Solar Charge Controllers

Many persons are interested in setting up solar panels to power their homes and businesses. However most persons aren’t aware of the aspects of how to set up a small off-grid solar system. In this post I take you through the basics of setting up a small off-grid solar system in your home or small business. After giving a brief introduction to the process of setting up a solar system to supplement of replace your dependency on energy from the grid, I will give you a basic overview of the many type of solar charge controllers available on the market today, and throughout the next three series of posts we cover each type of charge controller.

Solar Energy Basics

Let’s start with the very basics. The Sun is a star. I know it may be news to some persons, as my high school English teacher almost laughed at me when I told her that (true story). The Sun is a star that is very close to us.  The Sun lies in the center of our solar system and essentially keeps all the planets from floating off into space with gravity. The Sun is essentially a massive nuclear reactor that turns hydrogen into helium. This is a process known as nuclear fusion and produces a lot of energy both thermal and light that is responsible for sustaining all life on earth.

Now this light that strikes the surface of the earth is the key to solar power. Sunlight (and all light) is comprised of particles called photons. The solar panel is made up of a special material that reacts with the photons to produce electricity. These cells in the solar panel are therefore called photovoltaic cells.

Solar energy is a type of Alternative Energy System as other sources of energy such as wind and hydro based systems are also available. However as I am from the Caribbean where it is basically sunny all year round, solar energy is a key alternative energy source.

Now you understand the basics of solar power, lets move to the other section where we look at setting up the system.


Components of A Solar Energy System

A solar system comprises of five main sections. You have solar panels, solar panel mounting, a charge controller, an inverter and batteries for storage. Solar panels are the meat of a solar system, they would be like food to your body, they are responsible for capturing and carrying energy into the system. Each panel has a certain power output including things like an open circuit voltage and short circuit current. Another important aspect of the panel you will see is the Pmax rating, which we will discuss in a later section. Though we will discuss this in a later section. If you look at the back of your solar panel, you will see a label with these specifications.

Solar Panel Label

It is important to choose the right type of solar panel for your application. Basically there are three types of solar panels, there are monocrystalline, polycrystalline, and amorphous panels. Monocrystalline panels are best as they have the best power efficiency, though they may cost more than other types of panels.

A part of the system that persons overlook when they design their solar panel systems is they position and type of mounting they intend to use. There is the fixed mounting, adjustable mounting and solar tracker type mounting. The fixed mounting is the name given to the type of mounting where panels are affixed to one location such as the roof or a pole on the ground. For systems designed to power an entire home or building roof mount is the most sensible type of mounting to use. However if you are looking to supplement your existing supply, pole or grounding mount is most common. Adjustable mounting is usually used in temperate countries where the sun position would change according to seasons. There are also special mountings for panels in which the panel is mounted on a special platform that is controlled by a special microcomputer called a microcontroller that has sensors attached to it that track the sun. If you live in the the Caribbean, single axis trackers are the ones you should pay attention too as they track the sun from east to west.

The next part of your system you will need to know about is the solar charge controller. This part of the system is essentially responsible for charging the batteries in you solar power system. This is in my opinion the most important part of your system. This is akin to the circulatory and nervous system in your body. A proper solar charge controller is crucial to maintain good health within your batteries and will ensure they have a stable life. In later sections, we will discuss the types of charge controllers in more detail.

The inverter is the third part of your system. An inverter will convert the Direct Current (DC) that is in your batteries into Alternating Current (AC) that would power your devices. Most common small systems are based on either 12v or 24v systems. These would convert that 12 or 24v into 120v. Is is important when choosing your inverter to choose a pure sine wave inverter for your system. The power rating of your inverter is also an important aspect to look at, the more power the inverter can provide, the greater number of devices you can power.


The batteries are the most important aspect of the system. The batteries store energy in the system, so that you will be able to draw power if there is no sunlight such as night time for example.

Generally about 500W of power should be able to provide enough energy in your home to get lights, fans or probably power small TVs and charge electronic devices in case of a power outage.

Here are the three most popular questions I get asked.

Q: How long do solar cells last?

A: Solar cells last about 20-30 years depending on the type of cell and the quality.

Q: How much will it cost to implement a system?

A: Currently a small solar system can cost anywhere from US $1 500 (TT $10 170 at time of writing) for a bare minimum low power system to about $20 000 (TT $129 000) for a decent system that can power very small homes. For about $60 000 (TT $406 800) you can power a small business with all its equipment or a large home with all appliances. Keep in mind that according to where you live in the world (like the lovely Trinidad and Tobago) you would have to buy the panels in another country and ship them, which adds additional costs. Also when you install a system you get free, unlimited power for the next 20-30 years, and a system usually pays for itself in 3-10 years depending on your specific use case. Also advances in the technology will only drive costs down.

Q: How often will I have to change batteries?

A: The subject of batteries is a tricky one, so I put that in a section by itself.


