I believe that well burned campfire charcoal could also be used. Backpackers could have an extremely lightweight, wholly renewable and non CO2 generating light source.
From Wikipedia:Aluminum Air Battery
Aluminium batteries or aluminum batteries are commonly known as aluminium-air batteries or Al-air batteries, since they produce electricity from the reaction of oxygen in the air with aluminium. They have one of the highest energy densities of all batteries, but they are not widely used because of previous problems with cost, shelf-life, start-up time and byproduct removal, which have restricted their use to mainly military applications. An electric vehicle with aluminium batteries could have potentially ten to fifteen times the range of lead-acid batteries with a far smaller total weightt.
Step 1: Bill of Materials
An aluminum can. I use a soda can.
A piece of sandpaper. I happen to have some sticky sandpaper for a sanding block. Very convenient. This is used to remove the surface treatments from the can. This actually may be optional. I will experiment and update the instructable appropriately.
A sponge. This will be cut to fit inside the battery. I used a cellulose sponge from a 6 pack my wife had under the sink. A natural sponge may be more conductive.
A charcoal briquet.
Several readers have suggested alternative, readily available carbon sources. The most scientific sites I've visited have recommended activated charcoal for the carbon electrode.
- Britta disposable filters
- Aquarium filter charcoal
Rolling pin or short piece of pipe for crushing charcoal
Duck Tape - That's right its called Duck Tape, not Duct Tape.
Some copper wire or copper mesh. The effectiveness of this device is directly linked to connectivity between the copper drain and the carbon cathode. Also it is not possible to solder a lead to the aluminum.
I am using copper mesh and foil from K&S Metals to obtain connectivity. In the first release I tried a simple copper mesh/carbon combination that generates power but I am working on second generation and will update the instructable when it's available.
Step 2: Preparing the Anode
Cut the soda can by whatever means about 1.5 inches high or about 1 inch above the lower rim. Using your sandpaper sand the inside lightly to remove any anti-oxidant coating.
Prepare enough warm salt water solution to fill the container. Use warm water and enough salt that it won't dissolve. Stir the solution to faciltate dissolving the salt and set aside.
In the Mark II I will be attaching a copper lead to get a good electrical connection with the can. Since aluminum cannot be soldered we're going to secure our drain mechanically with tape. We're going to use Duck Tape which was invented during WW II by Johnson and Johnson when the U.S. Navy needed a durable waterproof tape for use in the field. I understand this position is akin to posing a solution to "who writ Shakespare" but there you have it.
Step 3: Preparing the Cathode
Take the sponge (dry is easier), invert the anode over it and draw a circle.the diameter of the can. Cut a circle from the sponge slightly smaller than the circle. The sponge will expand so you want to leave 1/8" or so all around.
Take the charcoal briquet and break it up into smaller and smaller pieces until you have a reasonably fine powder.
Put the briquet in a 1 gallon plastic garbage and break it with a hammer. Take care not to puncture the bag. I used a short length of PVC pipe as a rolling pin to grind down the charcoal. Remove any unburned particles of wood that you see.
The finer you make the powder the more power transfer will be available. Or at least that's my understanding. Its not clear at this point how much carbon is required in the cathode.
Step 4: Assembling the Battery..
Place the sponge inside the aluminum can and pour in the salt water gradually allowing the sponge to expand. When the water just comes to the top of the sponge place the copper mesh on top of the sponge making sure it lays flat.
Now carefully pour the crushed carbon onto the copper grid.
Attach one lead from the voltmeter to the can, attach the other to the copper mesh. In this particular shot you can see the multimeter showing 3.6 mA, the intro shot shows the .5V reading.
If I've been reading everything correctly I believe this can be hooked up to a "Joule Thief" to power an LED.
Look for continuing updates to this project to reflect the Mark II design.