pencil lead is an alternative, there is also carbon paper which I used in another project.

If you ask at a store for 'carbon pads' they cannot help you, try a pet store and ask for aquarium filters.

There are also carbon electrodes available at your local ACE (or other) hardware store, tell them you're looking for a replacement brush for an electric motor.

BlankBome7 (author)2010-03-09

so what did you do with the salt to make this work and did you put layers of alumine foil down or one

I layer of aluminum foil, the salt is dissolved in water to obtain a 5% solution. The salt will react naturally with the aluminum and does not require additional processing.

One layer of foil, one layer of paper towel and the carbon electode. Measure out about 300 ml of a 10% salt solution in a measuring cup and pour into the cell. the salt is merely dissolved in the water, no particular prep is done. This link points to another instructable of mine that contains detailed instructions for making a 10% solution. I didn't use iodized salt, not sure if that would make a difference.

More layers of aluminum will probably increase your current. You might try with and without layers of paper towel between each layer of aluminum.

Udon (author)egbertfitzwilly2010-11-03

More aluminium? Do you mean adding more SEPERATE layers of foil, or more sheets of foil at the same electrode? For more surface area?

egbertfitzwilly (author)Udon2010-11-03

More sheets of foil at the same electrode, its all about surface area in contact with the electrolyte. If you use layers of paper towel between them make sure the towels are saturated. You will that as the towels dry out your current will increase, or at least that's what I experienced when I used a sponge. There will be an optimal amount of fluid for the towels.

It might interersting to soak the towels in a saturated solution and allow them to dry partially then assembling the cell. This should eliminate the surface oxidation that occurs while the towel is drying and may give a better overall performance curve.

smeezekitty (author)2010-05-07

One correction: It isn't 'air' but rather a saltwater reaction.
Also zinc may work better then aluminum.

I look forward to reading your update on wikipedia.

BlankBome7 (author)2010-03-12

agian sir if you dont mind me asking im srry im new i dont know mutch but is that yellow wire a positve or negative please do tell my teacher is useing this site and this is my project i have chosen and the teacher is really anoying

The yellow wire is used to cross connect the batteries in series so it isn't positive or negative. It connects the positive terminal of one cell ( the aluminum foil ) to the negative terminal of the next cell ( the carbon filter ).

lucek (author)2010-01-22

from your own quote "between the fuel (on the anode side) and an oxidant (on the cathode side).The reactants flow into the cell, and the reaction products flow out of it, while the electrolyte remains within it. Fuel cells can operate virtually continuously as long as the necessary flows are maintained." the device you have here meets none of thees 3  criteria. there is no oxidant on the cathode side there are no reactants flowing into the cell and the products remain in the cell, nor is there a flow to be maintained.
as for your question if I have something useful to do. this is my break. and despite your Monty Python clip I'm not here for an argument. I read your instructable and comment, left some of my own and then was going to leave.

lucek (author)2010-01-21

by the way that was a failed argument from authority. you didn't show that "Lawrence Livermore National Labs" appears to agree with you let alone that the description was accurate.

lucek (author)2010-01-21

by your definition the fuel cell was invented in 1800. the fact is that a fuel cell has a separate fluid that supplies energy and the anode and cathode do not. what you are describing is know as a battery.

lucek (author)2010-01-21

we used to do this back in school with aluminum foil paper towels and pennies. pretty simple homemade battery. but what we'de do is make a stack penny foil paper penny etc. then put that in a empty film container. use a paperclip as the terminals. it was like a real battery.

egbertfitzwilly (author)lucek2010-01-21

That is a galvanic cell ( ) which is close relative and is actually a real battery....

necropolian (author)2010-01-02

Wonderful project, although I do not really understand it. does it recharge itself, or is it simply a fuel cell?
anyway, 4.5 for the rating.

Thank you for your kind words and I apologize if the overall operation is unclear. The fuel cell provides a constant output (which varies over time as the available aluminum is oxidized). This charge is routed through the battery. When the optical diode light levels fall enough current is routed to the light. I believe this current is composed of the fuel cell output plus the available battery voltage. When the sun comes up, or the lights go on, the optical diode blocks the current flow to the light and the battery is recharged by the fuel cell voltage during the day.

The number of things I don't like about this circuit would only just about fill the Grand Canyon. The next project will probably a battery pack charger with a more sophisticated circuit and a toroid for power amplification.

 so it acts like a not very effecient battery right? It charges the other one inside the light, then after a while it dies.

Actually it acts like a moderately efficient aluminum oxide fuel cell with a sodium chloride electrolyte generating very close to the optimal voltage. Like any fuel cell (or engine) it must be periodically refueled or it will stop working. If it is maintained it will work until the battery dies from overcharging.

If the electrolyte is refreshed it should run for a week to 10 days per sheet.

 If you somehow used thicker aluminum sheets, would it last longer. Or is it the carbon that is used up. And how would this be different than a battery? Is it that batteries dont need fillups on electrolyte?  

With thicker aluminum you would need to periodically use a more aggressive electrolyte, such as lye, to clean off the accumulated surface oxidation.

Yes, you are correct, the difference between a fuel cell and a battery is that a fuel cell can be refueled ( as opposed to being single use or recharged ). It's not that a battery doesn't need a fillup, its that it cannot be filled up even if one wants too.

lucek (author)egbertfitzwilly2010-01-21

no sir this is a battery. your definition of a fuel cell is a little off.
"Fuel cells are different from conventional electrochemical cell batteries in that they consume reactant from an external source, which must be replenished"
in this cell it remains a closed system replacing the anode isn't the same as having an external supply of fuel.

buteman (author)2010-01-17

Like this, great idea. Could I just ask did you mean 0.5% solution which 5gm in 1L is or 5%  i.e 5gm in 100mls?

egbertfitzwilly (author)buteman2010-01-17

Actually I used 5g in 1L, intended to use a 5% solution but ending up with a much milder electrolyte. I have not experimented with the salinity/energy ratio but I suspect the more salt the higher the current.

Herber (author)2010-01-07

Nice article.  I think that if you put 5gm salt in 1L water in step five you have a .5% solution? 5gm in 100ml would be 5%?

egbertfitzwilly (author)Herber2010-01-09

You're right, I mistyped that it should be 50 grams not 5.

ToniRose (author)2010-01-07

If you're not a science teacher you should be. What I like best about this ible is that it demonstrates a couple of simple concepts but can be revisited to add knowledge (ie, the toroid for amplification). Excellent!

egbertfitzwilly (author)ToniRose2010-01-08

Thank you for these kinds words but I'm afraid I'm much closer to W.C. Fields than Mr. Wizard. However I did hope that this project would be useful (and used) in classrooms. I think its suitable for a broad range of grades.

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