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What is a Solar Charge Controller? Selecting the Right Size of MPPT & PWM Charge Controllers

Types of Solar Charge Controllers – How to Select the suitable PWM & MPPT “Maximum Power Point Tracking” Charge Controller?

In the following article, we will discuss an Introduction to DC-DC power converters, Charge controllers, and MPPT in a Solar PV system.

DC-DC Converters:

DC-DC converters are widely used to convert unregulated or uncontrolled DC voltage to a regulated or controlled DC voltage level as shown in figure 1.

Fig 1: DC-DC converter

Other than the uncontrolled voltage to controlled voltage these converters convert the voltage from one level to another level (high or low). For example, we have a PV system that produces 24 V dc output voltage but the inverter AC output needs to be 230 V, so we require a higher input dc voltage at the inverter’s input.

So, to obtain that we connect a dc-dc converter in between the PV array and the inverter. These dc-dc converters play a very significant role in our solar PV system. They are used as charge controllers, maximum power point trackers, and acts as an interface along with PV source for different types of loads. Their application also includes power bus regulation, Current boosting, and noise isolation.

In the DC to DC converter both the input and the output side have a DC flow. It is possible to determine input DC power if we know the input voltage and current, similarly, we can determine the output power if we know the output voltage and current. Once we know the input and output power the efficiency of the power converter can be easily determined.

Let us take an example of a DC-DC converter where a battery of 50 V is connected, supplying an input current of 8 A. At the output of the converter the voltage measurement shows a voltage of 100 V and current measurement shows a current of 3.6 A. Determine the input and output power, power loss in the converter, and efficiency of the converter.

  • Input voltage = 50 V
  • Input current = 8 A
  • Output voltage = 100 V
  • Output current = 3.6 A

Thus, Input power = Input voltage × Input current

Input power = 50 × 8 = 400 W

Similarly, the output power can be determined as follows;

Output power = Output voltage × Output current

Output power = 100 × 3.6 = 360 W

The power loss in the converter can be determined as follows;

Power loss = Input power – Output power

Power loss = 400 – 360 = 40 W

Efficiency of the converter is determined as follows;

Efficiency % = (output power/input power) × 100

Efficiency % = (360/400) × 100 = 90 %

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Solar Charge Controller - MPPT vs. PWM - How to Select the Right Size Charge Controller

What is a Charge Controller and It’s Working & Function:

It is the controller that helps to control the charge flow from and to the battery. It maintains the long battery life and performance by preventing deep discharge and overcharge of the battery. When a PV module is connected to a battery through a charge controller, the charge controller would disconnect the PV from the battery to avoid overcharging.

Similarly, when a battery is connected to a load through a charge controller, the controller will disconnect the load from the battery if it detects over-discharge. Such an ability of the charge controller helps in prolonging the life and performance of the battery.

Overcharge and deep discharge of the battery are detected by measuring the voltage level of the connected battery. In overcharge, the battery voltage increases above a certain voltage level similarly in case of deep discharge the battery voltage decreases below a certain voltage level.

The charge controller can disconnect the battery in both these above-mentioned conditions. The charge controller also reconnects the battery when the voltage level has reached within the normal operating level.

Due to overcharge the voltage level of the battery reaches high and the charge controller disconnects the battery from the PV module (or charging DC source), but when the level of the voltage falls due to utilization of the battery by the load the charge controller detects this voltage drop and reconnects the PV module (or charging DC source) to charge the battery.

Such a similar thing can also observe in case of deep discharge when the battery gets cut-off (from load) due to a drop in voltage below a certain level. Now if the battery is an undercharging condition, the terminal voltage level will rise after a while due to the charging process. This rise in voltage level is detected and if it is above a low cut-off voltage level the controller will reconnect the battery to the load so that the load can utilize the energy stored in the battery.

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Types of Charge Controllers:

The following are the two widely used charge controllers;

  1. Maximum Power Point Tracking (MPPT) charge controllers
  2. Pulse Width Modulation (PWM) charge controllers

In the case of MPPT charge controllers, the voltage across the battery bank and the PV array is different. This type of charge controller operates at the maximum power point of the PV array to deliver the maximum possible power available from the irradiance.

The kind of charge controllers makes it possible to have PV array voltage higher than the voltage of the battery bank connected to our system. The advantage is that the higher the voltage lesser will be the current for the same flow of power. Thus, we can use the small gauge wire which reduces the cost of the wire in the system.

