Solar Charge Controller – How to Choose the Correct Option?

Sizing a Solar Charge Controller – How to Choose the Correct Option for Your Solar Power System

A solar charge controller plays a vital role in any solar power system. Essentially, the charge controller is the regulator that limits the rate of current that flows to and from the system’s battery bank. By controlling the flow of energy from your solar panel array, the solar charge controller can prevent overcharging issues and reverse the current flow when the solar panels are not generating electricity.

Despite the fact that the solar charge controller plays such an important role, few people know how to accurately determine the correct unit for their solar power kit.

How Can We Help?

To help you choose the correct solar charge controller for your specific setup, we will explain what function the controller performs and explore the two main types you can choose from. From there, we will go over the basics of charge controller sizing and explain how you can calculate array current and the load current, so you can determine your solar power system’s controller needs for yourself.

Why Do You Need a Solar Charge Controller?

In simple terms, a solar charge controller manages the power flowing into the battery bank from a solar array. They protect the expensive battery bank in a solar power system by preventing overcharging during daylight hours. They also prevent power from flowing backwards to the solar panels at night, which reduces the impact of phantom battery drain.

While some solar charge controllers have additional capabilities, like load control, managing the flow of power is their primary function. If you have invested in a battery bank for solar to store the energy, your system will require a solar charge controller.

What are the Two Types of Solar Charge Controllers?

Solar charge controllers are available in two separate configurations – PWM and MPPT. Understanding the differences between the two will help you determine the best option for the particular needs of your solar power system.

Pulse Width Modulation Charge Controller (PWM):

PWM solar charge controllers tend to be less expensive, as they feature less advanced technology. Essentially, a PWM acts as a simple switch, which turns on and off at set parameters to prevent overcharging issues.

Using a PWM, the solar panel, or panels, need to be at the same voltage as the battery, as you connect them (more solar panel connectors types) directly to the battery with just a switch separating them. Since the type of battery bank you are using will determine the output the solar panels can generate, there is a considerable loss factor, usually in the range of 30%.

Maximum Power Point Tracking Charge Controller (MPPT):

MPPT charge controllers are considered ‘smart’ units, as they feature a built-in computer, which makes them programmable and much more adaptable than PWM controllers. They function by measuring the voltage of the panels and down-convert it to match the voltage of the battery.

Since the power flowing into the battery from the charge controller is dropped to match the battery bank, the current can be raised, which means more of the available power is flowing from the panels to the battery. Basically, you can use a higher voltage solar array than the voltage of the battery, which opens up a wide range of options and allows you to make your solar power system far more effective and even scale it up.

The Key Features of a Solar Charge Controller:

• Overload Protection – Prevent excessive current that exceeds the amount the circuit can handle from flowing into your battery. This prevents overheating issues and the premature deterioration of your battery.
• Low Voltage Disconnection – Automatically disconnects non-critical loads from the battery when the voltage falls below the necessary amount. This prevents over-discharge issues.
• Prevention of Reverse Current Issues – When the solar panels are no longer generating power (usually at night), current can flow in the reverse direction, which means electricity will flow from the battery to the solar array. A charge controller prevents this from happening and prevents the unnecessary loss of power.
• Advanced Features – More advanced MPPT solar charge controllers often feature display screens, which allow users to monitor voltage and charge information. They can also permit multi-stage charging of the battery, meaning they will charge the amount of power sent to the battery based on its charge level, which helps protect the health of the battery.

Solar Charge Controller Sizing:

The first step in sizing your charge controller is determining whether you will be using a PWM or a MPPT controller, as they are sized differently.

Charge controllers are sized based on the current and voltage of your solar array and battery. You will want to choose one that is capable of handling the full current potential of your solar array, which will maximize your system’s efficiency and power potential.

Charge controllers are typically available in 12, 24, and 48 volt varieties. Amperage ratings will range from 1 to 60 amps.

Basically, you just need a charge controller that can handle more than your solar panels can generate. For example, if you use 12v solar panel at 14 amps, you would need a charge controller that could handle at least 14 amps. However, it is a good practice to exceed this minimum, as environmental factors can cause surges and spikes.

Sizing a PWM Charge Controller:

Since they cannot limit their current output, you need to match and preferably exceed the current of your solar panels. When searching for a PWM controller, look for the amperage and voltage rating and make sure these numbers exceed your solar array’s rating and that of your battery.

After you have identified the voltage of your system and found a charge controller that matches or exceeds that value, you need to look at your battery’s rated current. This is where we recommend choosing a charge controller that can exceed the amp rating of your solar array, as it can spike. A good practice is to exceed the amp rating by 25%, which means multiplying the amp rating of your solar panels by 1.25 and finding a charge controller with an amp rating that exceeds that number.

If finding a charge controller that matches and exceeds the nominal voltage of your system, the rated current of your battery, and the maximum solar input seems like a complicated task, do not worry, it is actually fairly straightforward once you have your numbers. However, it is worth noting that the entire process is easier if you opt for an MPPT charge controller.

Sizing a MPPT Charge Controller:

As explained above, MPPT charge controllers regulate output, so they do not need to be matched to the maximum output of the solar array. However, you will still want your MPPT charge controller to exceed the current potential of your solar array, so it can maximize the efficiency of your system.

Since MPPT controllers can lower voltage to match the battery bank’s voltage and increase current to make up for the loss of power, they are far more adaptable.

You will still want to make sure that your charge controller can allow your system to run as efficiently as possible, so divide the total wattage of the system by the lower voltage value between the solar array and battery. Follow Watts/Volts to determine the amps. So, if you were running a 900W solar array with 48V and your battery’s voltage was 24V, you would divide 900W/24V to get a value of 37.5A. Adding an additional 25% for potential current spikes would give you a total of 46.9A.

This would mean choosing a MPPT charge controller that could handle at least 24V and 50A.

Recommended Solar Charge Controllers:

If you are running a smaller scale solar power system and you are looking for a solid PWM charge controller, we recommend the:

The Zamp Solar - 30A 5-Stage PWM Charge Controller

For those looking for a more advanced option, we recommend the following MPPT solar charge controllers:

Outback Power - FlexMax FM60 MPPT Charge Controller

Victron - SmartSolar 100/50 Bluetooth MPPT Charge Controller