Choosing the Correct Charge Controller

Selecting an efficient and properly designed charge controller is key to the longevity and efficiency of your entire battery-based photovoltaic (PV) system. By optimizing the power coming in from your solar modules, you will get that much closer to offset your use of traditional grid power or another source of energy. In addition, you will be protecting your battery bank and thereby you protect yourself from any unforeseen and needless replacement costs. Your solar charge controller is an item well worth investing in and researching as you design your system. You'll need to choose an option that is scalable and appropriate for your power needs, as well as making sure that you have ample battery storage for the solar modules you have selected to install. Greentech Renewables can advise you on everything from optimizing your current system, to how to install your solar modules, to choosing the right equipment tailored to your needs.

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Solar charge controllers are rated and sized by the solar module array current and system voltage. Most common are 12, 24, and 48-volt controllers. Amperage ratings normally run from 1 amp to 80 amps, voltages from 6-600 volts.

For example, if one module in your 48-volt system produces 8.05 amps and two parallel strings of modules are used, your system will produce 16.1 amps at 48 volts. Certain factors such as light reflection or cloud effect at irregular intervals can increase current levels.  This is quite common. Therefore we increase the charge controller amperage by a margin of 25% bringing our minimum controller amperage to 20.13. We migrate over to our catalog and we find a 30-amp controller, which is a very close match. There is no problem going with a larger controller, other than the additional cost. This would allow you to expand the size of your system later on down the road if your load demands change or you find you need a little more power.

MPPT Charge Controllers

Top: Schneider Electric&#;s Conext MPPT 60 150 Charge Controller

Bottom: Outback Power's FlexMax 60, an MPPT Charge Controller

In the past, you would assume that the nominal voltage of your battery and your solar module array would be the same and that you would also choose that voltage for your charge controller. However, this school of thought is no longer commonly used as more efficient charging technology called Maximum Power Point Tracking (MPPT) has become widely available on many models of charge controllers. The primary feature of this technology is that it allows you to have a solar module array with a much higher voltage than your battery bank's voltage. The MPPT charge controller by design converts the higher voltage down to the lower voltage.

MPPT Charge Controllers have the added benefit of saving you a little bit of cash on wiring costs. A big advantage to having a higher voltage solar module array is that you can use smaller gauge wiring into the charge controller. Many times a solar module array can be over a 100 feet away (or more!) from the charge controller, keeping the cost of the wiring down to a minimum is usually an important target for the whole project. When you double the voltage (e.g. from 12 to 24 or 48 volts), you will decrease the current going through the wires by half each time which means you use much less copper, saving you money.

Example of Sizing an MPPT Charge Controller

For example, you could have a 3,000-watt solar module array that operates at 93.3 volts DC and your battery bank is 48 volts DC. MPPT charge controllers are rated by the output amperage that they can handle, not the input current from the solar module array. To determine the output current that the charge controller will have to handle we use the very basic formula for power in Watts:

Power = Volts x Amps

Here we know the power is 3,000 Watts, the battery bank is 48 volts, so:

3,000 Watts = 48 volts x Amps

which gives us:

Amps = 3,000 Watts/ 48 volts

Amps = 62.5A

We still want to adjust this value by 25% to take into account any special conditions that might cause the solar module array to produce more power than it is normally rated for (e.g. due to sunlight's reflection off of snow, water, extraordinarily bright conditions, etc). So, 62.5A increased by 25% is 78.13A. In this case, we'd probably choose an 80 Amp MPPT Charge Controller, like Outback Power's FlexMax 80.

Another Benefit of MPPT Charge Controllers

Because MPPT charge controllers can handle a higher input voltage from the solar module array than the battery bank's voltage, you can also use these charge controllers with solar modules that have voltages that don't match your typical system voltage (i.e. 12, 24 or 48V). For instance, you could have a solar module that has a nominal voltage of 31.1 volts and charge controller and battery bank that's 48 volts efficiently with an MPPT charge controller.

