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. Some of the larger trolling motors will require 36 volts and connecting together 3 x 12 volt batteries in Series.
Just Like Everyday Batteries
Connecting 2 or more 12 volt deep cycle batteries together need not 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 usually are no fireworks with these small, flashlight batteries. That is one big difference with 12 volt auto or marine batteries - touching a (+) to a (-) of the same battery can cause sparks! Be careful.
Using the same series process used with multiple battery flashlights, you are able to connect two 12 volt deep cycle batteries together in order to power a 24 volt accessory, like a trolling motor. Deep cycle batteries are used in this application because they are designed to deliver power over time, unlike a typical starter battery which is made to deliver a power jolt for starting a motor. Following is a diagram of how this might be done:
What Size Wire?
You need to use heavier wire than when connecting radios, live well pumps, or depth finders to the battery. Note the wires that run from the battery to the starter of your engine. Or note the wire that comes off your trolling motor. You should match that size as close to possible. It will likely be 6-8 guage.
By using this same sequence you can set up a 36 volt system. On freshwater fishing boats the most common use for either a 24 or 36 volt battery system is to power a trolling motor. Damage can occur to your trolling motor if you try to connect 36 volts and your motor is only rated for 24 volts. Read instructions carefully.
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 also commonly seen in RVs. Following is a diagram of how this might be done: