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Any well designed 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 of a trickle charger.
A solar panel pumps electricity into a battery that stores it. But the solar panel has no control over how much it does or how the battery receives it. A solar panel just performs under the parameters upon which it was built. It’s the charge controller, sometimes called a charge regulator that is positioned between the solar panel and the battery that regulates the voltage and the current and essentially halts charging activity temporally when necessary.
Whether you experience the warm sunny days of Phoenix, or the cool cloudy days of Seattle, picking the right charge controller for your 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 charge controllers come in 2 types: PWM and MPPT.
PWM Charge Controllers
PWM stands for Pulse Width Modulation, and is usually used in lower power systems. 1 panel, and a small tractor battery running a gate, for example is a great application for PWM, as the controller, battery and solar panel all have to match in voltages.
These controllers are easy to identify. The features describe different charging modes such as bulk charge, absorption charge, float charge, and equalize. These controllers are usually low cost, come with a host of other features, and work well for most applications.
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MPPT Charge Controllers
MPPT or Maximum Power Point Tracking is other type of charge controller. Its work is to collect a higher voltage from the panel. This charge controller will deliver 10%-30% more energy from the solar panels to the batteries than one without this feature. This is not because it makes the controller more efficient, instead it is just smarter. The panel may be rated for 19 volts, at 5 amps. The controller will take the 19 volts, lower it to 14.5 volts needed to charge the battery, but raise the AMPS to 8 or 9! Where as the PWM controller will never go above the rated 5 amps and the extra voltage is lost.
How a MPPT Controller Works
Every solar panel made has a different maximum power point. A point at which current (amperage) and voltage peak. This 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. 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.
You might think then, that only a MPPT controller is what you want, and, for a large system that is used a lot I would agree, but, PWM does have some advantages. For example, it has been documented that PWM controllers will help with battery sulfation (the buildup of sulfate crystals on the lead plates of batteries). This is a HUGE advantage in that a sulfated battery will only hold a fraction of it’s normal charge. MPPT controllers (usually) have an equalize function that deals with sulfation by taking the batteries to a high voltage (15.6 volts for a 12v battery is a common setting) for several hours.
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 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.
Most charge controllers 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.
Reverse current protection
Eliminates the current running in reverse, out of the battery, at night when the panels aren’t working.
a feature where the unit attempts to make all cells in the battery of equal value bringing them all up to full capacity. This activity also can stir up battery liquid keeping the acid and water mixed.
This feature allows the terminals to “look” and compare battery voltage to what the charge controller is putting out and have the charge controller compensate if necessary.
LVD or Low Voltage Disconnect
Some units have “Load” or “LVD” output which can be used for small load devices, like a small light. With LVD this output will automatically disconnect to keep from running the battery down too far. The automatic disconnect is most useful for sites where the small load is unattended. It isn’t recommended to connect inverters to this output directly on the charge controller unless it’s very small.
Overheating and Mounting
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.
The 3 Stage Unit Solar Charge Controller
The generally accepted industry standard on charge controllers today is the three stage unit meaning a three stage charge cycle. This type is put out by a variety of good companies. One and two stage units are old and rely on shunt transistors or relays. Basically “shunt” controllers just disconnect the panel. You don’t see many of these old ones around except perhaps in inexpensive solar “kits” (The “wow great value” with the included (crappy) charge controller!) or on old systems. Just know you can do a lot better if you run into one. (And if you get one you’ll probably want to replace it.)
Three stage units, the current standard, have three stages of charge – bulk, absorption and float. They don’t just simply turn things on and off. Many other features, many standard, others optional by model, may be included as well. Some of these features have great benefit to the battery
Knowing a thing or two about solar charge controller technology can go a long way when considering solar power. Low cost controllers may be just fine for your application provided they meet some basic technical standards.