At this point, I was tempted to continue along the same vein, discussing what you need to know, in terms of tools, supplies, and techniques, to work on motor vehicle electrical systems.
However, I realized it's equally important to understand the what as well as the how. So, I decided I should do things in parallel, bouncing back and forth between the what and the how.
Many people, when they set out to understand a complex system, start with the details and work upward and outward towards a complete picture. I know, from observation, that this approach works for some people. However, I've also seen evidence this can cause difficulties and create confusion and misconceptions.
On the other hand, some people prefer to start with the big picture, working downward and inward towards the details. I lean towards this approach, while understanding this approach can also cause problems similar to the previous approach.
In the computer software industry, the first approach is called bottom-up analysis and design and the second called top-down analysis and design. Decades ago, there were a lot of academic arguments regarding these two approaches, with most arguments boiling down to "my way is better than your way because ..."
I am definitely a top-down person so we're going to start with the big picture, breaking up a motorhome's total electrical system into cohesive parts (i.e., subsystems). We will then examine each subsystem, breaking up each subsystem into cohesive sub-subsystems. We'll continue this iterative process until we reach an understanding of individual circuits and components and exactly how they fit into the whole picture.
So, to start, the majority of all recreational vehicles (that includes motorhomes) have two electrical systems that are effectively completely separate. These are the 120/240VAC system and the 12VDC system.
(VAC = volts alternating current and VDC = volts direct current)
While the two systems are effectively separate, there are a few specific points where they are interconnected. One of these is battery chargers, which convert 120VAC into 12VDC suitable for recharging and maintaining batteries. Another is transformers and bridge rectifiers that convert 120VAC into 12VDC suitable for providing power to 12VDC lights and devices while connected to "shore" power.
("Shore" power is a term taken from boat people and describes being plugged into the same electrical grid -- on shore -- that provides electricity to homes and businesses.)
While battery chargers and transformers/rectifiers appear to be similar, they are functionally completely separate beasts and one cannot be substituted for the other without creating significant problems. (This substitution is the root of many of the problems found in older motorhomes.)
The third, and final, interconnection between the 120/240VAC and 12VDC systems is inverters, which convert 12VDC into 120VAC for household appliances and devices. The problem with inverters is, in order to produce 1 amp of !20VAC current, they have to draw 10 amps of 12VDC current from the battery. So, a deep cycle battery rated for 75 amp/hours (75 Ah C20 capacity) means it will last about 7-1/2 hours before being completely discharged when hooked to an inverter producing 1 amp of 120VAC. (This is a gross simplification for the purpose of avoiding a complex technical discussion.)
You should only connect inverters to deep cycle batteries or running vehicles with high output, fully functioning alternators. Connecting an inverter to an SLI (starter) battery is a quick way to severely shorten the life of that battery.
(SLI = starter/lights/ignition ... in other words, the standard motor vehicle battery versus a deep cycle battery.)
For now, we're going to ignore the motorhome's 120/240VAC system, primarily because it is essentially identical to a house's electrical system. Instead, we'll focus on the vehicle's 12VDC system.
In my next post, we'll start breaking down 12VDC systems into cohesive subsystems.
Oh, by the way ... 12VDC is a widely accepted but somewhat misleading term for motor vehicle electrical systems. 12VDC is nominal, with the actual voltage being roughly 14.5VDC.
