Batteries are an old invention, having been around for centuries, but they are ingenious and effective power sources. They are in widespread use today in a wide variety of applications. Part of the reason for batteries' longevity as a power source is that they have the advantage of being wired together in ways that utilise their combined characteristics. For example, two 12-volt batteries can be wired together in series to make a 24 volt-power source.
Use a wrench to attach a jumper wire between the negative terminal of one of the 12-volt batteries and the positive terminal of the second battery.
Connect the positive lead from the device that will be powered to the positive terminal of the first battery.
Connect the negative lead from the device that will be powered to the negative terminal of the second battery. Test the device.