How To Select The Correct Solar Charge Controller

In this article, I will show you how to select the correct solar charge controller to maximize solar energy production and properly charge your battery.

When thinking of a solar system that produces electricity at your home, solar panels, batteries and inverters always come first.

However, the solar charge controller is an essential part of your solar system.

First, I will describe how solar charge controllers work. Then, I will give you the key parameters on how to select the best charge controllers according to your needs. Finally, I will explain, and compare the two main technologies.

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What does a solar charge controller do?

Solar charge controllers have two functions:

  • Regulate the power output of your solar panel
  • Properly charge the battery

There are currently two types of solar charge controllers: PWM (Pulse Width Modulation) and MPPT (Maximum Power Point Tracking).

In the coming sections, I will describe how they work and what the main differences between the two technologies are.


How To Select The Correct Solar Charge Controller – 6 Key Parameters

As previously seen, the solar charge controller is the bridge between your solar panels and your batteries. Therefore, for an optimized set-up it has to match both solar panel array maximum outputs and batteries maximum inputs.

In this part, I will give you the keys to select the best solar charge controller by looking at its most important specifications: input voltage and current and output voltage and current.

1. PWM or MPPT solar charge controller?

The first step is to choose the type of solar charge controller. PWM and MPPT are the two technologies available on the market.

As previously seen, PWM is less efficient than MPPT (around 16% for small systems and more for larger PV array). Therefore, I could only recommend to purchase a MPPT solar charge controller. Indeed, what would be the point of buying a 100W solar panel and being able to use at max 80W, because the solar charger is limiting the power output. This would be a waste of money. So go for MPPT controller, 50% higher upfront cost, but you will definitely save over time.

For the next parameters, we will look at the specifications of the solar charge controller and focus on the most important.

2. Solar charge controller max. PV open voltage

This value is the maximum voltage that your solar charge controller can accept. Indeed, you will connect your solar panel array to the controller. In a solar panel array, all your modules are electrically connected, either in parallel, series or the most common is a hybrid parallel/series.

When connected in parallel, the voltage is the same and the current adds up. When connected in series, the voltage adds up and the current is the same.

3. Solar charge controller max. PV short circuit current

This value is the maximum current that your solar charge controller can accept from the solar panel array.

For example, if you have four 400W solar panels (40V, 9.8Amps), 2 series connected in parallel, therefore your maximum PV voltage is 80V, and maximum current: 19.6 Amps.

For this system, you will need a solar charge controller that can accept at least 80V and 20Amps, for example the Vectron SmartSolar would be a good choice.

4. Solar charge controller battery voltage

Not only the solar charge controller will optimize the power output of your system, it will also charge your battery. Therefore, you have to make sure that it can accept your battery type and voltage (12V, 24V or 48V).

Most of the MPPT controllers automatically detect the voltage of the battery and can charge both lead-acid and LiFePo4 batteries.

5. Solar charge controller rated battery charge current

After making sure that your solar charge controller is compatible with your battery voltage, check the maximum battery charge current of the solar charge controller. The higher this value, the faster the charge of your battery.

LiFePO4 batteries can acheive quick charge, usually they accept 50 to 100Amps from the solar charge controller, for a total charging time of 2 to 3 hours. If your solar charge controller can only deliver 10Amps, then you won’t be able to enjoy the quick charge benefits.

6. Solar production and battery monitoring

Finally, monitoring your production and storage capacity is of great interest to regulate your energy consumption and for maintenance purposes. After a few years, you will have a great database that could help you to adjust the size your system in function of your needs.


Solar charge controllers: Why do you need them?

You might wonder why you would need a solar charge controller, knowing that solar panels produce DC power and your battery also works in DC mode.

To better understand how solar charge controllers work, let’s first have a closer look at the outputs of a solar panel and then how to properly charge a battery.

Solar panel outputs

A solar panel is an assembly of solar cells. In a solar module (the one on your roof), multiple solar cells are connected in series, usually 36, 72 or 96 for the latest solar panels.

Each solar cell has an output voltage of 0.5V to 0.6V that would mostly remain the same whatever the sunlight. Consequently, a solar module with 36 cells will have an output voltage of around 18V, for 72 cells: 36V and for 96 cells: 48V.  

how to select a solar charge controllers
Multiple individual solar cells in a solar panel

On the other hand, the current output of each solar cell will vary depending on the sunlight intensity.

The power output (W) of a solar panel is obtained by multiplying the voltage of the panel (U) by the output current (I). Therefore, to maximize the power output, you have to maximize U and I.

