The most basic functionality of a solar power system is solar panels collecting energy from the sun and storing it in batteries so that you can use it whenever you’d like. However, you can’t simply connect your solar panels directly to your batteries and expect them to charge. To get the most out of your solar panels, you’ll need a charge controller to charge your batteries efficiently. The most efficient type of charge controller is the maximum power point tracking or MPPT charge controller.

Let’s take a look at how they work and what benefits they provide.

What is Maximum Power Point Tracking?

Before we dive into how MPPT charge controllers work, let’s explain how they get their name.

The voltage at which a solar panel produces the most power is called the maximum power point voltage. The maximum power point voltage varies depending on environmental conditions and the time of day.

MPPT charge controllers get their name because they monitor the solar panel and determine the maximum power point voltage for the current conditions. This function is called maximum power point tracking, or MPPT for short.

Tip: Refresh on Amps, Volts, & Watts and their differences.

What Is An MPPT Charge Controller? 

Solar panels and batteries have different optimal operating voltages. Not only that, these voltages fluctuate. An MPPT charge controller is a DC-DC converter that maximizes the efficiency of a solar system. It does this by optimizing the voltage match between the solar panel array and the batteries.

For example, depending on the state of charge, a 12-volt battery has a nominal voltage that ranges between just over 10 volts and just under 13 volts. Furthermore, the voltage required to charge a 12-volt battery ranges between 13.5 and 14.5 volts depending on the charging phase.

mppt charge controller wired into rv

On the other hand, the optimum output voltage of a solar panel varies depending on the panel’s temperature, time of day, how cloudy it is, and other environmental factors. For instance, under ideal conditions, a 250-watt solar panel may have an optimal operating voltage of 32 volts. As the panel heats up in the sun or on a hot day, the optimal voltage may drop to as low as 26 volts.

The rated panel voltage must be higher than the battery voltage to accommodate for these voltage drops in the panel and the increased required battery charging voltage. Without an MPPT charge controller, this voltage differential leads to a lot of wasted power.

What Is The Difference Between MPPT and PWM Charge Controllers? 

To better understand how this voltage difference causes inefficiencies, let’s first examine the other common type of solar charge controller. This controller is the pulse width modulation (PWM) charge controller. 

PWM controllers use a transistor switch that rapidly opens and closes as needed to regulate the charge current going into the battery. Since PWM controllers can’t modulate the voltage, they pull the output voltage of the solar panel down to match the battery voltage. Let’s look at an example.

A 250-watt solar panel may have an optimal or max power voltage (Vmp) of 32 volts and a max power current (Imp) of 7.8 amps. (32 volts x 7.8 amps = 250 watts)

Using a PWM controller, your panel will still produce 7.8 amps. But the voltage will drop to match the battery at 12 volts. Now, your panel is only providing 94 watts instead of 250 watts. (12 volts x 7.8 amps = 94 watts) 

Pwm solar charge controller diagram

How MPPT Charge Controllers Work

As we mentioned before, MPPT charge controllers are DC-DC converters. This means they regulate the charge current into the battery like a PWM controller. But, they also convert the voltage coming out of the panel to match what the battery needs. Let’s look at an example of how this drastically improves efficiency.

Using the same 250-watt panel, the MPPT controller allows the panel to operate at the max power voltage (Vmp). Now the power going into the controller is the full rated 250 watts.

The output from the controller to the battery still needs to match the battery at 12 volts. But the current increases to 20.8 amps allowing you to utilize the full 250 watt potential of your panel. (12 volts x 20.8 amps = 250 watts)

MPPT Charge controller diagram

For simplicity, these examples assumed a 100% efficient conversion in the charge controllers. In reality, a small amount of power is lost as heat during the conversion.

Benefits of an MPPT Charge Controller

More Efficient at Using Power

On a properly sized solar power system, it’s not uncommon to see up to a 30% increase in efficiency by switching to an MPPT controller. This efficiency increase is even more significant on systems where the solar panel voltage is much higher than the battery voltage, like our example above.

large solar system on rv

Best for Large Systems

Utilizing an additional 20-30% of power out of your system becomes more advantageous as the size of your system grows. For this reason, MPPT controllers are often best used on large systems and may not be worth it on smaller, simpler setups.

Better in Cloudier Environments

The maximum power point tracking feature of MPPT controllers is a huge benefit in cloudy environments where the max power point of the solar panels will be fluctuating all day. 

Are MPPT Solar Charge Controllers Worth It?

MPPT charge controllers are more expensive than PWM controllers. The added cost of upgrading your controller may not be worth it on small, basic systems. However, on larger systems or in locations with unstable weather conditions, the increased power and efficiency gained by using an MPPT controller will likely more than makeup for the added cost of the controller.

Nobody likes to waste power. MPPT charge controllers help you get the most out of your solar panels without worrying about changing weather conditions or making sure you perfectly sized your solar panels to your battery voltage.

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