Batteries can be wired in an electrical circuit so that their voltages add together. This is necessary when you don't have a single battery that provides the voltage needed for a particular electrical device. This is an especially common problem with marine equipment, which frequently uses 24 volts, while many batteries are only 12 volts.

Step 1

Connect the battery's positive terminal to the positive terminal (power lead) of an electrical device with a battery cable. Connect the battery's negative terminal to the electrical device's negative terminal (ground connection) with another electrical cable.

Step 2

Observe that this arrangement provides 12 volts of current. If the electrical device is a 24-volt trolling motor, the battery will only provide the motor with 12 of the 24 volts of electricity it needs to start the boat.

Step 3

Add a second 12 volt battery to the circuit. Disconnect the cable from the starter's ground connection and connect it to the second battery's negative terminal. Connect a battery cable from the second battery's negative terminal to the starter's ground connection.

Step 4

Calculate the total voltage in the circuit from step three. This circuit has the batteries wired in series, which means that the voltages add together. The total voltage in the circuit will therefore be 24 volts, and the batteries can run the starter.

Step 5

Use 12-volt batteries to run a 12-volt and 24-volt devices. For example, a boat may have a 12-volt starter for the main engine and a 24-volt trolling motor. By wiring a pair of 12 volt batteries in series, you can use the same batteries to perform two different tasks.