P(W) = U(V)*I (Amps)

Below, you can see a solar I-V curve in function of the sunlight. Solar panels are said to be non-linear power generators. Indeed, If the voltage is too high, the output current will drop, such as the output power.

Battery charging

There are two types of batteries currently used for solar systems: the deep-cycle lead acid (AGM and GEL) and the LifePo4 lithium batteries.

Most of the recent solar chargers can accommodate both of them. For deep-cycle batteries you’ll find mostly 12V batteries, and for LiFePO4: 12V, 24V and 48V.

Without going into too much details, they both follow a three step charging profile: bulk mode, absorption mode and float mode. With each cycle having a specific voltage, for a 12V battery: between 13V and 14V.

In the end, you can see that on one hand we have solar panels with output voltages of 18V, 36V or 48V, and on the other hand, batteries that require no more than 14V. Therefore, the job of the solar charge controller is to match the output voltage of the solar panel with the appropriate battery charging voltage.

Let’s now have a closer look how both technologies can achieve this goal.


How PWM solar charge controller works?

PWM solar charge controller working principle is fairly easy, it acts like an electronic switch.  

The controller will first look at the battery charging voltage. If the battery requires 13V to charge in bulk mode, then the solar charge controller will impose 13V to the solar panel output: switch is ON.

As the battery is charged, it will reach the absorption mode (higher voltage) and the PWM controller will flicker between ON and OFF.

Finally, when the battery reaches float mode, the controller will switch OFF.

How MPPT solar charge controller works?

An MPPT solar charge controller can be seen as a smart DC-DC converter. First, it will maximize the power output of the solar panel (this is the smart part) and then convert it to the requested voltage for proper battery charging (DC-DC converter).

Let’s see in more details how it works:

Perturb and Observe (P&O): the heart of MPPT solar charge controller

Most of the MPPT solar charge controllers use the Perturb and Observe algorithm.

We saw that the power output of a solar panel is the product of voltage and current.

P(W)=U(V)*I(Amps)

The solar panel output current is function of the sunlight irradiation, and we can vary the voltage of the panel by modifying the resistance (R), thanks to Ohms’ law:

U(V)=R(Ohms)*I(Amps).

This is the job of the MPPT solar charge controller: it will vary the voltage at high frequency and analyse the power output with one goal: maximizing it.

First the solar charge controller set a voltage (U1) and measures the power output (P1). Then shortly after, it set another voltage U2=U1+ΔU and measures the power (P2). If P2 is higher than P1, the algorithm will try to impose a higher voltage U3 = U2+ΔU. On the contrary, if P2 is lower than P1 it will impose a lower voltage.

In the end, the solar charge controller will permanently adapt the output voltage to get the closest to the maximum power point as shown in the figure below for various sunlight irradiation.  

solar charge controller
I-V curves at various irradiation, the black line is the maximum power point (MPP)

DC-DC converter

After getting the maximum output from the solar panel, it will convert down the voltage to match the battery charging requirements with minimal loss.

MPPT vs PWM: which is the best?

To answer this question, let’s take an example, we will consider a 100W solar panel with 18.78V output (max. 5.32 Amps output) and a 12V battery that requests 14V to charge.

PWM solar charge controller:

The PWM solar charger will just reduce the solar panel output to 14V, therefore it will get 14V*5.32= 74.5W of maximum power to charge the battery.

MPPT solar charge controller:

The MPPT solar charger will nearly get the maximum output of the solar panel, around 18V*5amps= 90W (PV power loss at high temperature). And then convert down (DC-DC) to match the voltage of the battery: 14V*6.4amps=90W.

In the end, you’ll get 16% more power from the MPPT solar charger compared to PWM.  

Given this information, we could only recommend PWM solar charge controllers for lower budget systems, as PWM chargers are around 2 times cheaper than MPPT.

Let’s now see how you can select the best solar charge controller depending on your needs.

Final thougths

In this article, I showed how important the solar charge controller is for your solar set-up. Select the correct solar charge controller to optimize solar production and battery charging. Have a close look at your solar panel array maximum outputs and your battery inputs (voltage and current). Avoid PWM solar charge controller, they are inefficient and will waste a lot of your solar production.

Finally, don’t forget the future, you might want to add extra panels or battery, therefore I would advice to oversize your solar charge controller, so you won’t need to buy a new one.

Dr Metaye has a PhD in chemistry from Ecole Polytechnique, France. He is a renewable energy expert with more than 10 years of experience within the research world. During his career, he supervised more than 150 projects on clean energy. Off-grid smart systems, solar energy, battery and the hydrogen economy are among his specialties.

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