Keep in mind that MPPT charge controllers have a maximum system voltage limit that they can handle from the solar module array. It's important that you make sure there is no condition that the solar module array voltage will go above this limit or you could potentially harm the controller. You want to make sure that the open circuit voltage of the solar module array does not go above this value. You also want to give yourself a little bit of a margin for safety to take into account for the potential that an array's voltage will actually increase the colder it gets. If you give yourself a 25% margin of error you will be alright.

Here's an example:

We'll use twelve 31.1 volt SolarWorld 250 Watt solar modules with four parallel strings of three in series for a nominal voltage of 93.3 volts and a 48-volt battery bank. We'd like to use a Schneider Conext MPPT 60 150 charge controller. If we look at the module's specification page we see that each module has an open circuit voltage of 37.8V. That means the array has three times that because there are 3 modules in series. So the array open circuit voltage is 37.8V x 3 = 113.4V. We'll increase this by a safety factor of 25% and we get 141.75V. Now we'll look at the Conext MPPT 60 150's specifications and we see that it can take a maximum of 150 volts. 141.75V < 150V, so you&#;re good to go!

story written by Bob Gudgel & Kim Silva, MidNite Solar

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What is the most important factor in choosing the perfect solar charge controller for your project?

The most important job of all solar charge controllers is to properly charge the batteries and to give them as long a life as possible. There are two types of charge controllers:

• Pulse width modulation (PWM)

• Maximum power point tracking (MPPT)

The difference between these two types of controllers is that the PWM is not as efficient the MPPT. The MPPT is the most common these days and can gain you up to 30% more power than the PWM controllers. The MPPT controllers also allow the strings of panels to be connected in series for higher voltages, keeping the amperage lower and the wire size smaller, especially for long-wire runs to the PV array.     

When picking a charge controller there are a few steps that you must follow to make sure that you get the right controller for the job. The best thing that you can do is to use the manufacturer&#;s sizing tools that are offered on their web sites. The other option is to give the manufacturer a call &#; their salespeople will usually be happy to help you to pick the best controller.

If you need to make some quick calculations, the following information will be needed to manually figure out the amperage of the controller needed: 

• The wattage of the solar array

• The battery-bank voltage (12, 24, or 48). Typical bank voltage because inverters are offered in these voltages.

• Now Ohm&#;s Law comes into play: Amps x Volts = Watts

Example: 3,000-watt array/48-volt battery bank = 62.5 amps, so you would need a controller capable of 62.5 amps. Most controllers out there are either 60, 80 or 96 amps so you would pick the controller with the next higher rating. In this case, it would be the 80 amp controller.

Now if you know the amperage of the controller, and you would like to figure out how the maximum solar array wattage that can go into the controller, you would also use Ohm&#;s law:

Example: 80 amp controller x 48 volt battery bank = 3,840 watts of solar panels. Note that most of the controllers will allow a bit more wattage to go into the controllers. This is where the sizing tools or a call to the manufacture can help out.

The next thing that you must ensure is that we do not exceed the input voltage the controller can take. Again the manufacturer will dictate what the input voltage should be included in the design. Temperature and open-circuit voltages have to be considered. Since PV open-circuit voltage (Voc) goes higher as temperature drops, you will need to make sure the controller&#;s input voltage ratings can handle this in the cold of winter.  Manufacturer&#;s sizing tools will give you the best design for the controllers.

There are many makes and models of charge controllers available, but it&#;s best to get the most options for the best price. The following is a list of features available on charge controllers &#; but not all controllers. The best will offer every option.

• 150,200,250,600 V

• Manual and auto EQ

• Built in GFP and arc fault

• Online status monitoring

• Hyper VOC extend VOC limits

• 12-72 volt battery charging

• Solar, wind and hydro MPPT modes

Disclaimer: We cannot provide advice on your specific project needs. Please reach out to charge controller manufacturers for more information, or assist each other in our comment section, below. 

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