On the other hand, the Pulse Width Modulation (PWM) charge controllers have identical voltage across the PV array and the battery bank connected to the system.

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Various Features of the Charge Controller:

The various voltage and current level of the charge controller can be defined as follows;

  1. Nominal system voltage: This voltage represents the voltage at which the charge controller and the battery operate in a solar PV system.
  2. Nominal Load current: This represents the maximum load current that a charge controller should handle.
  3. Nominal PV array current: This represents the maximum PV array current that the charge controller should be able to handle. It is the short-circuit current of the entire PV array. While designing a safety factor of 1.25 is considered for variation in determined short-circuit current under non-STC (Standard Test Condition).
  4. Charge regulator set points: The function of the charge controller is to charge and discharge the battery, it senses the terminal voltage (i.e. state of charge or commonly known as SOC) and decides either to disconnect it from the load to avoid the deep discharge or to disconnect it from its PV array source to avoid the overcharge of the battery. Such a controller has set points upon which it takes decisions either to connect or disconnect the load or charging source (i.e. PV array).
Features of the Charge Controller
Fig 2: Features of the Charge Controller
  1. Voltage regulation setpoint (VR): This represents the maximum voltage up to which a battery can be charged without getting overcharged. If this setpoint is reached then the controller will disconnect the battery bank from the PV source or it may regulate the current supplies to the batteries.
  2. Voltage regulation hysteresis (VRH): This represents the difference between the VR and the voltage at which the charge controller will reconnect the battery to the PV source for charging. If this difference is very small then the control will be oscillatory (connect and reconnect frequently) which will eventually result in deterioration in the performance and life of the battery. But having a difference might also lead to some overcharging in each cycle. So, a balance has to be made while stating the VRH. The VRH also helps us understand how effective is the charge controller at charging the battery.
  3. Low voltage disconnect (LVD): This represents the minimum voltage that up to which discharge of a battery is allowed without getting into the deep discharge. This is also known as the depth of discharge (DOD) of a battery. It is strongly recommended to avoid discharge below this level to avoid the deterioration of the life and performance of the battery. The charge controller can disconnect the battery from the load if it detects the LVD and avoids the deep discharge of the battery.
  4. Low voltage disconnect hysteresis (LVDH): This represents the difference between the LVD and the voltage at which the battery can be reconnected to the load. It is not kept too small, as this may result in frequent connection and disconnect. Which can further reduce the life of the battery.

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How to Select Proper Rated Solar Charge Controller?

The following two examples shows how to select a right size solar charge controller for solar panel and array system having the appropriate nominal current rating in amperes at given rated nominal voltage and load in watts.

Example 1:

Let us now take an example to understand the above parameters, a living room has the following DC loads which are rated at 24 V;

  1. Three 20 W lamps
  2. One 25 W fan

All the above-mentioned loads are powered by two parallel connected PV modules, each PV module has a maximum power point current IMP of 5 A and short-circuit current ISC of 7 A. What will be the nominal system voltage, nominal PV array current, and Nominal load current of the charge controller?

Total DC load = (No. of lamps × Wattage of each lamp) + (No. of fans × Wattage of each fan)

Total DC load = (3 × 20) + (1 × 25) = 60 + 25 = 85 W

The nominal system voltage of the charge controller is the same as the rated voltage of the load and the PV array (Nominal system voltage of the charge controller = 24 V)

Nominal PV array current = 2 × 7 (short-circuit current of each PV module is 7 A and are connected in parallel)

Nominal PV array current = 14 A

A safety factor of 1.25 is considered for variation in determined short-circuit current under non-STC (Standard Test Condition).

Considering the safety factor of 1.25 the Nominal PV array current is 1.25 × 14 = 17.5 A

Nominal load current = Total DC load / Nominal system voltage = 85 / 24

Nominal load current = 3.5416 A

Thus, the charge controller should be able to handle a current of about 3.5416 A at the output side.

Example 2:

Let us take another example to practice it; an auditorium has the following DC loads which are rated at 12 V;

  1. Three 30 W lamps
  2. One 20 W fan

All the above-mentioned loads are powered by two parallel connect PV modules, each PV module has a maximum power point current IMP of 3 A and short-circuit current ISC of 5 A. What will be the nominal system voltage, nominal PV array current, and Nominal load current of the charge controller?

