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.
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. Civic Solar 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.
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!