RV solar systems are more than just panels on the roof. Any well designed RV solar system has a Charge Controller or Regulator as part of the system. It is the heart of the solar system. Without it, the batteries will not be properly charged and damage to the battery pack will occur. Or, at the very least, shorter battery life will result. One should NEVER EVER connect a solar panel directly to a battery pack without a controller….with perhaps one exception that will be covered in another article.
Whether you experience the warm sunny days of Phoenix, or the cool cloudy days of Seattle, picking the right charge controller for your RV solar system is important. There are plenty of good products on the market today, and a few great ones. Charge controllers of average quality work well, are very reliable if installed correctly, and won’t break the bank. There are a few basic differences in technology that are important to consider before buying. With this knowledge, a well informed consumer can make the right choice easily.
Solar Voltage Regulators.
Fortunately for consumers, the old days of low-tech Solar Voltage Regulators are gone. While these devices worked fairly well at charging your RV batteries, long term use shortened the RV’s battery life. The technical reasons for this are too lengthy to discuss here, but they have to do with the accumulation of sulphate deposits on the battery plates.
The latest Charge Controller technology to be brought to the RV industry employs MPPT or Maximum Power Point Tracking. A charge controller with MPPT (a technology borrowed from the commercial and residential solar industry) will deliver 10%-30% more energy from the solar panels to the RV batteries than one without this feature. This is not because MPPT makes the controller more efficient, instead MPPT is just smarter. Every solar panel made has a different maximum power point. A point at which current (amperage) and voltage peak. An MPPT controller is designed to sense this point and adjust itself for maximum output. This point is referred to as the ‘knee of the curve’ and is shown on the IV graph found on almost every solar panel datasheet. MPPT controllers are more expensive than PWM types. A good MPPT charge controller can cost $250-$700 depending on the power rating. So, the added value of the energy produced, versus the added cost, must be considered. Depending on the size of the system, it could make as much difference as adding another panel. In some cases updating just the controller itself to MPPT is about the same cost as adding another solar panel to the system with the same benefit and cost. Something to consider for those with older controllers.
All Solar Charge Controllers have a power rating. This is usually measured in AMPS. It is the maximum amount of electrical current the controller can handle without failing. A label is found on the back of every solar panel will detail the maximum amount of amps the panel will output. This is usually expressed as ISC or short circuit current. Since most RV solar panels are wired in parallel, the amperage from every panel is added together. The total solar panel current or battery current should not exceed the maximum power rating of the charge controller. It is a good system design practice to oversize the controller by 20%. The controller will run cooler, be more reliable, and last longer. Power does come at a price. Therefore, the higher the power rating of the charge controller, the more it will cost.
Environmental concerns and placement.
Solar charge controllers are usually mounted inside the passenger compartment of the RV and recessed in a wall cavity. While this installation looks good, it can sometimes be problematic. On larger RV solar systems where 20-40 amps of current flows through the charge controller, a great deal of heat is produced. Heat must be dissipated otherwise the charge controller will fail. Recess-mount a charge controller inside a hollow, unventilated wall cavity and then raise the ambient temperature to that of an ordinary summer day, and the charge controller will quickly overheat and fail. This is a very common problem. Instead, consider a surface mount controller with a good heat sink and mount it inside one of the storage lockers or somewhere good ventilation is found. Since some controllers are not waterproof, care must be taken to select just the right location. Oh, and never, ever locate the charge controller in the battery compartment. There are many reasons for this, the biggest of which are corrosion caused by battery off-gassing and the potential for fire should there be a spark.
Most charge controllers for RV Solar systems have some sort of display. Some have simple blinking lights, others have fancy digital readouts. Both work well, but the digital displays will cost you almost double. For those of us that like gadgets, blinking lights just won’t do. We have to know how much power is being made at any given moment. So, if you must have a fancy display inside the passenger compartment, purchase a model that supports a remote control display instead. Sure, it costs a few more dollars, but the remote control display generates virtually no heat and can be safety mounted in a hollow wall cavity without fear of high temperature failures.
Knowing a thing or two about solar charge controller technology can go a long way when considering solar power for your RV. Low cost controllers may be just fine for your application provided they meet some basic technical standards. Or, if you seek the best charge controller available, you are now armed with the knowledge you need to select the controller that’s right for you.
About the Author
James Mannett is solar power expert, RV enthusiast, and owner of CEA Solar in Sun City, AZ. James conducts seminars on solar power use, and exhibits RV solar products at various shows around the country. He assists people in solar system design and provides components and advice for solar do-it-yourselfers. James can be contacted by writing to firstname.lastname@example.org or by visiting www.rvsolarnow.com.