Creating a battery from a lemon is a common project in many science text books. Successfully creating one of these devices is not easy.
Batteries consist of two different metals suspended in an acidic solution. Copper and Zinc work well as the metals and the citric acid content of a lemon will provide the acidic solution. Batteries like this will not be able to run a motor or energize most light bulbs. It is possible to produce a dim glow from an LED.
The lemon: A large, fresh, "juicy" lemon works best.
The nail: Galvanized nails are coated in zinc. I used a 2" galvanized common nail.
The penny: Any copper coin will work.
Creating the battery: Insert a penny into a cut on one side of the lemon. Push a galvanized nail into the other side of the lemon.
The nail and penny must not touch.
This is a single cell of a battery. The zinc nail and the copper penny are called electrodes. The lemon juice is called electrolyte.
All batteries have a "+" and "-" terminal.
Electric current is a flow of atomic particles called electrons. Certain materials , called conductors, allow electrons to flow through them. Most metals (copper, iron) are good conductors of electricity. Electrons will flow from the "-" electrode of a battery, through a conductor, towards the "+" electrode of a battery. Volts (voltage) is a measure of the force moving the electrons.
I have connected a volt meter to our single cell lemon battery. The meter tells us this lemon battery is creating a voltage of 0.906 volts.
Unfortunately this battery will not produce enough current (flowing electrons) to light a bulb.
To solve this problem we can combine battery cells to create higher voltages. Building more lemon batteries and connecting them with a metal wire from "+" to "-" adds the voltage from each cell.
The two lemon batteries above, combine to produce a voltage of 1.788 volts. This combination still does not create enough current to light a small bulb. Note the red wire connecting the batteries is joined from "+" (penny) to "-" (galvanized nail).
Four lemon batteries create a voltage of 3.50 volts. We should be able to light up a small device like an LED (Light Emitting Diode).
Note the connecting wires go from "+" to "-" on each battery.
To turn on an LED you must determine the "+" and "-" connections. If you look closely at the red plastic base of an LED you will notice a "flat" spot (indicated by arrow above). The wire that comes out beside the flat spot must connect to the "-" side of a battery, the other wire to the "+" side.
Important information about LED's: LED's are designed to work at very low voltages (~ 2V) and low currents. They will be damaged if connected to batteries rated at over 2 volts. LED's require resistors to control current when used with batteries rated at over 2 volts. Lemon batteries produce low current. It is OK to connect an LED to a lemon battery.
In the above image, electrons flow from the "-" (nail) end of our lemon battery through the LED (making it glow) then back to the "+" (penny) end of the battery. This is an electronic circuit. The LED glows dimly with this configuration.
Improving your battery.
The quality of the copper and zinc can be a problem for a battery like this. Pennies in particular are rarely pure copper.
Try substituting a length of 14 gauge copper wire (common house wire) for the penny. Experiment with different lengths and configurations of electrodes. Other sources of zinc and copper may be found in the plumbing supply department of a hardware store.