Total DC load = (No. of lamps × Wattage of each lamp) + (No. of fans × Wattage of each fan)

Total DC load = (3 × 30) + (1 × 20) = 90 + 20 = 110 W

The nominal system voltage of the charge controller is the same as the rated voltage of the load and the PV array (Nominal system voltage of the charge controller = 12 V)

Nominal PV array current = 2 × 5 (short-circuit current of each PV module is 5 A and are connected in parallel)

Nominal PV array current = 10 A

A safety factor of 1.25 is considered for variation in determined short-circuit current under non-STC (Standard Test Condition).

Considering the safety factor of 1.25 the Nominal PV array current is 1.25 × 10A = 12.5 A

Nominal load current = Total DC load / Nominal system voltage = 110W / 12V

Nominal load current = 9.1666 A

Thus, the charge controller should be able to handle a current of about 9.1666 A at the output side.

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Maximum Power Point Tracking (MPPT):

The load connected to a PV module determines the power delivered by the module, take a look at the I-V and the P-V curve shown in figure 3 below.

Characteristics of Maximum Power Point Tracking (MPPT)
Fig 3: Characteristics of Maximum Power Point Tracking (MPPT)

It can be observed from the above figure that at the short circuit condition i.e. at V = 0, maximum current is delivered by the module known as short circuit current ISC­. But if we gradually increase the voltage across the load by varying the load the power delivered to the load also increases.

So, the increase in the voltage causes the power to increase up to a certain point, the point beyond which the increase in voltage further causes a decrease in the power is called as Maximum Power Point (MPP). So, the I-V curve of a PV module has a point that corresponds to the maximum power known as Maximum PowerPoint or in short MPP.

It is required that the load connected to the PV module should be operated at a voltage and current which corresponds to this maximum power point to obtain the maximum power from the PV modules. The operating point is the point of intersection of I-V characteristics of PV modules to a load.

Manufacturers rated their PV modules for peak power output. But the output power of PV modules not only depends on the available solar irradiance but also on the combination of voltage and current. For example during mid-day when the sun is high the module will not deliver the power if it is in the open circuit or short circuit condition.

So, there is an operating point on the I-V curve where the product of voltage and current will deliver the maximum power. But this maximum operating point changes with the change in the intensity of the radiation falling on the solar PV modules. Thus, to obtain that maximum power there are electronic devices that will ensure that the PV modules will operate at maximum power at all the level of irradiance throughout the day. This idea of operating the PV module at its maximum power is called Maximum Power Point Tracking (MPPT).

Practically there are changes in the I-V curve of the PV module due to the change in the intensity of the radiation falling on the module. Thus, it is not possible to keep the PV operated at the MPP for a chosen load. The solar radiation is less at around 9 a.m. and it gradually increases till noon. This increase in the intensity of the radiation will cause the I-V curve of the module to change as shown in figure 4 below.

I-V Curve of MPPT Solar Charge Controller
Fig 4: I-V Curve of MPPT Solar Charge Controller

This results in the change of the operating point for a given load. The operating points for 1 p.m., 11 a.m., and 9 a.m. are denoted by A, B, and C respectively. But the maximum operating points for 1 p.m., 11 a.m., and 9 a.m. are denoted by A’, B’, and C’ respectively.

Thus, if we need to obtain maximum power from the PV module the operating points A, B, and C should be brought closer to A’, B’, and C’ respectively and this is done by an MPPT device. The MPPT device does the job of getting the operating point closer to the maximum power point at a different level of solar radiation.

It helps in extracting the maximum power available from the PV module under any irradiance and temperature. It makes the use of an MPPT algorithm and an electronic circuit to get the job done. The idea is based on the principle of matching the impedance between the PV module and the connected load which is essential to transfer the maximum power.

Thus, when the impedance of the PV source and the load matches maximum power is transferred from the PV source to the load. If the ratio of module voltage at maximum power to module current at maximum power matches the impedance of the connected load, maximum power transfer takes place.

But practically it is not possible to have a matching of this ratio to the impedance of the load, hence the MPPT device does that operation of impedance matching to deliver the maximum power at available irradiance and temperature. Manufacturers combine the functions of the charge controller and MPPT into one device which is widely known as the MPPT charge controller. Both MPPT and the charge controller are two different and independent functions but are widely used as one device to serve two purposes.

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Sun-tracking and MPPT for Maximizing the Power Output:

The sun-tracking is not the same as the MPPT tracking, it is a mechanical tracking of a solar PV module in such a way that the sun ray’s incident on the modules is always perpendicular. The module should mechanically face the sun to obtain maximum power during that time of the day.

