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PWM vs MPPT Solar Charge Controller

Batteries are frequently charged with solar energy using both PWM and MPPT charge controllers.A solar array is basically connected to the battery by the PWM controller, which functions as a...

Batteries are frequently charged with solar energy using both PWM and MPPT charge controllers.A solar array is basically connected to the battery by the PWM controller, which functions as a switch. As a result, the array's voltage will be lowered to a level that is comparable to the battery's voltage. The more expensive (and complex) MPPT controller will change its input voltage to extract the most power possible from the solar panel array before transforming that power to meet the fluctuating voltage needs of the battery and load. Consequently, it basically separates the array and battery voltages, allowing, for instance, a 12V battery on one side of the MPPT charge controller and panels wired in series to produce 36V on the other.This article will describe in detail the differences and usage of solar controllers.

The current-voltage curve and the power-voltage curve of a solar panel

The examples throughout the following pages are based on an average 100 W / 36 cell monocrystalline solar panel, with the following specifications:
100 W panel 36 cell:

Pm 100 W Temp. coeff. of Pm  γ -0.45 %/°C
Vm 18 V  Temp. coeff. Of Vm ε -0.47 %/°C
Im 5.56 A Temp. coeff. Of Im  δ 0.02 %/°C
Voc 21.6 V Temp. coeff. Of Voc β  -0.35 %/°C
Isc  6.12 A Temp. coeff. Of Isc α  0.05 %/°C

Table 1: Specifications of the solar panel as used in the examples below

The current-voltage curve of this panel is shown in figure 1

Solar Charge Controller

Fig 1: Current-voltage curve of a 100W / 36 cell solar panel Standard Test Conditions (STC): cell temperature: 25°C, irradiance: 1000 W/m², AM: 1.5

From this basic curve the power-voltage curve can be derived by plotting P = V x I against V.The result is the blue curve in figure 2 below.

Solar Charge Controllers

Fig 2: Current-voltage curve (brown) and power-voltage curve (blue, P = V x I)

Fig. 3: The area of the blue rectangle is proportional to the product Pm = Vm x Im

Obviously, the power obtained from the panel is zero when it is short circuited (0 x Isc = 0) or when no current is drawn from the panel (Voc x 0 = 0). In between those two zero power points the product P = V x I reaches a maximum: the Maximum Power Point (Pm = Vm x Im).

The importance of the Maximum Power Point can be visualized as follows: The product Vm x Im is proportional to the area of the rectangle shown in figure 3. Pm is reached when the area of this rectangle is at its largest. Figure 4 and 5 show two less optimal results obtained when power is harvested at a voltage which is too low or too high.

Solar Charge Controllers

Fig 4: Less power harvested: voltage is too low

Fig 5: Less power harvested: voltage is too high

The maximum output of a 100 W solar panel is, by definition, 100 W at STC (cell temperature: 25°C, irradiance: 1000 W/m², AM: 1.5). As can be seen from figure 3, in the case of a 100 W / 36 cell crystalline panel the voltage corresponding to the Maximum Power Point is Vm = 18 V and the current is Im = 5.56 A. Therefore 18 V x 5.56 A = 100 W.

Conclusion: In order to get the maximum out of a solar panel, a charge controller should be able to choose the optimum current-voltage point on the current-voltage curve: the Maximum Power Point. An MPPT controller does exactly that. The input voltage of a PWM controller is, in principle, equal to the voltage of the battery connected to its output (plus voltage losses in the cabling and controller). The solar panel, therefore, is not used at its Maximum Power Point, in most cases.

What Is an MPPT Charge Controller?

MPPT stands for Maximum Power Point Tracking. MPPT charge controllers use advanced algorithms to track the maximum power point of the solar panel array. This means they adjust the load and voltage to ensure the solar panels operate at their optimal efficiency, maximizing the energy harvested from the sunlight. MPPT controllers are known for their ability to extract more energy from the solar panels, especially in situations where the panel voltage varies due to factors like shading or temperature changes.

As shown in figure 6, the voltage Vm corresponding to the Maximum Power Point can be found by drawing a vertical line through the top of the power-voltage curve, and the current Im can be found by drawing a horizontal line through the intersection of the Vm line and the current-voltage curve. These values should be equal to the values specified in table 1. In this example Pm = 100 W, Vm = 18 V and Im = 5.56 A. With its microprocessor and sophisticated software, the MPPT controller will detect the Maximum Power Point Pm and, in our example, set the output voltage of the solar panel at Vm = 18 V and draw Im = 5.56 A from the panel. What happens next? The MPPT charge controller is a DC to DC transformer that can transform power from a higher voltage to power at a lower voltage. The amount of power does not change (except for a small loss in the transformation process). Therefore, if the output voltage is lower than the input voltage, the output current will be higher than the input current, so that the product P = V x I remains constant. When charging a battery at Vbat = 13 V, the output current will therefore be Ibat = 100 W / 13 V = 7.7 A. (Similarly, an AC transformer may supply a load of 4.4 A at 23 VAC (4.4 x 23 = 100 W) and therefore draw 0.44 A from the 230 V mains (230 x 0.44 = 100 W)).

 Solar Charge Controllers

Fig 6: MPPT controller, graphical representation of the DC to DC transformation Pm = Vm x Im = 18 V x 5.6 A = 100 W, and Pbat = Vbat x Ibat = 13 V x 7.7 A = 100 W

What Is a PWM Charge Controller?

