Yash
PWM and MPPT are solar charge controllers used in the solar power system with battery backup.
Although it is a small component, it plays a crucial role in the proper functioning of the system.
Its right configuration and type help in producing optimum output and the smooth functioning of the system.
Therefore, it is very important to choose the right type of solar charge controller for your home.
Before doing so, let’s understand its function and work.
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What is a solar charge controller?
A solar charge controller is required in almost all solar systems which require battery backup.
The solar charge controller is an electronic device when placed between the solar panels and the battery regulates the voltage coming from the solar panels to the battery.
The function of a solar charge controller
a) It regulates the battery voltage and prevents the battery from overcharging
The solar charge controller checks the voltage of the battery.
When it sees that it is low, it allows a full flow of current from the panels to enter into the battery.
When the battery voltage reaches a certain level, it reduces the flow of energy to prevent overcharging of the battery.
opens the circuit and the charging of the battery from the panels gets stopped.
When the voltage of the battery drops due to less sunlight or an increase in electrical consumption, the charge controller again allows a full flow of energy to enter into the battery.
In this way, it regulates the charging of the battery.
b) It blocks the reverse current from the battery to the solar panels
During the night, the solar panels do not produce current and hence are at lower potential with respect to the battery.
We all know that current flows from higher potential to the lower (Battery to the solar panel in this case).
There is a chance that current will leak from the battery to the panels and will drain it.
But the diode-connected inside the solar charge controller allows current to flow in one direction that is from solar panels to the batteries.
It restricts the backflow of the current.
Hence, it stops the reverse flow of the current and prevents the draining of the batteries.
Types of solar charge controllers
The solar charge controllers which are commonly used in solar power systems are:
- PWM (Pulse Width Modulation) solar charge controller
- MPPT (Maximum Power Point Tracking) solar charge controller
Both of these charge controllers are based on sophisticated technologies and they adjust their charging rate based on the charging level of the battery.
Working of PWM solar charge controller
The PWM solar charge controller works by adjusting the charge from the solar panels according to the battery’s condition and its recharging requirements.
The charging rate is higher and pulses are wider when the battery is nearly discharged and the charging rate and the width of the pulses keep on decreasing as the battery reaches its maximum charging level.
It can keep the battery fully charged state for an indefinite period of time.
This helps in avoiding heating and gassing of the battery and results in high charging efficiency with healthy battery life.
When sizing the system the voltage specification of the PWM charge controller must match the voltage of the battery.
Whereas the normal solar charge controller also called shunt controller, stops charging the battery when it is fully charged and re-charges again when the battery drops to a certain level.
Working of MPPT solar charge controller
The MPPT charge controller captures the voltage difference between the panel voltage and the battery voltage.
It detects the voltage difference and converts the excess voltage from the solar panel into the equivalent amount of the amperage.
In that way, the same amount of power reaches the battery which is being generated by the solar panel.
Understand the difference between the PWM and MPPT solar charge controller
Assuming, I have a 140-watt solar panel that has the following specifications under STC (Standard Test Conditions):
- Vmp: Maximum voltage of 17 volts under STC
- Imp: Maximum current of 8.24 ampere under STC
Again assuming, I use this panel to charge a 12volts/100 amp-hr battery.
*(A 12 volts battery generally requires around 14 volts from the panel to charge it).
I need to connect the solar charge controller between the solar panel and the battery.
First I will use PWM and then MPPT.
Thereafter, we will learn the difference between the two.
Using PWM solar charge controller
The PWM solar charge controller brings down the solar panel voltage from 17 volts to 14 volts.
Thereafter, it will start charging the battery.
In this process, we lost 17 volts – 14 volts = 3 volts.
The PWM solar charge controller cannot utilize this voltage difference.
Finally, it allows 14 volts and 8.24 ampere to charge the battery.
I can find the power going inside the battery using the above two values.
Power = voltage x current
= 14 volts x 8.24 amperes
= 115 watts
These 115 watts are going inside the battery.
The solar panel is producing 140 watts but the battery is getting 115 watts.
A loss of 25 watts in this process.
