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What is a Solar Charge Controller? | Solar Know How

solar charge controller

If you are like me in 2014, I had no clue about making solar energy useful for powering the electrical systems in my home. I knew that with solar panels you can convert solar energy into electrical energy for use in the home, caravan, cabin, boat or other off-grid situations. What I did not know is exactly how it is done. And I sure had no idea of what a solar charge controller was. In this article, I have tried to present everything I have learned and continue to learn about solar charge controllers in one place for you to learn. You do not need to scout around the web like I had to do to find answers to all my questions. So this is my long answer to the question: “What is a solar charge controller?”

What is a solar charge controller and What does a solar charge controller do?

A solar charge controller is simply a device used to regulate the way solar panels apply electrical charge to your batteries. It is the charge controller that regulates the charging of the batteries. It detects when to allow the maximum available charge that the solar panels can produce to be applied to the batteries and it also detects when the battery is fully charged and cuts off the charge supplied by the solar panels.

Some solar charge controllers have some other functions like providing an output connection where you can attach your D.C (direct current) loads. This means that you can connect lights and other devices that use DC directly via the charge controller to the solar panels without using an inverter. Some also provide USB outlets to connect your USB devices directly to your solar panel. These are secondary functions of the solar charge controller though.

Why Solar Charge Controller?

I have explained it above. For those who like videos better here is a video, I found helpful in understanding what a solar charge controller is and does.

What are the different types of solar charge controllers?

There are two main types of solar charge controllers. The Pulse Width Modulation (PWM) Solar charge controller and the Maximum Power Point Tracking (MPPT) solar charge controller. Before the PWM charge controllers, there were some other types of charge controllers but you are not likely to run into any of those these days. If you are looking for the best mppt solar charge controller, best PWM solar charge controller or best solar charge controller we have something for you.

PWM Solar Charge Controller

These charge controllers are cheaper and simpler in their operation. When connected, the PWM charge controller would match the voltage of the solar panel to that of the battery bank, and then charge at the battery bank with the rated current of the solar panel. For example:

A 12 volts  100 watt solar panel rated at about 19V and 5.2A should output 100 watts of power to your battery array. If you use your batteries until they are 11.5 volts and you want to charge them. The PMW solar charge controller will match the solar panels voltage to that of the battery bank that is 11.7 V but keep the current at 5.2A.

That way you get 11.7 x 5.2 = 60 Watts going into your battery bank instead of 100 Watts. The good news though is that, the power going into your battery array increases as it gets charged.

Now let me tell you in user language – The PWM solar charge controller, charges much slower when your battery is depleted than when it is almost full. If you do not deplete your batteries a lot, then you would not notice this. In an off-grid situation where budget is not unlimited, you will find yourself depleting your batteries quite a bit. When I first started out, I had only one 12 Volts battery and we used it every night till the inverter shut it down at 11.5 Volts. Do I need to tell you that it took very long for us the charge the battery back to 13Volts. Most batteries get full at 13.7 – 14.1 Volts.

The MPPT Charge Controller

I guess the engineers discovered that the PWM charge controllers were wasting too much energy. They developed the MPPT charge controllers. These controllers still “match” the voltage of the battery array, but the MPPT charge controller will increase the current so that it outputs the same power that it is receiving from the solar panels.

For example,

In our example above, the MPPT controller will maintain 100 watts by increasing the current to 8.5A instead of 5.2 to give approximately 100 watts.

The MPPT charge controllers are still quite expensive but in many situations, they are cheaper on the long run. However for smaller systems, you may still get good service with the PWM charge controllers.

Which Solar Charge Controller is best PWM or MPPT?

In the video below the battery voltage is quite high (13V), so the PWM and the MPPT solar charge controllers are not too far apart.

If the battery bank was depleted to about 11.5volts the PWM charge controller would have been supplying 11.7 V x 3.3A = 38.6 Watts instead of 53Watts.

Note also that some manufacturers just label their solar charge controllers as MPPT even when they are PWM. In such cases, it’s best to go with a known company that is well known and has a name to protect.

What is the difference between PWM and MPPT

This video was helpful in answering this question. I already mentioned it above, however, you can always learn more from this.

Using higher Voltage Solar Panels with lower voltage battery array – How MPPT Solar charge controller can help

I have a 3000 watt  – 12 volt Inverter. That is the first item I got in my solar powered system. I got it because it was on sale and I was leaving for a developing country. To cut the long stor short, a 12 Volt inverter forces you to use a 12Volt battery array. That mean all the batteries I get must be connected in such a way that the total output is still 12 Volts.

The problem with this is that the higher wattage solar panels are 24volts or 48 volts solar panels. For example a 300 watt solar panel would most likely be a 24V panel.

If  you have a 12 volts battery array and you connect a 24Volts solar panel to it, you loose the extra voltage.

The interesting part of this is that with a PWM solar charge controller, a 24V 250 Watt solar panel may give you less than 100 watts to charge your 12 volts battery array.

A 24V – 250 Watt solar panel can be are rated as Voc – 30V and Isc – 8.4A

If you connect a PWM solar charge controller to a battery bank at 11.5Volts all you get from your solar panel is

11.5V x 8.4 A = 96.6A. This is a waste of solar panel investment.

With an MPPT solar charge controller however, you get

11.5V x (30/11.5 x 8.4) = 11.5 V x 21.9 A= 251W (assuming no losses)

If you have an MMPT solar charge controller you have the room to get higher wattage solar panels at 24V or 48V without loosing to much power.

How to select a Solar Charge Controller – What size of Solar Charge Controller do I need?

There are two major values you need to know before buying a solar charge controller. They are:

  1. What is the voltage size of your battery array and inverter
  2. What is the maximum current your solar panel array can deliver?

This video explains it very well.

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