In its simplest form, a solar battery needs just one solar cell in order to produce electricity, and for a kids' project, it never hurts to keep it simple. According to sci-toys.com, instead of using silicon, you can make your own photoelectric solar cell material---cuprous oxide---by heating up a burner-sized piece of copper sheeting on an electric stove top. First, clean the copper with soap and water to remove oils, and rub away any impurities with a wire brush or sandpaper. Then heat the material on high until it passes through several stages of molting and color changing, eventually developing a thick, black coating called cupric oxide.
Let the copper cool, and eventually this black coating will flake away, leaving behind a red layer called cuprous oxide. Cut out another piece of copper the same size as the first, but leace this piece leave unprocessed. Place both pieces of copper in a bottle or jar, bending them gently if necessary, and take steps to prevent them from contacting each other. Next, dissolve a few tablespoons of salt into some hot water (from the tap), and pour it into your container. Leave the tops of the copper plates sticking out at least an inch above the water. You have now completed your solar cell, and you can help your child measure the electric current it produces with a multimeter and two alligator clip leads. Simply connect the copper sheet with the cuprous oxide to the multimeter's negative terminal, and the plain copper sheet to the positive terminal. According to sci-toys.com, this simple solar cell counts as a battery, as can store energy and will usually display a few microamps of current on the multimeter even in the dark.