What is a Solar Charge Controller? How Does It Work?

Photovoltaic systems are the most common system among solar power generation systems. Photovoltaic systems are also divided into two: off-grid and grid-tied. The solar charge controller used in off-grid photovoltaic solar energy systems is very important for system efficiency. In this article, we will examine this important device.

What is a Solar Charge Controller?

The solar charge controller is a significant device for off-grid solar PV systems, which is placed between the battery and the solar panel, ensures efficient charging of the batteries, and prevents reverse currents that can go from the batteries to the panels. The transistors in its circuit act as switching and do not allow any current to pass unless the voltage of the panels is higher than the battery voltage.

How Does the Solar Charge Controller Work?

Since the solar charge controller is between the solar panel module and the battery group, its main task is to prevent these two system components from damaging each other. In other words, when the total output voltage of the solar panels exceeds the total battery voltage with the circuit in this device, the system switches to the charging position and the controller allows current to flow from the panels to the batteries. However, when the output voltage of the solar panels falls below the battery voltage, it does not allow current flow from the batteries to the panels.

The solar charge controller is a kind of DC-DC regulator. In other words, it stabilizes the voltage and current that cannot be produced stably in solar panels, allowing the batteries to charge more efficiently. Generally, batteries with a voltage of 12 V need a voltage of around 14-14.5 V to be fully charged. If the 18-20 V voltage from the solar panel is not regulated, the battery will be damaged by overcharging. The controller regulates this high voltage by reducing it to the battery charge level.

What Does a Solar Charge Controller Do?

Solar Charge controllers,

• Protects solar panels by preventing reverse current from flowing from batteries to panels.
• When the batteries are fully charged by the panels, it prevents the batteries from overcharging by cutting the power from the panels.
• It charges the batteries by stabilizing the variable voltage and current generated in the solar panels. This ensures that the batteries are charged efficiently.
• It also prevents the excessive discharge of the batteries.

Types of Solar Charge Controllers?

Solar charge controllers are divided into two PWM solar charge controllers and MPPT solar charge controllers.

PWM Solar Charge Controller

PWM (Pulse Width Modulation) is short for “pulse width modulation”. Instead of a constant output, the PWM solar charge controller sends short charging pulses to the charging battery. The controller determines the length and interval of the pulses according to the state of charge of the battery. If the battery is discharged, the current pulses that the device sends to the battery are long and continuous. But when the battery is charged, these pulses are short and sent at intervals of a few seconds.
PWM is the most widely used controller in the solar energy sector because it is cheap. Although it is a very ideal system for small off-grid applications, MPPT is generally preferred in larger systems.

MPPT Solar Charge Controller

MPPT (Maximum Power Point Tracking), which stands for “Maximum Power Point Tracker”, charges the batteries by converting the electricity generated from the PV panels into the voltage and current level most suitable for the operation of the system. When the voltage value reaches a certain level, this device monitors the voltage of the battery pack and accordingly allows current flow from the panels to the batteries or cuts off charging.

The MPPT solar charge controller can convert overvoltage into the current. This reduces the time required to charge the batteries. This increases the efficiency of the system by ensuring that the system always works effectively.

Although the MPPT solar controller is more expensive than the PWM solar controller, the benefits it provides to the system make the price difference insignificant. Therefore, MPPT controllers are generally preferred in off-grid applications of 5 kW and above.

How to Select a Solar Charge Controller?

The following should be considered when selecting the charge controller;

• Total output voltage and short circuit current value of solar panels,
• Total voltage of the battery pack.

Since the total short circuit current of the solar panels will increase at some times due to the increase in radiation and temperature, 1.25 times the total panel output current constitutes the charging current capacity of the solar charge controller. We can find this with the formula below.

ISCC	: Short Circuit Current (A)
SP	: The number of solar panels
ICCC	: Charge current of the solar harge controller

The reason for dividing the share by 2 in the formula is that in off-grid solar systems, solar panels are connected in 2 strings. For example, a 36-panel system is divided into two strings as 18 panels. These 18 panels are connected in parallel with each other and then the system connection is completed by connecting in series with the other 18-panel string.

As it is known, parallel connection increases the current, that is, the maximum current on the module is formed by the sum of the current produced by each panel when 18 panels are connected in parallel with each other. Therefore, the total number of panels is divided by two in the formula.

Let’s consider a system in which 36 solar panels with 270 W power will be used. Let the output values of the battery group be 48 V/1200 Ah. According to this information, let’s determine the most suitable solar charge controller for our system.

Table 1. Technical information of a 270 W polycrystalline solar panel

Maximum Power-Pm [W]	270
Open Circuit Voltage-Voc [V]	38
Short Circuit Current-Isc [A]	9.21
Maximum Power Voltage-Vm [V]	31
Maximum Power Current-Im [A]	8,71
Module Efficiency-η [%]	16,45

According to Table 1;

Short circuit current of the solar panel, i.e. ISCC =9.21 A

Open circuit voltage of the solar panel, i.e. Voc= 38V

Number of solar panels i.e. SP =36

9,21 . 36 . 1.25 . 0.5 ≤ ICCC

207 ≤ ICCC

As seen in the calculation above, the current capacity of the charge controller in our system is 207 A. Since the battery voltage in our system is 48 V, a 48V/207A controller is required. However, since there is no controller with such a capacity, 3 pieces of 48V/70A charge controllers must be used.

Most of the inverters used now contain a solar charge controller within their own structure. Although it usually varies according to the inverter capacity. Charge controllers with the models 12V/24V/48V and 50A/60A/70A/80A are included in the inverters. If the amount of current generated in the system exceeds the capacity of the charge controllers in the inverters, this problem can be solved by adding an external charge controller.

Incorrect selection of solar charge controllers is one of the biggest causes of fires in photovoltaic solar energy systems. Because the circuit inside the control devices that cannot carry the current from the solar panel breaks down and loses its function. Then, when the panel voltage drops, it causes a fire in the solar panels due to the current flow from the batteries to the panels. It can also cause the batteries to swell.

You can specify your opinions, suggestions, and questions about the solar charge controller in the comments section.