Wiring Two Batteries in a Series Circuit
If you would like to use two batteries in your circuit, there is two ways to hook-up the batteries - in series or in parallel. If you choose in series, than you are stringing the batteries together in a line where the (+) terminal of one battery is connected to the (-) terminal of the second battery. If you join two 12-volt batteries in series you will get 24 volts. Now you have more power, but you have changed the voltage and this can really damage circuits in a 12-volt system.
Certain electrical systems found in some industrial, marine, and aircraft applications require 24 volts or even higher. These systems are designed to operate with higher voltage than a regular car or truck.