*Warning for Non-Electricians and Non-Electronics Experts*: The information here is enough to let you understand how a system works. It is not enough for you to design your own system. There are other components such as circuit breakers and surge arrestors that must be considered before designing your own system. Things such as the voltage of the system must also be considered when designing your system as it can affect wire size and you can cause serious damage to yourself and property if you do not design your system properly. If you are considering designing your own system, let a professional do it for you, it will save you time, money and risking your safety.  You have been warned.


A Note on Batteries

The type of battery you use in your solar power system matters. You cannot take any old automotive lead acid battery and toss it in there. Basically the batteries that is in your car is known as a “starter battery”. It is designed in such a way that it can provide a large current for a short period of time. These batteries are designed to be kept at almost full charge constantly, and not fall below a minimum voltage (usually not less than 80% capacity).

The batteries that you want to use in your solar power system are called “deep cycle” batteries and are designed for discharging almost completely (down to max of about 30%) and then recharging. This is critical for solar systems where you may be using your battery power at night and the system has to wait until morning to charge back up.

There is also Lithium-ion batteries that are designed to replace lead acid batteries, but currently due to restrictions with shipping these batteries and cost it may not be as feasible to invest in these kind of systems. You may also not be able to use the low cost charge controllers with lithium-ion batteries due to their chemistry and you will damage them. There are special “smart” lithium ion batteries that are designed to be used with regular chargers, but the cost factor must be considered.

Types of Solar Charge Controllers Overview

The solar charge controller is a critical aspect of your solar energy system. The charge controller you use will determine how much energy you are able to squeeze out of your system. There are basically three types of solar charge controllers. There are On/Off charge controllers, PWM based charge controllers and MPPT based charge controllers. The On/Off charge controllers are the cheapest and least efficient. The PWM type charges are a good balance of cost to performance and a good fit for small systems. The MPPT tracker is the most complex and expensive type of charge controller that is available today.

“On/Off” Charge Controllers

The most basic and simple type of solar charge controllers are what are known as “On/Off” controllers or switching type charge controllers. These types of charge controllers charge the battery until it reaches a certain point. When that particular voltage is reached, the charging circuit switches off, and allows the battery to discharge, when the battery reaches a certain discharge point, the circuit switches on to charge the battery again.

Theses type of charge controllers are used on very low power systems, usually less than 5A charge controllers. These are very simple to design and thus have a very low cost attached to them.


PWM Based Charge Controllers

Pulse Width Modulation or PWM solar charge controllers is the name given to a type of charge controller that utilizes PWM to perform charging. Let’s discuss PWM a little. As you may know in digital electronics circuits operate a two voltages either high or low, 5v (or 3.3v or 2.5v or 1.8v or 1.5v) and 0v. The particular voltage that is the on voltage is known as the logic level of the device. Regardless of the actual logic level voltage, a digital system is usually  capable of representing only one of these voltages. However PWM changes this.

What PWM does it is allows the digital circuit (in the case of charge controllers a microcontroller) to vary the width of the pulse of current coming out of the circuit in such a way that it appears to be a value between 0 and the maximum logic level value of the circuit. We must remember in the analog world we live in, things aren’t precise and PWM allows us to sort of simulate these analog voltages we may need. This has applications in things such as in light dimming and motor speed control.

In out charge controlling circuit, the microcontroller within the circuit runs a series of steps called an algorithm that gradually decreases the PWM value of the voltage going to the battery thus increasing charging efficiency, prolonging battery life and preventing the battery from over heating and gassing. It is a good step up from using a switching charge controller.

PWM chargers are good enough for most small systems, as they have a good cost to performance ratio. I recommend them for dealing with systems from 6A – 30A, which is the typical use cases for charge controller. In the Caribbean, these are a good choice as the availability of sunlight year round makes up for the inefficiency of this charge controller in relation to the next type we will cover.

MPPT Based Charge Controllers

The Maximum Power Point Tracking  solar charge controller are the best charge controllers today. They are designed to maximize the power output from the panel. These charge controllers are critical in countries that have seasons, where for example in winter the days would be shorter and you would need to maximize the time to charge your batteries.

The MPPT tracker uses excess voltage put out by the panel and converts it into excess amperage. Remember the current is responsible for charging the battery, the more current, the higher rate of recharge, however batteries have a certain maximum rate of recharge that they are specified for.

Solar panels also have a particular voltage they produce, more sun means more voltage. Less sun would produce less voltage. A “12 V” panel can produce upwards of 20 volts when bright sunlight is shining on it. Yet the battery at full charge is at 14 volts. The MPPT charge controller would take this excess voltage and convert it into excess current to charge the battery at a faster rate. Remember due to cloud cover and sunlight intensity etc, the charge controller would have to constantly adjust the power produced by the panel varying voltage to current and finding the right power point.

The maximum power point is the “Goldilocks” point where it has just the right voltage to current output, or the best output rather given power output from the panels. Thus MPPT charge controllers are expensive.

One must be careful when buying “MPPT” trackers on the aftermarket as many of these trackers that claim to be MPPT are really PWM or even switching charge controllers.


Hopefully now you have a good understanding about solar power systems and the types of charge controllers that are available. In the next post we will design a  switching solar charge controller from readily found components.


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