There are a few things that you'll need to know to answer this question. The first is about current, resistance, and voltage. Current is the actual amount of electricity flowing through the circuit. Resistance is anything that tries to stop or slow down the electricity. Voltage is a little trickier. Voltage is the potential to move. If a charge has a higher voltage, it is more likely to move. So if you have a high potential, then there will be more current because it will be easier for the charges to flow.
The second important thing relates currect, resistance, and voltage together. It is called Ohm's Law. Ohm's Law says that the voltage across something is equal to the current times the resistance. That makes sense because if you have a lot of current going through something that has a very high resistance, the charges need a very high voltage to get through.
The last imporant thing before we tie everything into your question is the concept of series and parallel. Series means things are in a line and parallel means they are side by side. This is important because of the way things in series and parallel work together.
Now, back to your question. 2 batteries in series have one special effect, their voltages will add up. So by putting two 1.5 volt batteries in series, you have a total voltage of 3 volts. On the other hand, if you put the batteries in parallel, they do not add up. You still have 1.5 volts. You might say, "Why do we need 2 batteries if it doesn't change anything?" The reason for 2 batteries is to provide more current or to make the device run longer.
Each electronic device you put batteries into needs a different amount of current to work. This is where Ohm's Law comes in. The designer of the device figuers out how much voltage he needs in order to get the correct current. Then he can put batteries in series to get the correct voltage. He can also put batteries in parallel to make the device run longer.
(republished on 07/22/06)