In some college some-where there is a professor droning on and on aboutseries and parallel and even series/parallel circuits. I was following Kenny around and picking on him and accidentally ended up in the back of one of these classes. I don't really remember much of it though because I fell asleep and woke up in the middle of some kind of "art" class, if you know what I mean.
Anyway, you might be asking yourself, "Self, why do I care if something is in series or in parallel?" Well, I'm glad you asked-you see aside from being armed with more knowledge than your buddy on the bar stool next to you, it is handy to know the difference and how to hook each one up. You can hook anything up in series or parallel, from lights to batteries to speakers, the trick is to know why you'd wanna hook it up one way or the other.
Take speakers for example. If you hook them up in series, you will end up with less sound coming out because the resistance the stereo sees is greater. However, if you hook them up in parallel, while it will be louder, you run the risk of shorting out your radio. Just like the turn signal flasher, the radio wants to see some resistance to function right. How much resistance depends on the unit you are using.
Take lights for another example; if you wire lights up in series and one bulb goes out, all the lights on that circuit will go out (think of old-school Christmas lights). So wiring them up in parallel is a good way to go. Some Jeeps have their turn signal lights wired in a series/parallel setup which allows the front signal to blink, and then the side signal in an alternating pattern.
Now, if you were to wire up batteries in series, you end up adding voltage. Many military vehicles have two standard 12-volt batteries wired in series to provide 24 volts to the radio equipment. However, if you wire up two 12-volt batteries in parallel, you end up adding the rated amp/hour ratings like in many diesel trucks that need more cranking power to get started.
It sounds complicated, but it really isn't. Just check the pictures below and you'll come away with a new understanding of a pretty simple wiring idea and be able to apply it down the road.
 Most car speakers are 4 ohms,...  Most car speakers are 4 ohms, but recently audio manufacturers have been going to 2-ohm speakers. Well, if you have an older head unit or amp and want to add newer 2-ohm speakers, just add two speakers wired in series, or end- to-end (negative to positive, positive to negative). If you have older 4-ohm speakers and your new head unit wants to see 2-ohms, just wire the two speakers up in parallel (all negatives together, all positives together). |  If you have a military vehicle...  If you have a military vehicle that needs 24 volts, wire your batteries in series as shown here. If you bust something on the trail, hook the batteries together in series and you can weld. However, you typically need three or more batteries to strike a good arc. Make sure to disconnect the batteries from the vehicles before wiring them up in series and welding though! |  On the other hand, if you...  On the other hand, if you live in a cold climate or just have issues cranking your engine over in the morning, hooking two batteries up in parallel could be for you. In this example, we keep the 12 volts that the vehicle needs to run, but we would get 110 amp/hours out of them, which means if we had something that drew 5 amps, we could run it for about 22 hours before killing both batteries (divide the number of amp/hours by your amp usage for how many hours it'll run). |