Looking at MPPT solar charge controller in detail

15 mins 08 Jan 2021

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To keep a solar panel system working steadily and provide you with free electricity from a natural source, it takes more than just panels. To control the energy they bring in, you need a regulator that keeps your batteries and your whole installation safe. Right now the best thing you can find is a MPPT solar charge controller. What is MPPT, how it works and what are the advantages of such a device — we'll explain in a minute.

Charge controller keeps your battery safe

The main purpose of a regulator is to protect your batteries from being charged when full. If their charge is too low, the charge controller disconnects them, because it's not healthy for the battery to be used when empty. It also prevents the current from going back to the panels in reverse, which can happen at night if there isn't a regulator. What is more, the controller makes sure that the battery can handle the voltage of the panels. You can't connect a typical 60-cell 32V panel with 12V battery just like that — the battery is going to break, and the gases inside can even potentially explode and start a fire. Whatever panels you are using, you would always need a regulator of some kind (except really small panels of 1-5 W).

Alongside with MPPT-controllers, there are PWM-regulators (pulse width modulation). They simply limit the voltage of the panels to the level of the batteries and provide a steady trickle of pulses of energy to the batteries to keep them charged. Of course, this leads to the loss of some amount of energy. MPPT-controllers, on the other hand, are a little more complicated.

MPPT controller works like a DC-DC converter

So what is an MPPT controller? Maximum power point tracker controllers are the most modern and efficient ones: they first emerged on the market in 1985 and since then have become very popular. You can look at it as a smart DC-DC converter. Instead of just limiting the voltage of panels to match the one of batteries, MPPT controllers turn it into current and send it to panels, maximizing the efficiency of the system.

The rated performance of the panel that you've bought or plan to buy is measured at specific circumstances — so called Standard Test Conditions (STC). Panels perform best at 77° F and irradiance of 1000 W/m2. However, the amount of energy which solar panels produce during the day constantly changes — the irradiance of panels shifts with the position of the sun in the sky, temperature goes up and down. If it's getting hotter than 77 F°, the voltage goes down. When it's cold and sunny this is where panels go up to their maximum power voltage. MPPT-controller is a device that helps to deal with excessive voltage. In fact, the higher it goes, the more efficient MPPT regulator becomes.

How does it work in practice? Let's say you have a 60-cell panel. On paper it has a voltage of 24V — this is their voltage at the middle of the sunny day on a roof somewhere in continental US. However, its maximum operational voltage can go up to 32 V. The panel is connected to a 12 V battery. Without a regulator the battery will break, because voltages don't match. PWM-regulator would just cut the voltage coming from panels to 12 V at any point in time (unless the voltage of panels for some reason drops lower than 12 V) and the current would be at approximately 8 A. To the contrary, the MPPT-controller would use the excessive voltage of panels. In the time when panels reach their peak power voltage (or Vpp) of 32 V, MPPT regulator would balance it out: it would decrease the voltage to 12 V for battery and raise the current coming from panels up to approximately 20 A.

The efficiency of MPPT controllers is 90-95%

MPPT controllers do their best in cold weather

On average, MPPT controllers are 20-25% more efficient than PWM regulators: they make use of around 90-95% of energy coming from solar panels. PWMs should be used only when voltages of panels and batteries match. MPPTs can be used at any combination of a panel and a battery and they are more efficient when there is a significant difference between voltages. In fact, the only time when a PWM performs almost as good as a MPPT is when the peak power voltage (Vpp) of panels is only slightly higher than the voltage of panels — for example, when a 60-cell panel has 24 V, its Vpp is 32 V and the battery has 24 V). In theory, if you have two panels connected together, it's better to use an MPPT-regulator.

Since MPPT controllers make use of the excessive voltage of panels which builds up, for example, in cold sunny weather, it would be fair to say that these regulators pay for themselves best in winter. They let you use around 30% more energy from solar panels at that time of year, so you can be sure your house will be well-heated. Obviously, since MPPT controllers have more energy at their disposal during those days, they can charge an empty battery faster. They are the least efficient in summer when it's hot and bring you around 10% more energy than PWM-regulators. Judging by that, it makes sense to use MPPT-controllers in cold and cloudy states especially. If you're wondering how exactly weather affects the performance of solar panels, check out our article on how solar panels perform on cloudy days.

Make sure the controller fits you solar panel system

There are two main numbers to look for: maximum voltage (measured in Volts) and maximum charge current (measured in Amps). Maximum voltage of the controller should be higher than a sum of open circuit voltages (Vocs) of all connected panels. The charge that the whole array generates should not exceed the capacity of the controller and should be well suited for the battery.

Let's say, you have two 300W 24V panels with Voc at 42 V. The sum of Vocs therefore is 42V*2=84V, so the maximum voltage of the controller should be bigger. However, if the sum of peak voltages (Vpps) exceeds the maximum voltage of the controller it can also be potentially dangerous. Generally, the Vpp of a panel can be calculated by adding 5V to the Voc. In our example the sum of Vpps is 42*2+5*2=94V, so a 100V controller should do the job.

Next is the maximum charge current. Both of your panels generate 300/24=12.5 A*2 =25 A in the best case scenario. For this system you would need a regulator with a maximum charge of 30 A, maximum voltage of 100V and a 300Ah (amp/hour) battery. A 30A MPPT charge controller can cost around 200$-300$ and the bigger the maximum charge is, the more expensive it gets.

You can manage a controller via smartphone

Most solar charge controllers are equipped with a display that shows the charge of the battery and its current state. Of course, since MPPT-controllers are much more sophisticated than PWM-regulators, they often have more features. Some of them (like some models of Victron) can be connected to your laptop or smartphone via BlueTooth and controlled that way. There are special MPPT-controllers that can be used in situations when the voltage of your panels is lower than the voltage of the battery (for example, you have a 30-cell panel, whose voltage is 12 V with a peak power around 18V, and 24 V battery). These are called boost MPPT controllers — for example, Genasun is one of the brands that produce such regulators. That's rarely a problem for home installations, but sometimes applies to boats or solar vehicles, which have strong batteries and not a lot of space for panels. The most popular brands for MPPT controllers are MorningStar, Midnite, Victron. On our website there is a special section for battery charge controllers and you can take a look at models yourself.

Illustrations – Marina Fionova