MOst everything runs on 12V so you hook them up in series. That means you hook the positive of one battery to the negative of the other. the + and - terminals left open now deliver 12V. In series the voltage adds, and in parallel the voltage remains the same and the available current add.
Since it seems we're trying to come up with analogies for electricity so we can understand the concepts, the one that has always seemed most useful to me is the water tower analogy:
You have a water tower = battery.
The size of the water tower tanks it the capacity of the battery in Amp Hours. I think someone used Coulombs here, it's also a measure of capacity...one Coulomb is 6.242×10^18 electrons.
The hight of the water tower = voltage. The higher it is the more pressure (voltage)you'll have at a spigot on the ground.
By the way, ground is like a lake at the bottom.
You have a pipe from the tank to a spigot at the bottom. This is a conductor. The fatter the pipe and spigot, the faster you can get water out of the tank = amps. Fat pipe = low resistance; narrow pipe is high resistance.
If you have a fat pipe, but it has one little section where the pipe is very narrow, that's like the electrical component called a resistor.
For a given pipe size and tower hight, you can get a certain amount of water out of the tank = gallons per hour = amp hours.
If you put a paddlewheel in front of the spigot outlet, you can spin it to do work = Watts
For a given flow rate (gallons per minute, or Amp Hours), you still need to factor in how high the water tank is. The higher the tank the more work you can do per gallon = flow rate X pressure = Watts = current X voltage.
So, Watt/hours = amps x voltage/hours. Yes, to get amp hours from watt hours you divide by 12.
BTW, the reason I like this analogy is that you can also make analogies to give people the concept of a couple of other electrical components:
As I mentioned, a resistor is a narrow section of pipe, and it reduces the pressure in the pipe = a resistor creates a drop in voltage.
A capacitor is like a flexible rubber diaphragm in the pipe. It will stop the constant flow, but if you vary the pressure up and down the diaphram will flex and pass that change through to the other side. A capacitor blocks direct current (DC) but passes alternating current (AC or signals).
An inductor is a section of pipe made of a flexible material. Imagine a ballon with a pipe going in one end and out the other. Depending on the pressure in the pipe the ballon will get bigger or smaller. If you have rapid alternating pressures in the pipe, the ballon will tend to even them out at the output. An inductor is kind of the opposite of a capacitor, it passes DC but inhibits AC.
