How to Wire Two 12 Volt Batteries to Make 24 Volts

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.

With enough patience and the careful following of instructions, 12 volt batteries can provide 24 volts of battery of energy in no time.

Connect the Batteries

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. It's important to make sure you that your work space in clean and clear and free of any flammable materials like alcohol, gasoline, or other simple household items like flammable cleaning solutions as well as items of clothing, towels, and blankets that could easily catch fire if near a flame.

You should also be careful to be free of any standing or puddled water while working. It's very easy to be electrocuted when working with batteries and voltage near and around water. Working with any sort of electricity grounded or not, there is always some element of danger for shock, electrocution, fire, or worst. Proceed with caution at all times.

What's the Current of the Battery?

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.

Adding the Additional Battery

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.

How Much Power is Running to the Batteries?

Calculate the total voltage in the circuit. 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.

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.

As with any trial and error procedure, it's important to dispose of all used and unused materials properly. Tossing the materials in the trash could unknowingly create a fire hazard.