What Is a Solar Charge Controller, and Do You Need It?

Installing solar panels requires understanding the workings of many components: solar batteries, inverters wiring, conduit bending… If you’re going the DIY route, you could practically work as an electrician once you finish the installation! 

The charge controller is one component of a solar power system that confuses many people. A solar charge controller is necessary for most residential PV panel installations. Let’s explore what exactly a solar charge controller does and whether or not you’ll need one for your setup. 

What Is a Solar Charge Controller?

A solar charge controller is a device that regulates the energy that travels from the solar panels into the battery. Solar generators convert and store power in a battery, with the electrical capacity recharged by the solar panels. A solar charge controller regulates the electrical current to prevent the battery from electrical surges that can damage it and reduce its lifespan.

A solar charge controller is essential if your PV solar array feeds a battery bank. If you are on a grid-tied system, you probably don’t need a solar charge controller.

How Does a Solar Charge Controller Work?

A solar charge controller regulates the voltage transmitted from the solar panels to the batteries. 

Solar panels for a 12V battery system are usually rated for 17V. It may seem counterintuitive, but there is a good reason for it. 

Solar panels rarely output their full power rating due to clouds, dirt on the panels, or other environmental factors. So, if they were only rated at 12V, they would always be putting out less power — which a 12V battery cannot accept. 

A 12V battery at rest is around 12.7V, and a charging battery is around 13.6 to 14.4V. So, a solar panel must generate at least this much electrical output. 

A solar charge controller takes the electricity from the solar panel — around 16 to 20V — and downregulates it to the voltage the battery currently needs. This amount can range from 10.5V to 14.6V depending on the battery’s current charge, the temperature, and the controller’s charging mode. 

Charge controllers ultimately protect against battery damage. Inconsistencies in the electrical output, power surges, and other external factors can overcharge and damage a solar battery. 

Types of Solar Charger Controllers

There are two main types of charge controllers: PWM and MPPT. Neither is necessarily “better” than the other — each has advantages depending on climate, array size, and system components. 

While MPPT controllers typically cost more than PWM, the difference is negligible considering the total solar installation cost. Always choose a controller because it is the right tool for the job — not because it is cheaper.

PWM Charge Controllers – PWM (Pulse Width Modulation) controllers are generally smaller and less expensive than MPPT controllers. PWM controllers often come standard with small solar systems, such as RV and small cabin setups. 

When using a PWM controller, the voltage from the array needs to match the battery voltage. Off-grid solar panels (those rated at 17-18V) are required when using PWM controllers, which sometimes cost more than grid-tied panels (often rated at 37V).

PWM controllers work best in “ideal” conditions — warm, sunny weather. When the weather becomes colder, batteries operate at less efficient rates.

A PWM controller is not able to adjust voltages. Instead, it shuts on and off as the voltage from your solar array inevitably varies — this auto shut-off also results in some loss of power. 

MPPT Charge Controllers – MPPT (Maximum Power Point Tracking) controllers are more expensive than PWM, but they are significantly more efficient in many circumstances. 

MPPTs draw out the current at a rate based on the panel’s maximum voltage. They can utilize a higher-voltage array with lower-voltage batteries. You can use the mass-produced, lower-cost PV modules standard on residential homes. 

An MPPT controller can accept and modulate varying voltages. They harness excess power that a PWM would otherwise waste. 

Who Needs a Solar Charge Controller?

All off-grid solar systems require a solar charge controller to regulate the energy moving to and from the batteries. 

You won’t usually need a solar charge controller for grid-connected renewable energy systems. The utility company gathers any excess energy produced and utilizes the electricity. 

When Should You Use a Solar Charge Controller?

Almost all solar systems that utilize batteries will require a solar charge controller. Tiny solar setups are the only exception — 5-watt trickle chargers and similar devices will not need one. 

For example, many golf cart owners will keep their batteries charged over winter with a small panel. This setup does not need a charge controller between the panels and the golf cart batteries. 

If you are hooking up a full array of 400W panels, you will need an adequate solar charge controller (likely of the MPPT variety). 

Some solar solutions already have a built-in charge controller, such as the EcoFlow Portable Power Stations. The controller, batteries, inverter, power outlets, and everything else are part of the power station — you just need to add the solar panels. 

