From Sunlight to Storage

The Role of a Solar Charge Controller

• Introduction
• Independent Controller or Hybrid System
• MPPT vs. PWM: Understanding the Basics
• MPPT Controller Sizing and Configuration
• Adapting to a Unique Sun Path
• Control Or Be Controlled
• 12, 24 or 48?
• Conclusion

Introduction

With our little cabin design from the past three articles, (  From Cabin Lights to Water Pumps ,  Your Off-Grid Lifeline ,  One Array All Seasons .) We’ve now reached a point where things get a bit more complex, making it the perfect opportunity to get into some critical concepts before you start designing your own system.

A 12V battery bank paired with a 2400W solar array introduces inefficiencies, requiring a really large or multiple charge controllers to manage the high current demand. Upgrading to a 24V or 48V system reduces the current, simplifies the setup, and significantly improves overall efficiency.

We’ll explore this in more detail shortly, but first, let’s begin by discussing the essential role of the charge controller.

Once you’ve sized your solar array to meet your energy needs, the next critical step is selecting the right solar charge controller. This device ensures that energy from the solar panels is delivered to the battery bank efficiently and safely. Choosing the appropriate controller type and size is essential for system performance, battery health, and long-term reliability.

In this article, we’ll focus on MPPT (Maximum Power Point Tracking) charge controllers, which are the standard for most modern off-grid systems. We’ll explain why MPPT is the best choice for your setup, how it differs from PWM controllers, and guide you through the calculations to select the right MPPT controller for your solar array and battery bank.

With the right charge controller, your system will not only meet your daily energy needs but also perform reliably through winter’s low sunlight conditions. Let’s dive in and ensure your system is ready to handle the demands of year-round off-grid living.

Independent Controller or Hybrid System

Today, hybrid inverter-charger systems offer a streamlined alternative to traditional standalone charge controllers. These all-in-one units combine an inverter, charger, and multiple MPPT controllers, making them cost-effective and simplifying installation by reducing the number of components. Their great for grid-tie systems especially when using them in a battery backup scenario for in cases such as utility outages. However, they come with a trade-off for off-grid: the entire unit must remain powered for the solar array to charge the batteries. This continuous operation results in idle energy consumption.

For cabin owners who visit intermittently, this can be a concern. In such cases, the solar array must be large enough to compensate for the unit’s idle losses, especially during winter when sunlight is limited. Many cabin owners prefer to turn off their inverters while away, leaving only the charge controllers active to maintain battery charging. This practice minimizes energy waste and ensures their batteries stay topped up for their return.

That said, in systems with large solar arrays generating excess energy that can offset the inverter’s idle losses, hybrid systems can still be a practical solution. However, for smaller setups, separate components, a standalone charge controller and inverter, often prove to be more efficient, offering flexibility and minimizing energy waste.

In our design for this system, we’re working with a smaller array and a basic setup that faces challenges like limited sunlight until after 11 a.m., and almost no sunlight in winter due to the mountain’s shadow. To maximize battery performance under these conditions, we’ll opt for independent components, ensuring we get the most efficient use of the available energy.

MPPT vs. PWM: Understanding the Basics

There are two primary types of charge controllers: PWM (Pulse Width Modulation) and MPPT (Maximum Power Point Tracking). Here’s a quick comparison:

1. PWM Controllers
   • Simpler and less expensive.
   • Best suited for smaller systems with lower voltage panels.
   • Operate at the battery voltage, which can lead to power loss if panel voltage is significantly higher than battery voltage.
2. MPPT Controllers
   • More efficient, especially in colder or low-sunlight conditions.
   • Adjust voltage and current to maximize the power from panels to the battery.
   • Ideal for systems with higher-voltage panels or larger arrays like our setup.
   • Typically offer up to 30% more efficiency compared to PWM controllers.

For our system with a 2,400W array of 400W, 24V panels, an MPPT controller is the clear choice due to its ability to handle higher voltages and maximize energy transfer.

MPPT Controller Sizing and Configuration

To select the right MPPT charge controller, we need to calculate the following:

1. Maximum Input Voltage (Voc): The open-circuit voltage from the solar panels.
2. Maximum Current (Isc): The short-circuit current from the panels.
3. Total Power (W): The combined wattage of the array.

Step 1: Calculate Voltage and Current

Each 400W, 24V panel has a typical Voc of 40V and an Isc of 10A (values vary slightly by manufacturer).

• Voc of the Array: If wiring the 6 panels in series, total Voc = 6 × 40V = 240V. If wiring in parallel, Voc remains 40V, but current increases.
• Isc of the Array: For a parallel configuration, total Isc = 6 × 10A = 60A. For a series configuration, Isc remains 10A, but voltage increases.
• ( We must consider these numbers with having a battery bank voltage of 24 volts, we have a 12 volt bank so we will discuss this later. )

Step 2: Match Controller Specifications

A suitable MPPT controller should handle:

• Input Voltage: Higher than the array’s Voc (240V for a series configuration).
• Input Current: Higher than the array’s Isc (60A for a parallel configuration).

