Two 6-volt batteries can be wired together to create a 12-volt system for powering electrical devices. This is known as a series connection. This combines the voltage of the two batteries without increasing the capacity of the output, which is measured in amps per hour. The amperage stays the same with series wiring, but the voltage has a multiplying effect. For example, four 6V batteries wired in series will produce 24V.
Cut three lengths of wire long enough to connect the 12V system consisting of two batteries and an electrical device. Strip 1/2 inch of insulation from both ends on each piece of wire.
Attach the end of one piece of wire to the positive terminal of one 6V battery by unscrewing the post cap, wrapping the wire around the metal terminal, then tightening the post cap over it to secure the connection. The positive post will be marked with a plus (+) sign and is typically colored red.
Connect the other end of the same wire to the negative terminal of the second battery and secure the connection in the same manner.
Connect a second piece of wire to the negative terminal of the first battery and leave the other end free.
Secure the third length of wire to the positive terminal on the second battery and leave the other end free.
Connect the two free ends of the wires to a 12V electrical device. Attach the wire from the positive terminal first, then the negative wire. The device will operate with 12V drawing on the doubled power of the two batteries.
Be sure the device being connected with this battery system is rated for 12 volts. An electrical motor, for example, will burn out quickly when run at a higher voltage than designed.
An 18-gauge wire is an acceptable size for running either 6V or 12V circuits.
As a standard safety procedure, do not touch the terminals of the batteries at the same time, regardless of the voltage.