Step 1: Parts and Tools
A film canister (or similar container)
Tissue paper or kitchen roll
Carbon rod (Pencil led or battery rod)
Electrolyte (Salty water or other salt in water)
As for tools,
Multimeter if you want to check it.
Step 2: Making the Cathode
Start by cutting a strip of foil just slightly shorter than the film canister, but long enough to make a full ring inside with plenty of overlap.
Tape a wire to the foil so that it won't be facing inwards, but is against the side of the canister.
Try to curl the foil so that it will easily fit into the canister. Pulling it gently over a sharp edge does this nicely.
Insert it into the canister and leave it for now.
Step 3: Making the Anode and Electrolyte Medium
The anode needs to be surrounded by electrolyte, which we achieve by saturating tissue paper with salty water. I use bicarbonate of soda in water, but any salt in water will do.
Start by taking two sheets of kitchen roll or other tissue paper and folding them into a narrow band slightly shorter than the film cannister. You may need to trim the height with a pair of scissors.
Now place the carbon rod along with a stripped wire end on one end of the paper. Roll the tissue paper as tightly and as neatly as you can.
On to the next step!
Step 4: Filling the Battery
Next we insert the tissue roll into the canister carefully, making sure the foil is straight around it and not creased.
Push it all in tightly, as the lid has to shut and seal the electrolyte in.
Now to fill the canister. Pour the electrolyte in slowly as it has to be absorbed into the tissue. when it begins to overflow, drain off the excess and wipe it down.
Step 5: Seal the Cannister
Pull the wires right through and clip the lid onto the canister fully.
Now all that is left is to test it!
Step 6: Test the Battery
If you want to be precise, measure the voltage and current with your multimeter.
My battery produces between 0.8 and 1.1 volts at around 2-5mA. Using a joule thief you could run an LED from this. A larger surface area and stronger electrolyte concentration would improve the battery, but that is up to you to try...