Solar Charge Controller circuit & working principle of ON/OFF charge controller

In this article, we are going to learn about the solar charge controller. There are different types of solar charge controllers in the market. All these have different working principles. But the basic principle is the same. In this article, we will learn the basic principle of the solar charge controller and little details with a circuit diagram. I hope this article will be helpful to you. So let’s start our Solar Charge Controller circuit.

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What is a solar charge controller?

In a solar power system, energy is harvested from sunlight and stored in a battery; then, the battery gives us power backup when required. This is very simple. But the problem is, each battery has a limit of taking charge and being discharged. That is why we need a controller to control both the charge and discharge limit. Otherwise, the battery will be damaged.

Working principle of Solar Charge Controller:

A charge controller has a basic operation of sensing and switching the electrical connection between the solar panel, battery, and load. Although this mechanism differs from controller to controller (we’ll discuss this later) but you can say this is some kind of switch-like relay switch. There is a switch between the solar panel and the battery and another switch between the battery and to load. Besides, it senses the battery voltage and panel presence. That’s it in a very simple way. Check this block diagram of the Solar Charge Controller circuit.

Here SW is the switch. This switch can be electronic or relay type depending on the controller situation. But let’s guess it is as simple as a switch. Now, two voltage need to be sensed, that is why two voltage sensor is required. Although for the advanced charge controller, there are some other functions for the basic one, this is all we need to have.

So how the charge controller works?

First of all, the controller senses the battery condition. If it needs to be charged, it turns on the switch between the solar panel and the battery. Afterward, if the battery is fully charged the controller turns this charging switch (between solar panel and battery) off. Besides, the controller keeps the switch (between the battery and load) on and if the battery is discharged below a certain level, it turns this load switch off. This is how the charge controller works.

Sometimes in a large charge controller, the load switch part is not available. Because in large power systems, other devices like inverters are used which already have their own discharge protection. But in small charge controllers, loads are directly connected with the charge controller.

Types of solar charge controllers:

Based on operation principles, solar charge controllers are three basic types. These are

• ON/OFF Charge controller
• PWM Charge controller
• MPPT charge controller

ON/OFF Charge controller:

The on/Off charge controller is the most basic and easy one. It simply uses a simple switch as the block diagram explained earlier. Usually, MOSFETs are used as the switch. As there is no extra current control mechanism in the ON/OFF charge controller so it can not manage the battery charging capacity. That is why this type of charge controller is not so efficient.

Pros	Cons
Simple circuit, less maintenance	Poor battery management
Lowest in price comparing other controllers	Poor charging efficiency

PWM Charge Controller:

In the PWM charge controller, PWM (Pulse Width Modulation) is used to control the charging current. This is an extra feature compared with the ON/OFF Charge controller. To fully utilize the charging capacity of a battery the battery should be charged in three steps.. Bulk, Absorption, and float. Only with PWM control, this 3 steps charging control is possible. Which improves the battery charging efficiency as well as improves backup time with battery life.

Pros	Cons
Improved battery charging ensures longer battery life	Low power harvesting capacity than MPPT
Low price	Moderate charging efficiency

MPPT Charge Controller:

In the MPPT charge controller, the DC/DC converter (SMPS) is used. Usually, a Buck converter is commonly used. The switch-mode power supply is much more efficient in power converting. That is why it is more efficient in switching. Besides, the MPPT charge controller uses MPPT algorithms to get the maximum power from a solar panel. I’ve posted an article on this. Please check this article here. I’ve explained detailed information about MPPT in that article. That is why I’m not discussing the circuit of the MPPT charge controller here.

Pros	Cons
Maximum power harvesting from solar panel	Complex circuitry
Excellent battery and load management	High Price

In this article, we will discuss the first two types of solar charge controllers.

Circuit diagrams:

There is no difference rather than the switching signal between an ON/OFF and PWM charge controller. The common circuit diagram is like this; where the current flow direction is shown. Based on the current flow, MOSFETs are used in different positions.

Now, keep a note that most of the MOSFETs are voltage biased and the gate to source voltage Vgs is 10V for full contact. That is why, to switch these MOSFETs properly, we need to use an optocoupler. Opto-coupler works like a voltage shifter here as well as an isolator too.

