There are three sources of 12 volt DC power in most recreational vehicles, the chassis alternator, the converter, which converts 120 volt AC shore power to 12 volt DC power and in some cases solar panels. All provide power to run lights and other devices when they’re available as well as to charge batteries for when they’re not. We’ll cover the basics here and you can look for a deeper dive into solar power in a future post.
Converter
When 120VAC power is available a device called a converter turns AC power into DC power that charges the batteries and at the same time provides DC power to whatever DC components that are running such as lights, stereo, etc. Some RVs have inverter/chargers that in addition to providing DC power to charge the batteries also are capable of taking that DC power and turning it into 120VAC power to run your AC devices.
Chassis Alternator
When the engine is running in a motorhome or when your trailer is electrically connected to a tow vehicle the engine’s alternator will provide 12VDC power to charge the house batteries and power DC systems. Charge rates may vary from dozens of amps down to a trickle charge depending on the alternator capacity, wire size, distance from the engine to the house batteries and the condition of any connections along the way.
Solar Power
Solar panels have been used on RVs for many years. My parent’s 2000 Bounder diesel motorhome had a small panel that didn’t generate much power but was at least able to maintain the batteries while it was parked. Since then solar capabilities in RVs have blossomed to the point where you can set yourself up to pretty much live off-grid while in your camper if you want to. All you need is enough solar panels along with batteries to store the power they produce.
RV Solar panels are usually rated at 100-watts although they do come in sizes ranging from 50 to 200 watts. The watt rating is calculated based on optimal solar conditions which generally require the sun’s rays to be striking the panel at an angle of close to 90 degrees. They will put out some level of power when any light is hitting them, but output drops significantly. Any clouds, shade, etc. will negatively impact output. For horizontally-mounted panels in the summer you’ll get 4-5 hours of near full output from your panels from around 10 am to around 3 pm. Before and after that output will tail off as you get closer to dusk or dawn. In winter you’ll probably want to have your panels angled to get closer to a 90 degree angle to the sun and given the shorter daylight you’ll get less power from your panels.
Solar panels are wired so that they charge your house batteries. In order to avoid damage to the batteries a solar charge controller is required to drop the voltage produced by the panels (up to 18V) down to the proper level to charge the batteries. There are two types of charge controllers, pulse-width modulation (PWM) and maximum power point tracking (MPPT). PWM controllers work by rapidly turning the power on and off to control the voltage to the battery while MPPT controllers take the higher voltage DC input from the solar panels, convert it to high-frequency AC and then convert it back to a DC voltage that matches what the battery needs. PWM controllers are a good low-cost option while MPPT systems are more efficient by 20% or more.
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