The efficiency of the system is reduced by:
{(140 – 115)/140} x 100 %
= 18 %
And
the charging time is increased to:
= (12 volts*100 ampere-hr)/ (115 watts)
= (1200 watt-hr)/ (115 watts)
= 10.4 hours to charge the battery (assuming the output power from the panel is constant).
Although, the PWM solar charge controller protects the battery from overcharging.
But it cannot utilize the voltage difference.
Battery charging using MPPT solar charge controller
The MPPT solar charge controller detects the voltage difference and utilizes it.
It doesn’t let this voltage difference gets wasted.
It provides maximum power to the battery and reduces its charging time.
When I use the MPPT solar charge controller, it converts the voltage difference into equivalent amperage.
Using the following mathematical equation:
Power Input = Power Output
17 volts x 8.24 amperes = 14 volts x 10 amperes
*(Ignoring losses like conversion loss and heat loss).
First, the MPPT solar charge controller brings down the voltage from 17 volts to 14 volts.
Second, it increases the current from 8.24 amperes to 10 amperes.
{Output current = (17 volts/14 volts) x 8.24 amp}
So that the power remains the same.
Hence, the battery gets the same power (140 watts) despite the difference in the charging voltage and the voltage produced by the panels under STC.
The charging time of the batteries, in this case, will be:
= (1200 watts-hr)/140 watts
= 8.6 hours (Assuming the power from the solar panel is constant).
Hence the charging time is reduced when compared with the charging time using the PWM charge controller.
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The MPPT solar charge controller detects the variation
Hence, the battery gets the same power (140 watts) despite the difference in the charging voltage and the voltage produced by the panels under STC.
The charging time of the batteries in this case will be:
= (1200 watts-hr)/140 watts
= 8.6 hours (Assuming the power from the solar panel is constant).
Hence the charging time is reduced when compared with the charging time using the PWM charge controller.
The MPPT solar charge controller detects the power variation
The voltage and the current from the panel are the function of the temperature and the amount of solar radiation falling on the surface.
As both of these parameters change continuously which causes the output power from the panel to vary throughout the day; the MPPT charge controller detects the maximum power produced by the solar panel and transmits the same to the battery for charging.
Even though the MPPT charge controller is more expensive than the PWM charge controller but paying a high price is worth it in a view of the increased efficiency of the system.
Which is the best? PWM or MPPT
At first, it looks that MPPT solar charge controller has a cutting edge over PWM solar charge controller.
However, each technology has its own advantages which are based on multiple factors such as:
- Site location
- Temperature
- Size of the array with respect to the load
- System size
- Cost of system.
Let us understand their efficiency and the advantages under a different set of conditions:
a) Location
The angle at which sun rays striking at the solar panels depends on the location and the season.
The less amount of energy is absorbed by the solar panel when the sun rays come at the shallow angle and hence resulted in less output power from the solar panel.
Here the MPPT technology is suitable for harvesting the maximum amount of solar energy.
b) Temperature
As the temperature drops the Vmp of the solar module increases causing more differential between the battery voltage and the panel voltage.
As MPPT charge controller has the ability to track or detect the excess voltage and covert the difference into the equivalent amperage.
Therefore in lower temperature conditions, the MPPT technology has better harvesting power than that of PWM technology.
c) Array size with respect to the load
In a situation where the size of the panels is large enough relative to the power drawn by the load, the battery will remain in the full charge state almost all the time.
Here the PWM charge controller is suitable and economical as compared to the MPPT charge controller.
d) When array size is small
A small power solar system is better suited to the PWM controller because it operates at the constant harvesting efficiency irrespective of the size of the solar system.
Whereas, the harvesting efficiency of the MPPT charge controller is low with a small solar power system. So, in low-power applications, a PWM charge controller is a better and more economical option.
e) Cost
PWM charge controllers are less expensive than MPPT charge controllers.
So PWM charge controllers have a cost advantage over their counterparts.
So, you can decide which type of solar charge controller is suitable for your solar system by analyzing the above-mentioned parameters.
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Conclusion
Finally, we saw that the right type of solar charge controller (PWM or MPPT) depends on the location, temperature changes, size of the system, and budget.
An MPPT charge controller may be doing well in one location but is not effective in another.
Similarly, the PWM charge controller may not suit well in every size of the system.
It is you who can decide the best one for your home after considering all the above factors.