PWM, or Pulse Width Modulation, charge controllers regulate the flow of energy from the solar panels to the batteries by periodically connecting and disconnecting them. As the battery voltage rises, the PWM controller adjusts the width of the pulses to reduce the charging current, preventing overcharging. While PWM controllers are simpler and more cost-effective, they are better suited for smaller systems and applications where efficiency is not a critical factor.

 Solar Charge Controllers

Fig 7: PWM charge controller
In this case the charge voltage imposed on the solar panel can be found by drawing a vertical line at the voltage point equal to Vbat plus 0.5 V. The additional 0.5 V represents the voltage loss in the cabling and controller. The intersection of this line with the current-voltage curve gives the current Ipwm = Ibat.

A PWM controller is not a DC to DC transformer. The PWM controller is a switch which connects the solar panel to the battery. When the switch is closed, the panel and the battery will be at nearly the same voltage. Assuming a discharged battery the initial charge voltage will be around 13 V, and assuming a voltage loss of 0.5 V over the cabling plus controller, the panel will be at Vpwm = 13.5 V. The voltage will slowly increase with increasing state of charge of the battery. When absorption voltage is reached the PWM controller will start to disconnect and reconnect the panel to prevent overcharge (hence the name: Pulse Width Modulated controller).

Figure 7 shows that in our example, with Vbat = 13 V and Vpwm = Vbat +0.5 V = 13.5 V, the power harvested from the panel is Vpwm x Ipwm = 13.5 V x 6 A = 81 W, which is 19% less than the 100 W harvested with the MPPT controller. Clearly, at 25°C a MPPT controller is preferable to a PWM controller. 

What’s the Difference?

Efficiency: MPPT controllers are generally more efficient in converting solar energy into usable electricity. They can deliver up to 30% more energy compared to PWM controllers, especially when the weather conditions are less than ideal.

Panel Configuration: MPPT controllers are highly adaptable to various solar panel configurations, including series and parallel setups. This flexibility makes them suitable for larger installations.

Voltage Handling: MPPT controllers can handle higher voltage panels and are better equipped to deal with variations in panel voltage, such as those caused by shading or temperature differences.

Cost: MPPT controllers are more complex and tend to be pricier than PWM controllers. However, the higher efficiency and increased energy production can often justify the additional cost.

System Size: If you have a smaller solar system with a limited budget, a PWM controller may be sufficient. For larger systems or situations where every bit of energy matters, an MPPT controller is a better choice.

PWM vs MPPT: How to Choose

Choosing between PWM (Pulse Width Modulation) and MPPT (Maximum Power Point Tracking) charge controllers depends on various factors, including your solar system setup, energy requirements, and budget. To make an informed decision, let's delve into the differences between PWM and MPPT charge controllers and explore how to choose the right one for your needs.

System Size and Voltage:

   - PWM: PWM charge controllers are suitable for smaller solar systems with lower voltage panels. They are commonly used in 12V systems and are cost-effective options for basic setups.

   - MPPT: MPPT charge controllers are more versatile and can handle higher voltage panels. They are recommended for larger systems, especially those with multiple panels or panels wired in series to create higher voltage arrays.

Efficiency:

   - PWM: PWM charge controllers are less efficient compared to MPPT controllers. They regulate the charging process by intermittently interrupting the flow of energy from the panels to the batteries, which can result in some energy loss.

   - MPPT: MPPT charge controllers are highly efficient as they dynamically adjust the voltage and current to maximize power output. They can capture more energy from the solar panels, especially in low-light or fluctuating conditions.

Cost:

   - PWM: PWM charge controllers are generally more affordable upfront, making them a good choice for budget-conscious users or smaller systems.

   - MPPT: MPPT charge controllers are more expensive due to their advanced technology. However, their increased efficiency can lead to higher energy savings over time, which might offset the initial cost.

Environmental Conditions:

   - PWM: PWM charge controllers are better suited for situations where the solar panels are consistently exposed to optimal sunlight conditions and temperature variations are minimal.

   - MPPT: MPPT charge controllers shine in environments with varying sunlight intensity, shading, or temperature fluctuations. They adapt well to changing conditions and can maintain high efficiency.

Long-Term Savings:

   - PWM: While PWM controllers are cheaper upfront, their lower efficiency may result in slightly higher long-term costs due to energy losses.

   - MPPT: Despite the higher initial cost, MPPT controllers can provide greater long-term savings through improved energy capture and conversion, leading to reduced reliance on grid power.

 Note:  If you have a smaller budget and a simpler solar setup, a PWM charge controller might suffice. However, for larger systems, varying conditions, and a desire for optimal efficiency, an MPPT charge controller is a more suitable choice. It's important to assess your energy needs, system size, and environmental factors to determine which type of charge controller aligns best with your goals.

In conclusion, while both PWM and MPPT charge controllers have their merits, the choice ultimately depends on the specific needs of your solar power system. If you're aiming for higher efficiency and energy production, especially in larger systems, an MPPT controller is a wise investment. On the other hand, if you're working with a smaller system and looking for cost-effectiveness, a PWM controller can serve your needs. Bateriapower.com offers a range of high-quality charge controllers, including both PWM and MPPT options, to suit various solar setups and help you make the most of your solar energy. If you are still looking to compare it practically, you are most welcome to watch the range of PWM and MPPT Batteries of Bateria Power.

 

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