There are two methods to connect two 12 volt batteries. The series connection adds the voltage of both 12 volt batteries for a total of 24 volts. In a parallel connection, the voltage remains the same but the overall amperage of both batteries is added together. In other words, if two 250 ampere batteries are connected in parallel, the total amperage is equal to 500 amperes.
Make a series connection to the two 12 volt batteries. Attach the positive (+) and negative (-) terminals from each battery together. Use the ½-inch end wrench to tighten the battery cables and clamps to the battery terminal posts.
Connect one of the battery cables to the positive terminal on battery number one. Attach that cable to the negative terminal on battery number two. Use the second battery cable to connect the negative terminal on battery number one to the positive terminal on battery number two. The total voltage for the series connection will be 24 volts.
Make a parallel connection for the two 12 volt batteries. Attach the positive (+) terminals together and the negative (-) terminals together.
Use one battery cable. Connect the two positive terminals on both batteries together. Attach the second battery cable to both negative terminals on the two batteries. The batteries are now in a parallel connection. The voltage remains the same--12 volts--but the amperage is added together.
Connect only like batteries together. In other words, never connect a deep cycle battery and a regular car battery in series or parallel. Only attach deep cycle batteries to deep cycle batteries, and car batteries to car batteries. Mixing types of batteries may cause permanent damage to the storage devices.