If the modules are not perpendicular to the sun rays falling on it most of the sunlight would be reflected from the modules. The solar module produces maximum output power for given sunlight when the angle of the light and the module are perpendicular to each other (i.e. 90o) as shown in figure 5.

When the angle of the incident of light is less than or greater than 90o as shown in figure 5 then it will produce output power lower than the maximum output power capability of the module. When the light falls on an angle greater or lesser than 90o some part of the light is reflected, and the light utilized by the module is less than the actual falling on it.

This results in a reduction of the output power generated by the module. It is due to this reason that we must have mechanical sun-tracking to generate maximum possible electricity.

Sun-tracking and MPPT for Maximizing the Power Output
Fig 5: Sun-tracking and MPPT for Maximizing the Power Output

Specifications of the MPPT Charge Controller:

PV Input

Maximum input power: This represents the maximum power that the MPPT charge controller can handle from the connected PV array.

Maximum open circuit voltage: This represents the maximum open-circuit voltage that the MPPT charge controller can handle.

MPPT tracking voltage range: This represents the voltage level range that the MPPT charge controller can handle.

DC Output to the Battery

Nominal battery voltage: This represents the voltage at which the battery operates in a connected system.

Voltage regulation setpoint (VR): It is the maximum level of voltage up to which we can charge a battery without causing the overcharge. Once this level has reached the charge controller will either disconnect the battery from the PV source or will regulate the current delivered to the connected battery.

Low voltage disconnect (LVD): It represents the minimum voltage up to which the discharge of the battery is allowed without causing the deep discharge. Also known as Depth of discharge (DOD). When the battery level reaches the DOD level the MPPT charge controller disconnects to avoid overcharging.

Maximum charging current: It represents the maximum current that an MPPT charge controller can handle from the PV array. It is a PV array short circuit current. While designing a safety factor of 1.25 is used due to variation at non-STC operations.

DC Load Control

Nominal voltage: This represents the charge controller’s maximum load voltage which it should be able to handle.

Maximum current: This represents the charge controller’s maximum load current which it should be able to handle.

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How to Select the Right Size MPPT Charge Controller?

Let us take some examples to understand the above-mentioned specifications numerically.

Example 3:

Consider a 500-watt PV array that operates at 24 V DC and has a battery bank of 12 V DC. Determine an MPPT charge controller rating s for this given system.

  • Solar PV array wattage = 500 W
  • Solar PV array operating voltage = 24 V
  • Battery pack operating voltage = 12 V

The input power to the MPPT controller is 500 W, the solar PV array is connected at the input side of the MPPT charge controller and the battery is connected at the output side of the MPPT charge controller. Thus, the battery acts as a load to the system. The data specifies the output voltage. Assuming 100 % efficiency we can determine the output current for its load.

Power = Voltage × Current

Current = Power / Voltage = 500W / 12V = 41.66 A

Thus, we would require a 12 V, 41.66 A MPPT for the above system, we can increase the current value by 25 % considering some conditions that occur causing the panel to produce more power. Thus, we can take it as 52 A. So, 12 V, 52 A MPPT charge controller would be suitable for the above system. Note that the MPPT charge controller should be able to handle the open-circuit voltage and the voltage at the maximum power point of the connected PV array.

Let us take another example where we have to design a 140 WP Solar home system with a PV module of 70 W having an open-circuit voltage of 20 V and voltage at maximum power point of 16 V. The voltage of the battery bank is at 12 V. Determine a suitable MPPT charge controller rating for this home solar design.

Let us connect the available PV module in series.

Thus, the open-circuit voltage of the system would become = 2 × 20 = 40 V

Voltage at maximum power point would be = 2 × 16V = 32 V

Peak power of the system would be = 2 × 70W = 140 W

The input power to the MPPT charge controller is 140 W if we assume 100 % efficiency. And the available battery voltage at 12 V, then the current to the battery pack can be determined as follows;

Power = Voltage × Current

Current = Power / Voltage = 140W / 12V = 11.66 A

Thus, we would require a 12 V, 11.66 A MPPT for the above system, we can increase the current value by 25 % considering some conditions that occur causing the panel to produce more power. Thus, we can take it as 15 A. So, 12 V, 15 A MPPT charge controller would be suitable for the above system.

Again, it is important to note that the MPPT charge controller should be able to handle the open-circuit voltage and the voltage at the maximum power point of the connected PV array.

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