What Is a Solar Charge Controller?

What Is a Solar Charge Controller?

A solar charge controller regulates the flow of energy from a solar panel to a battery system. It prevents too much power from flooding the battery and shortening its life.

It also keeps current from flowing back into the solar panel during the night (electricity always flows from high voltage to low). It’s an essential part of any off-grid solar setup that uses a battery backup.

MPPT (Maximum Power Point Tracking)

A microprocessor-controlled MPPT controller is capable of adjusting its output to match the voltage and current of your battery system. This can help to maximize energy efficiency and increase power production compared to standard charge controllers. The technology works by finding the maximum power point voltage for your solar panels and matching it to your battery system. It can also correct for changes in temperature and light conditions. This allows you to get the most out of your solar panel configuration, even if it has multiple panels connected in series.

REDARC’s range of MPPT solar chargers feature industry-leading power conversion efficiency and are ideal for large off-grid solar systems, caravans, campervans, boats and more. The microprocessors used in the latest models can shut down for a few microseconds to ‘look’ at the condition of your panels and battery to see how much power they can generate and make any necessary adjustments.

Most MPPT solar charge controllers are designed to operate at the maximum input power rating, which is usually listed on their datasheet. However, some manufacturers, such as Victron Energy and Morningstar allow oversizing of up to 150% (Nominal rating x 1.5) so that you can get the most out of your solar panel. This helps to extend the life of your batteries. However, we recommend avoiding oversizing as this will cause the controller to operate at higher temperatures which can reduce its lifespan.

PWM (Pulse Width Modulation)

PWM solar charge controllers use a basic electrical switch that opens and closes rapidly (hundreds of times per second) to modulate battery charging. However, the varying on and off durations of these switches pull the solar panel voltage down to match the battery absorption charge voltage, which reduces panel power output and operating efficiency.

The solar charge controller is a crucial component of your solar energy system, and it’s important to size it correctly for the amount solar charge controller 12v of current your solar panels will produce. Oversizing the charge controller could lead to damaging your batteries and voiding your warranty.

Solar battery charge controllers prevent overcharging and discharging of your batteries. They also help maintain a desirable battery state of charge and protect your batteries against damage from shading and low-light conditions. They also include short-circuit, open-circuit, reverse polarity and overload protection to ensure that your solar system is safe.

Although MPPT solar charge controllers are more expensive than PWM, they provide better performance in specific conditions and can save you money in the long run by maximizing your system’s energy production. To determine whether an MPPT solar charge controller is right for your application, it’s important to carefully analyze the realized efficiencies, system operation and site conditions of your specific project. If you’re not sure, consult with an expert at Morningstar and learn more about the differences between MPPT and PWM solar charge controllers.

Sense Terminals

Usually, a charge controller has a load output terminal which is used to supply power for the small DC appliances such as light bulbs in a solar power system. Some also have a low voltage disconnect (LVD) feature that is capable of automatically shutting off the load when battery voltage drops to a safe value. Before connecting your DC appliance to the charge controller’s load port make sure to calculate its total operating and inrush current. This is important to avoid damaging the charger by supplying it with an amount of current that is higher than its rated capacity.

The sense terminals on a charge controller carry very little current, only 1/10th of a milliamp at most. What they do is look at the voltage at the bat+/bat- terminals and compare it to the battery terminal voltage, if there is a voltage drop between them the controller will raise its voltage to compensate.

The most common type of controller are the basic 1 or 2 stage types, which use relays or shunt transistors to manage voltage in one or two steps and basically disconnect the solar panel when a specific voltage is reached. They are still in use on some smaller systems and are relatively inexpensive and reliable. The newer PWM and 3-stage controllers from Xantrex, Morningstar, Blue Sky etc are the standard nowadays but you might still see the older shunt/relay types in some cheaper solar systems.


The first step is to ensure that the solar charge controller is rated to Household Power System handle at least 25% more amps than the maximum current output of your solar panels. This is important because solar panels can put out more power than their rated voltage when the sun is bright. A solar charge controller that is not rated to handle this can burn out quickly, and if it does, you will lose the benefit of your expensive solar panels.

Next, connect the DC load to the solar charge controller. Make sure that you follow the manufacturer’s instructions for proper polarity. Also, be sure to install the solar charge controller in a place that will allow for adequate heat dissipation and cable connections. The location should also be free from dust and rain, as the controller will experience a temperature increase while operating.

Finally, connect the solar array to the solar charge controller using a pair of line switches between your PV panel and your controller, and a second pair of switches between your solar array and your batteries. Make sure that you turn off the switch between your PV panels and your controller before turning on the one between your batteries and your controller. This will prevent the controller from burning out when your battery is charging too fast.

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