Start with a voltage experiment; this makes it easier to choose other experiments, based upon the energy your homemade battery produces. Connect the two wires from a voltmeter onto the two electrodes --- it doesn't matter which wire touches which electrode. If you don't get a reading on the meter then it's worth swapping the wires over. Have a look at the voltmeter and make a note of the voltage reading. If you have more than one homemade battery, repeat the experiment to see which battery produces the highest volts and make a note of it.
Use a low voltage flashlight. It's best to try and match the voltage labeled on the flashlight bulb to the voltage you recorded from your homemade battery. Connect two wires to the light bulb terminals and the opposite ends to the electrodes on your homemade battery. The light illuminates. Repeat the process on other homemade batteries to compare brightness. The batteries producing the highest volts illuminate the bulb the brightest. If the voltage is too high the bulb will blow.
Increase the voltage from your homemade batteries by wiring them in series. This means the voltage from each battery gets added together. For example, if one battery produces 1 volt and another 2 volts, if you wire them in series you get 3 volts.
Cut a few strips of wire. You need two strips to connect to the device you're going to power and then strips of wire to connect between the batteries. The number of wire strips to connect the batteries is always one less than the number of batteries so if you are combining three batteries you need two strips. Attach a wire to the first battery then connect the opposite end to the other battery. Make sure you connect alternate polarities. For example, if you are using copper and steel electrodes, then connect one end of the wire to the copper electrode and the opposite end to the steel electrode. Connect the wires that go to the device you are going to power to the remaining electrode on the first battery and the remaining electrode on the last battery. Attach the opposite ends to the terminals on the device you want to power and you've increased the voltage from your batteries.
If you've connected a few batteries in series, you can operate a more powerful electrical device, such as a small motor. Attach the wires from the batteries you've wired in series to the terminals on the motor. The faster the motor turns the more voltage your batteries are producing. However, a motor can use a large amount of energy, so it may not operate for long.
Line up several homemade batteries and connect a flashlight bulb to each. For example, put a citrus fruit battery and a potato battery next to each other. Start a stop-watch and see which homemade battery lasts longer. This means you can work out which is the best homemade battery and make more of them.