ON/OFF type Solar Charge Controller circuit:

And PWM type Solar Charge Controller circuit:

As you see, there is no basic difference rather than the charging signal. Now, you can make a control circuit for either the ON/OFF one or for PWM one. Using a micro-controller makes everything easier than using analog circuitry. If the micro-controller becomes complex to you, you can use a simple OP-Amp-based control circuit. But it is recommended to use a micro-controller as it is a wide option to design your control system.

As you’ve seen the basic circuit I hope you have got the concept. Now, as we need to control everything so we need some kind of controller and some sensing circuit for sensing the voltages or presence of voltage. For that, we can modify the circuit diagram a little like this:

Here the left part is controlling the charging and the right side controlling the load or discharge. And a voltage regulator L78L05 is used to get a regulated 5V supply.

Basic operation:

Analog circuit needs lots of tuning to get an optimum result. Also, all the components are not ideal. Small tolerance of the resistors can displace the working point. That is why you need to tune the circuit properly. But as simple Op-Amps are very cheap in the market, so the total cost is pretty low.

But on the other hand, using a small micro-controller makes this very easy to control and tune. If we convert this analog circuit into a micro-controller based circuit then it will be like this:

If replaced by a microcontroller:

See? Using a small 8 pin micro-controller makes the control circuit lot easier than the analog circuit. Also, we can control the charging and discharging level at whatever we want. Simple programming will work fine with this circuit and the total cost is also much lower than MPPT and PWM. Even if a programmer wishes to make a PWM signal using this small micro-controller that is possible.

You may read this: PWM Solar Charge Controller with PIC12F675

Coding:

/*******************************************************************************
Program for, Simple ON/OFF charge controller
Program Written by_ Engr. Mithun K. Das; mithun060@gmail.com
MCU:PIC12F675; X-Tal: 4MHz(internal). Compiler: mikroC pro for PIC v7.6.0
Date: 28-03-2021; © labprojectsbd.com
*******************************************************************************/
#define  charge_pin     GP0_bit
#define  load_pin       GP4_bit
#define  ch_led         GP5_bit
#define  ld_led         GP2_bit
#define  solar_sense    GP3_bit

unsigned int battery=0;
int i=0;
bit charge;
bit load;


void main() 
{
  TRISIO = 0b00001010;//set I/O ; 1=input, 0=output
  GPIO = 0x00;
  CMCON = 0x07;//comparator off
  ANSEL = 0b00000010;//AN1 analog only
  ADCON0 = 0x05;//AN1 channel selected
  while(1)
  {
     //read battery voltage
     battery=0;//clear previous data
     for(i=0;i<20;i++)
     {
        battery += ADC_Read(1)*27/100; // *5/1023*(10+2.2)/2.2*10*calibration
        Delay_ms(10);//keep a delay for smooth measurement
     }
     battery/=20; // make an average result from 20 samples.
     
     //Note: here rather than taking floating point variable, unsigned int
     //is used and while calculating, eliminated fractions to reduce further 
     //RAM & ROM use.
     
     
     //charge control
     if(solar_sense)
     {
        if(battery>145)charge = 1;//charge high cut
        if(battery<132)charge = 0;//charge reconnect
        
        if(charge)//full charge condition
        {
           ch_led = 1; //full charge indication
           charge_pin = 0;//keep off charging mosfet
        }
        else
        {
           ch_led = ~ch_led; // Toggle ch_led for charging indication
           charge_pin = 1;//keep on charging mosfet
        }
     }
     else
     {
        charge_pin = 0; //do not charge
        ch_led = 0; //indicator off
        charge = 0;//reset charge
     }
     
     
     //load control
     if(battery>126)load = 1;//load reconnect
     if(battery<116)load = 0;//load disconnect
     
     if(load)
     {
        load_pin = 1; //load MOSFET on
        ld_led = 1;//LOAD on indication
     }
     else
     {
        load_pin = 0; //load MOSFET off
        ld_led=~ld_led;//LOAD off indication
     }

  
  }//end of while(1)
}//end of void main




//End

The code is very simple for the ON/OFF charge controller. If you have a mikroC compiler you can build the hex file but if you do not have any, you can download this hex from here.

Conclusion:

To make a PWM charge controller we need to change only the charging signal. In that case, you can code yourself to add software PWM. I’ll try to publish the PWM one in my next article. Please subscribe and stay with this blog. So that you can get the updated project notification.

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