How to Size Charge Controllers Correctly?

Solar charge controllers come in various sizes for arrays of varying voltages and currents. Choosing the wrong one can lead to power loss and inefficiency. 

First, you’ll want to check the voltage rating on the charge controller. Most PWM controllers are rated for 12 or 24V, while MPPT controllers can handle 12, 24, 36, and 48V systems. Robust off-grid energy solutions like EcoFlow’s Power Kits come with an MPPT charge controller and 48V battery (or batteries) built-in.

Most charge controllers have an “amps” rating. Smaller PWM controllers may be rated at 10, 20, or 30 amps. MPPT controllers are often rated at higher amps — 80 or 100 amps are common — to accommodate larger PV arrays. 

To determine the potential amps that a solar array can output, we need to make a simple calculation: 

Amps = Watts / Volts

Let’s say we have an 800-watt array running at 12 volts. We can plug these numbers into our equation: 

Amps = 800 watts / 12 volts = 66.67 amps

The system could produce up to 66.67 amps. A charge controller rated below this amount can overload and malfunction. For this example, you would want a charge controller rated at 70 amps. 

You’ll also want to check that your batteries are compatible with the charge controller. Lithium-ion and lead-acid batteries utilize different technology. Most controllers are designed for one battery type or the other. 

Control Set Points vs. Battery Types

Most charge controllers operate at different voltages depending on the current state of the battery. For instance, a PWM controller may charge the battery most of the way, then reduce the voltage for a final trickle charge. The level at which the controller changes voltage is called a control set point. 

Different battery types require varying methods of charging. Lithium-ion batteries utilize a three-stage charging system: precharge, constant current, and supplementary. 

The precharge stage uses a low current for batteries that are nearly dead. Then, the constant current stage provides a steady supply at full power. Finally, the supplementary stage keeps the lithium battery at maximum charge. 

Lead-acid batteries utilize three main charging stages: bulk, absorption, and float. The bulk stage sends maximum power to the batteries until they hit around 80-90% capacity. For the absorption stage, the current begins to drop. Finally, the float stage provides a trickle charge to keep the batteries topped off. 

Why Are Displays and Metering Important?

Many solar charge controllers now feature an LCD. The display allows the user to monitor essential system vitals, such as battery charge percentage, current voltage, and time remaining on the battery at the current load. Some basic controllers for smaller systems will omit the LCD screen as the information may be unnecessary. 

Other systems like the EcoFlow DELTA 2 have intelligent monitoring and smart app control. The in-built metering system lets you see the input and output levels of the battery and other critical information, including the battery’s vitals, charge time, and more, all on the smartphone app. 

Understanding Control Set Points vs. Temperature

The temperature has a significant effect on battery charging. The energy in batteries flows with more ease while in warm temperatures. The battery has a harder time moving energy around as it gets colder. 

Most control set points are set for room temperature operation. Temperature compensation is featured in most charge controllers to adjust the voltage for various temperatures. Some controllers have built-in temperature sensors, while others utilize a remote sensor. 

Some charge controllers even allow for custom set points based on temperatures. Battery manufacturers each recommend a different adjustment based on the temperature, so this feature enables the homeowner to dial in their system. 

Common Features and Settings on a Charge Controller

Charge controllers for residential applications will almost always have an LCD to convey essential information. Many controllers will allow custom set points to work well with your battery bank and climate. 

Most charge controllers have built-in protection against reverse polarity, overload, short-circuiting, and other standard electrical issues. 

Advanced technologies integrated into premium controllers will even allow remote monitoring on a smartphone and Bluetooth operation. Software like the EcoFlow smart app enables you to manage these features from a smartphone. 

Conclusion

Building your solar system can be challenging, as it requires you to understand the basics of electricity. However, putting the system together is manageable once you learn the essentials. 

A solar charge controller is at the center of your solar system. It bridges the gap between your PV array and your battery bank. Make sure you choose the correct controller to prevent any issues down the line. 

All-in-one solutions can be helpful if the electrical jargon is too much for you. The EcoFlow Solar Generators and Power Kits are a great way to switch to solar, with a built-in MPPT controller and smart app to make metering and regulating your energy use even easier.

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