Electrical systems primer
RV Electrical Systems - a primer
The following was written as a series of responses by RV Forum member Lou schneider to a member request for basic knowledge on RV electrical systems. As such, it's not intended to be an extensive technical explanation, but more of an introduction for the newcomer to RVing who has no knowledge of these systems. The details vary between one RV and another, but the general principles are the same. Motorized RVs (motorhomes, vans and buses) have an additional "chassis 12 volt system" that other RVs such as trailers don't have. The draft article was edited by RV Forum staffer Gary Brinck who also added the section on solar battery charging.
There are two 12 volt systems in a motorhome or motorized RV. The chassis battery starts the main engine, runs the headlights, dashboard lights, the dash radio and most likely anything else that you turn on from a switch on the dashboard. Some may also have map lights over the dashboard, a pair of front mounted windshield fans and perhaps some low-level lights entry way or aisle lights; If these lights are left on, they will drain the chassis battery and the motorhome won't start. Certain safety related systems are also powered from the chassis battery, e.g. the motorized entry steps.
All the rest of the 12 volt loads are powered from the house batteries. They are dedicated to running the house part of the vehicle. Anything in the house that works without being plugged into power or running the generator runs on the 12 volt system. All the house lights, the furnace fan, exhaust vents, the water pump, the circuit boards in the furnace, refrigerator and water heater run on the house batteries.
Most of the time, the house and chassis systems are completely separate. You can totally drain the house batteries and still have full power in the chassis battery to start the main engine. However, it's not a good idea to run any battery completely dead.
The engine alternator charges the chassis battery while the engine is running. It may also charge the house batteries via a relay or isolator. There is usually a switch on the dash that you can push to momentarily boost a dead chassis battery from a good house battery to start the engine if the chassis battery is dead. In some coaches there is also a battery charge controller that enables the house charging system to also charge the chassis battery. This prevents the chassis battery from going dead when camping for an extended period. These periods where both sets of batteries are being charged are the only times the house and chassis systems are interconnected.
The converter manufactures 12 volts to run the house electrical needs and recharge your house batteries whenever 120VAC power is available - including when you are running the generator. In some makes of RVs, it does not charge the chassis battery and in that case you will have to be careful about what you run from it while boondocking.
The third electrical system is the 120 volt system. It's active whenever the rig is plugged into power or you're running the generator. Basically, it's main purpose is to supply power to the converter so it can make 12 volts to run your lights, furnace, exhaust fans, etc. and recharge your house batteries.
The 120 volt system also runs the large appliances like the air conditioner(s), the electric mode for the refrigerator and hot water heater, and anything you plug into a 120 volt outlet, like a kitchen appliance, hair dryer, VCR, TV, etc.
All RVs have a converter/charger - it takes 120 VAC in from shorepower (or the generator) and creates 12 volts to run the house and charge the house batteries.
An inverter is an option and is not in every RV. It does the opposite - takes 12 volts from the house batteries and creates 120 VAC so you can run 120 volt stuff without starting the generator or connecting to shore power. Some inverters also work like a converter (producing 12 volts) when you're connected to another source of 120 volts, like being connected to shore power or running the generator.
An inverter is handy, but it can't produce more power than the battery can store. If you have an inverter, it's probably connected to only some of your 120 volt circuits. It can't power the air conditioner, the electric element in the hot water heater or the electric mode of the refrigerator, for example. If you have a washer/dryer, these also use more power than your batteries can store.
An inverter is good for running low power 120 volt equipment like a TV, VCR, satellite receiver, your computer, etc. If you have a high power inverter, you can also run a high power device like the microwave for the few minutes it takes to heat up a meal. But high power, long duration loads take more power than your batteries can store.
Your RV may also have solar panels that generate 12V power when the sun is bright. The solar panels act as an auxiliary battery charger, restoring the power you are using from the house batteries and can extend the time you can go without charging from shore power or generator. There may also be a small solar panel that helps maintain the charge on the chassis battery, but these are intended only the replace the gradualy power loss that all batteries suffer from when not in regular use. And of course, solar panels generate no power at all on cloudy days or at night, so you need enough battery storage to provide for your 12v needs during those times.