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Step 5: What Capacity of Charge Controller Do I Need -sizing charger

This article will help you choose from varying sizes and types of Charge Controller for your PV system. It will specifically show you how to calculate the capacity of Charge controller that will work best in your projects.
Sizing Charge Controller

In this tutorial, I will help you choose from varying sizes and types of Charge Controller for your PV system. I will specifically show you how to calculate the capacity of Charge controller that will work best in your projects. This tutorial is the fifth step in six-step tutorials on how to design a solar PV system.

If you jumped into this article from a google search or somewhere like that, I strongly recommend you to visit the following articles:
step 1: Determining your Power Consumption Requirements.
Step 2: How Many Panels Do I Need. 
Step 3: How Many Batteries Do I Need.
Step 4: What Capacity of Inverter Do I Need.

The information in those articles is very critical to understanding this fifth step. More so, if you’re a complete solar energy beginner, I recommend Fundamentals of Solar Energy and Everything You Need to Know About Solar Electricitythose articles are written for you in mind.

Before you bring out your calculator, I want to begin the tutorial by explaining what we mean by “Charge Controller”(popularly called charger) and its function in the solar PV world. After that, I will make you understand different types of charge controllers in the Nigeria market. Then, I will show you the international best practice on how to calculate the capacity of the charge controller you need in your PV system.

Let’s get going!

What is a Charge Controller?

charge controller usually refers to as a Charger, is an electronic device that regulates and control the rate at which battery is charged and discharged. Charge Controllers performs the following functions in a PV system:

1. It prevents overcharging

2. It protects the battery against overvoltage 

3. It improves battery performance or lifespan

4. It makes the battery user-friendly rather than a safety risk during charging and discharging.

Types of Charge Controller

There are two main types of charge controllers based on their principle of operation. They are MPPT – Maximum Power Point Tracking and PWM – Pulse Width Modulation charge controllers.

MPPT – Maximum Power Point Tracking

The principle operation of the MPPT controller is to draw current out of the panel and convert the nominal voltage of the battery to “maximum power voltage”. MPPT chargers have the capacity to convert the voltage of the battery to maximum power voltage possible. That’s why they are called DC to DC inverters.

For Illustration, let’s say we have a panel that has the ratings in the table 1 below:

Solar Panel Ratings
Table 1: Panel Rating

And we have a battery that has a nominal voltage of 12volts. 

MPPT charger will take Im (Current Max) of 8.15Amp from the panel, and convert 12V of the battery nominal voltage to 31.9Vm (Voltage Max) of the panelThe max output will be 259.985Wp. That is: Im multiply by Vm = 31.9V X 8.15A = 259.985Wp.

PWM charger will handle the operation differently. Check it out!

PWM – Pulse Width Modulation

The principle operation of a PWM controller is to draw current out of the panel and the nominal voltage of the battery. PWM chargers don’t have the capacity to convert the voltage of the battery to the voltage max like the MPPT charger.

For Illustration, let’s say we have a panel that has the ratings in table 1 above.

PWM charger will take Im (Current Max) of 8.15Amp from the panel and 12V of the battery nominal voltage (no conversion). The max output will be 97.8Wp. That is: Im multiply by V = 12V X 8.15A = 97.8WpWp. So, with PWM charger, we will lose about 162Wp

Pros and Cons of MPPT Vs PWM

The only reason why you can choose PWM over MPPT is when you are on a very tight budget. And when the output of your installation is very small. 

Otherwise, MPPT chargers can produce a better charge rate up to 30% more efficient when dealing with larger panel arrays on your system. MPPT will outperform a PWM in both cold and hot temperature. It charges faster than PWM chargers.

Ratings of Charge Controllers

The solar charge controller is typically rated against Amperage and Voltage capacities. The voltage ratings of most PV charge controllers in the Nigeria market today come between 6 to 60 volts. Their amperage ratings can be between 1-60 amps. Select the solar charge controller to match the voltage of the PV array and batteries and then identify which type of solar charge controller is right for your installation. Make sure that the solar charge controller has enough capacity to handle the current from the PV array. So, if your solar system’s volts were 12 and your amps were 14, you would need a solar charge controller that had at least 14 amps.

How to calculate the Capacity of Charge Controller You Need

According to international best practice, the sizing of a solar charge controller is to take the short circuit current (Isc) of the PV array and multiply it by 1.3

Therefore, solar charge controller rating = Total short circuit current of PV array X 1.3

Therefore, charge controller rating = Total short circuit current of PV array X 1.3.

Practical Example

Recall that the Total Watt-hour per day used by our appliances in step 2: Sizing Solar panel is 6,656Wh (after catering for losses in the panel). You may like to go back to step 2 to find out how we arrived at the figure. In that step two, we calculated and found out that we needed 4 (four) panels of 260Wp each for the installation.

Each of the panels is rated as below:

Pm (W) = 260wp

Vm (V) = 31.9V

Im (A) = 8.15A

Isc(A) = 8.71A

Voc (V) = 38.1

To size the charge controller, we’ll take Isc (Current Short Circuit) of each of the panels and multiply them by 1.3.

Therefore, 8.71A (Isc) X 4(panels) X 1.3(standard practice of giving extra room for charger) = 45.292A

So, we can comfortably use 45A by 12v MPPT charge controller for the installation.


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