Example: A controller rated for 250V input voltage and 60A output current is ideal. Today we will use the Victron 150 / 100 like the one in this articles feature picture as an example, but we will connect it to our array in series parallel and we will explain this next.

Adapting to a Unique Sun Path

When we first explored the property and assessed the cabin, we noticed the sun’s unique path over the roof. On those sunny fall days, the sunlight began breaking through the trees around 11 a.m., illuminating one side of the roof. By about 1 p.m., the sun shifted, casting its rays more directly onto the opposite side of the roof for the remainder of the day.

With this pattern in mind, we determined that the best setup would be to mount three panels on one side of the roof and three on the other, creating two separate arrays to maximize solar exposure throughout the day.

To ensure efficiency, each set of three panels will be connected in series, producing a string voltage of approximately 120V. These two strings will then be wired in parallel, combining their power output. With the array voltage reaching 120V, we’ll select a charge controller rated for at least 150V to handle this configuration safely. The current will remain at around 60 amps, as the array consists of six 10-amp panels in total.

If the customer anticipates potentially expanding the system in the future, we might consider sourcing a larger charge controller upfront to accommodate additional panels. However, in this example, we’ll stick with a controller sized appropriately for the current setup. Should expansion be desired later, it’s straightforward to add a second controller to manage the additional panels. This approach provides flexibility while keeping the initial system design efficient and cost-effective.

Control Or Be Controlled

Remember when we discussed the limitations of a 12-volt battery bank in the introduction? Running a 2400-watt solar array on a 12-volt battery bank, while technically possible, is far from an ideal design. This situation often arises when a system starts small and expands over time, but the original 12-volt bank is never upgraded. Since the battery bank operates at 12 volts, the inverter must match this voltage as well.

In older systems, it’s common for us to see large arrays paired with multiple charge controllers to handle the high current. However, expansion eventually reaches a point where it becomes impractical. The system’s components, like a 15-year-old inverter or aging batteries, may hit their limits, signaling the need for an upgrade. Transitioning to a 24-volt or 48-volt system not only enhances performance but also allows for better scalability and efficiency.

When designing a solar system from the ground up, planning for future expansion is essential. A 12-volt system has inherent limitations, and for larger arrays, a 24-volt or 48-volt configuration is far more efficient and sustainable. Unless it is a Van, Boat or RV with limited panel space we don’t really install 12 volt systems anymore.

12, 24 or 48?

To calculate the charge controller requirements for each battery bank voltage (12V, 24V, and 48V), we use the following principles

1. 12V Battery Bank

• Total Array Current: I = 2400W ÷ 12V = 200A
• Charge Controllers Needed: If we use an MPPT charge controller such as a 150V/100A model we need: 200A so 100A per controller = 2 – 150/100 charge controllers.

2. 24V Battery Bank

• Total Array Current: I = 2400W ÷ 24V = 100 A
• Charge Controllers Needed: A single 150V/100A charge controller can handle this configuration since the array current is within its limits.

3. 48V Battery Bank

• Total Array Current: I = 2400W ÷ 48V = 50 A
• Charge Controllers Needed: A single 150V/50A charge controller would be sufficient. Alternatively, a higher-rated controller as we were using before the 150V/100A could be used for added capacity or future expansion. in fact you could double the size of your array.

The confusion arises because the current values discussed earlier (60A for parallel or 10A for series) are based on the output of the solar array, not the input to the battery bank.

Why the Numbers Differ:

• The 60A from the array reflects the current at the panel level.
• The 200A for the battery bank reflects the adjusted current after the charge controller steps down the voltage. This is why the apparent “mismatch” exists, it’s a result of the voltage conversion process performed by the MPPT charge controller.

This highlights why a 12V battery bank can be problematic for large solar arrays. Handling 200A requires thicker cables, more robust charge controllers, and overall less efficiency compared to using a higher-voltage bank (e.g., 24V or 48V), which would reduce the current significantly.

Conclusion

In our little cabin design, we’ve seen how the initial choice of a 12-volt battery bank can shape the entire solar system, and not always in the most efficient way. While such setups are common in the real world, they highlight the importance of understanding how battery bank voltage impacts scalability and performance. By envisioning a system that can grow over time, we can create a design that outlives its components especially key parts like batteries and inverter chargers, which we’ll get into in our next discussion.

It’s important to remember that these articles cover the foundational basics of solar design, but there are many additional factors to consider. These include adding mathematic safety margin percentages, cold-weather effects on voltage rise, proper cable sizing based on conduit use and cable lengths, and ensuring all calculations comply with local electrical codes.

Here, our goal is to build a foundational understanding so you can confidently evaluate equipment when starting your solar journey. When it’s time to move forward with your system, enlisting the help of a professional will ensure your setup is both safe and efficient.

IOTG Solar: Helping you design smarter solar systems through education.

IOTG Solar…

Keeping you powered through education.