Learn how to connect two 12 volt batteries to make one 24 volt battery unit. This is a common application needed to supply power to a trolling motor requiring 24 volts. In fact some of the strongest trolling motors will require 36 volts.
Connecting 2 or more 12 volt deep cycle batteries together can be intimidating. First: stop to think - you are used to connecting 2 or more standard AA, AAA, C or D dry cell batteries together in a series for a flashlight for example. Some flashlights will hold 4-6 "D" cells end to end. This means that the (+) end touches the (-) end of the next battery. Each battery = 1.5 volts. A flashlight that holds 6 "D" cells delivers power to the light source of 9 volts: you add the volts together in this "Series" application. If you accidently short a cell out by inadvertently doing something that causes the positive end to touch its negative end there is usually are no fireworks with these weaker batteries. Of course you want to avoid this as it can damage these cells too.
With 12 volt marine and auto batteries that are used here, shorting across one battery post to the other can and will cause sparks. Care should be exercised to avoid this. Now, using the same series process used with multiple battery flashlights, you are able to connect two 12 volt deep cycle batteries together and can produce a 24 volt battery unit. Following is a diagram of how this might be done:
It is important to remember that when connecting 2 or more batteries of any kind you must use the same battery with the same specifications.
24 Volt Unit Made by Series Connecting 2 X 12 Volt Batteries
What size wire should you use? Choosing a wire similar to the wire that runs from your main battery to the fuse box is a good choice. We here at PFT typically choose a 6 or 8 guage wire.
By simply adding a third deep cycle battery in this sequence you can make a 36 Volt Battery Unit. Immediate damage can be done to electrical devices that are not designed to run on 24 or 36 volts. On fresh water fishing boats about the only applications for these units will be your trolling motor. Be sure to read your instruction manual before connecting any electrical device to a multi-marine battery set up like this to make sure that device calls for the voltage you are making.
There may be times that you want power that lasts longer, but want to maintain delivery of only 12 volts. For example on larger vessels, with multiple electrical devices demanding power at the same time you will see 2 if not more deep cycle batteries connected together.. You can accomplish this by "Parallel Connecting" two or more 12 volt batteries. This application is often seen in an RV. Following is a diagram of how this might be done.
12 Volt Unit Made by Parallel Connecting 2 X 12 Volt Batteries