Solar Controller Basics

The solar controller

Every energy collection system needs a brain.
  Otherwise things could get nasty real quick. A solar array of any size -even one panel- does to. Unless of course you decide you want to sit there with your multimeter in hand ready to disengage the circuit when your battery voltage gets to 12.8v/DC. Lucky for us solar controllers are pretty simple little devices and are therefore pretty inexpensive.

Basic Solar Controller

  A solar controller needs to do a few things as previously mentioned.

These functions include but are not limited to;

•   Keep your battery form overcharging      

  Most of the time the solar controller will be factory preset to a maximum voltage and you will have little choice in the matter. The industry standard appears to be borrowed from the automotive world at a safe 13.8 volts. This most likely isn't the optimum voltage to charge deep cycle batteries as efficiently as possible but it is a safe level. Some solar controllers have “field adjustments” that can allow you to up the anti a little to a higher setting. Voltage goes downhill so to speak, it will equalize itself across two points. Therefore you have to have a higher voltage to bring up a lower voltage. -i.e. to charge a 12 volt battery you need a more than 12v source- The highest voltage you ever want to charge an AGM battery at is 14.1 volts due to the bad things that happen if they gas, but more on that later.

• Handle unexpected spikes  

  Solar controllers are usually rated at the maximum amperage they will tolerate before failure. This number is actually a little lower than the true tolerance though due to government guidelines that require solar controllers to handle at least 25% more than their rated capacity. Early on there was an effect of cloud movement discovered that concerns us here and that is how cloud sides can temporarily focus higher levels of light on a solar panel and substantially spike production levels. But you shouldn't skimp in this area and purchase a controller that's 25% less than you will expect to need just because solar controllers aren't cheap.

•  Supply your needs for the present & future

   If you have to upgrade your solar controller every time you need to make an unforeseeable upgrade in your array you will be out of substantially more cash than if you just super size early. The difference in cost from one solar controller to another that is twice the size is far less than two solar controllers. You would be surprised at the number of people who commit to spartan electrical lifestyle in the beginning but wish for a little more "comfort" later.

• Stop back feed to the panel

  The last function of a basic controller is to stop your battery from discharging through the panel when there is no sunlight available and no power is being produced. (12.8v goes downhill to 0v remember :) The voltage loss is pretty small but newer panels are manufactured with the intention of the solar controller stopping the night time back feed.

 Relay solar controller

   The simplest of the lot are designed to just open the circuit with a mechanical relay (disconnect the wires) to keep the battery from going into meltdown. They are pretty simple, cheap to make, and are the oldest technology. The drawback here is that your panels go on producing energy without you collecting it. It leaves your panels on an open circuit and the voltage output spikes harmlessly. On another note any excess voltage above about 13.8 volts your panels produce gets lost to creating useless heat. Most panel modules will produce 17-18 volts in direct sunlight so the excess is just burned up through resistance.

PMW Solar Controller

  A newer version of solar controller employs solid state design -transistors- and goes under the generic acronym of PMW controllers. PMW stands for Pulse Width Modulated and with PMW controllers the opening and closing of the circuit happens so quickly you won't even notice it. By all outward appearances the voltage will stay at one level. With the speed at which this happens the controller will effectively keep the battery at a predetermined level. The advantage of this type of controller is the lack of moving parts to wear out and give you trouble in the future. There are also no contacts or points to get scorched. One drawback here is complexity for the sake of convenience.

MPPT or Muppet Solar Controller

   More advanced than the last we have the MPPT controller. MPPT stands for Maximum Power Point Tracking with this little puppy you gain 15% or more in efficiency due to its ability to convert excess voltage that the other controllers just loose to heat resistance into current your battery can use. This means that a solar panel or panels that produce 18 volts at 10 amps will give you 180 watts instead of the 120 watts you could expect from the older style solar controller. This feature can be a real asset in the colder months of the year when there just isn't sunlight for as long each day, but due to the colder temperatures your solar panels are working just a bit more efficiently. You need to up the amperage to help offset the loss of light you are experiencing and your batteries can absorb the extra power by taking more amps in a shorter time frame.