This is really simple, but it’s amazing how many people screw it all up!
In series, volts increase; amps remain the same.
In parallel, amps increase; volts remain the same.
In Parallel: you connect the Positive(+) of one 12v battery to the Positive(+) of the another 12v battery. Connect the Negative(-) of one to the Negative(-) of the other. You will then have still have a 12 volt battery, but with greater amp hour capacity. This is now an ordinary 12V battery, except that instead of being in one “box,” it is in two boxes.
In Series: If you were to hook two 12 volt batteries in series, you’d have 24 volts. Clearly not the thing to do unless you have a bus conversion or custom rig that uses 24Volts. However, many RVers use 6 volt (usually golf cart) batteries. E.G., Two 105AH 6v in series would still = about 105AH but @ a nominal 12V.
Wiring in Series:
To visualize it easier. Start with a simple block diagram. Two 6V batteries.
On left battery, place negative(-) at left end, place Positive(+) on right end.
On right battery, place Negative(-) on left end, place Positive(+) on right end.
Draw a line from (+) on left bat to adjacent (-) on right battery.
This is now an ordinary 12v battery, except that instead of being in one “box” with cells all connected in series on the interior, it is in two boxes joined with a cable. It’s still a single 12 volt battery, electrically, so START THINKING OF IT THIS WAY and don’t confuse yourself by thinking of it as battery 1 and battery 2.
At this point, you’ve got two unused battery posts — just like an ordinary 12 volt bat; one neg that goes to chassis ground and one positive that goes to normal 12v isolator/supply/etc.
Series/Parallel:
Just repeat the series step above with two more 6 volt batteries and you end up with two 12V batteries. Think of it this way instead of as four 6V batteries! You now have two negative(-) unused posts. Connect them together (just as you would when connecting two ordinary 12v batteries in parallel). Repeat for the two unused positive(+) posts.
It’s really quite simple. The problem many people have is in thinking that this is very complicated. It’s not.
The only time you think of the batteries as four 6V batteries is when you disconnect them for maintenance and cleaning. And then, only to make absolutely certain that you don’t screw up when putting them back together.
Toward this end, it’s essential that you clearly label posts and cable ends!