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, that 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) 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 voltages need to be sensed, that is why two voltage sensors are 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 does the charge controller work?

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 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-step 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 conversion. 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 other 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:

The primary distinction between these circuits is the method of charging control. This can be achieved using a basic on/off switching mechanism or a more advanced PWM approach. Employing a microcontroller offers substantial benefits over analog designs, simplifying the implementation and providing greater design flexibility. Although op-amp circuits offer a less complex alternative if microcontroller programming presents a challenge, microcontrollers are generally preferred due to their versatility and broader range of control options.

Having reviewed the fundamental circuit, you should now grasp the basic operating principles. To achieve comprehensive system control, including voltage and presence sensing, the circuit can be further refined as shown below:

Charging is controlled by the circuitry on the left, and load/discharge management is handled by the circuitry on the right. An L78L05 voltage regulator provides a stable 5V supply.

Basic operation:

Analog circuit needs lots of tuning to get an optimum result. Also, all the components are not ideal. A 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.

On the other hand, using a small microcontroller 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:

This demonstrates how an 8-pin microcontroller can significantly simplify control circuitry compared to traditional analog methods. The digital approach offers greater flexibility in managing charging and discharging parameters through simple software implementations. This solution is also more cost-effective than dedicated MPPT or PWM charge controllers. It’s worth noting that the microcontroller can also be programmed to generate PWM signals directly.

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

Code explanation:

This code implements a basic charge controller for a solar power system using a PIC12F675 microcontroller. Here’s a breakdown of its functionality:

1. Pin Definitions and Initialization:

• Defines symbolic names for various I/O pins:
  • charge_pin: Controls the charging MOSFET (likely connected to the gate).
  • load_pin: Controls the load MOSFET (likely connected to the gate).
  • ch_led: Charge indicator LED.
  • ld_led: Load indicator LED.
  • solar_sense: Indicates presence of solar voltage (connected to a voltage divider or comparator output).
• Sets up I/O pins:
  • TRISIO: Configures specific pins as input (1) or output (0).
  • GPIO: Sets initial state of all output pins to 0.
  • CMCON: Disables the internal comparator.
  • ANSEL: Sets only AN1 (GP3) as an analog input pin.
  • ADCON0: Selects AN1 (channel 1) for the ADC conversion.

2. Main Loop:

• Battery Voltage Reading:
  • battery variable is initialized to 0 for each iteration.
  • A loop reads the ADC value from channel 1 (AN1) 20 times and accumulates the results in battery.
  • A delay (Delay_ms(10)) ensures a stable reading between samples.
  • The final battery value is an average of the 20 readings.
  • Note: The code uses integer calculations to avoid using floating-point numbers, saving memory.
• Charge Control:
  • If solar_sense is high (indicating solar voltage presence):
    • charge bit is set to 1 if battery is above 145 (high voltage cut-off for charging).
    • charge bit is set to 0 if battery falls below 132 (reconnect charging).
  • Based on the charge bit:
    • If charge is high (battery fully charged):
      • ch_led is turned on to indicate full charge.
      • charge_pin is set low to turn off the charging MOSFET.
    • Otherwise (battery needs charging):
      • ch_led is toggled for a blinking charging indication.
      • charge_pin is set high to turn on the charging MOSFET.
  • If solar_sense is low (no solar voltage):
    • charge_pin is set low to stop charging.
    • ch_led is turned off.
    • charge bit is reset to 0.
• Load Control:
  • If battery is above 126, the load bit is set to 1 (reconnect load).
  • If battery falls below 116, the load bit is set to 0 (disconnect load).
  • Based on the load bit:
    • If load is high (load can be connected):
      • load_pin is set high to turn on the load MOSFET.
      • ld_led is turned on to indicate load connection.
    • Otherwise (battery voltage too low for load):
      • load_pin is set low to turn off the load MOSFET.
      • ld_led is toggled for a blinking load disconnect indication.

This code provides a basic control strategy for charging and load management based on battery voltage and solar presence. However, it lacks some features for a robust system:

• No Hysteresis: The thresholds for charging on/off and load on/off are very close, leading to rapid switching near the trigger points. Implementing hysteresis (a small voltage difference between the on and off thresholds) would prevent this.
• No Over-current Protection: The code doesn’t limit the charging current or load current. This could damage the battery or system components if exceeded.
• No Temperature Compensation: Battery voltage varies with temperature. The code doesn’t compensate for this, which could lead to inaccurate charging and load control.

For a more reliable system, consider adding these features and using a more sophisticated control algorithm.

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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