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What is Maximum Power Point Tracking and How Does it Work?

 What is MPPT?

mppt controller

 mppt

 Maximum Power Point Tracking, frequently referred to as MPPT, is an electronic system that operates the Photovoltaic (PV) modules in a manner that allows the modules to produce all the power they are capable of.

 MPPT is not a mechanical tracking system that “physically moves”the modules to make them point more directly at the sun.

 MPPT is a fully electronic system that varies the electrical operating point of the modules so that the modules are able to deliver maximum available power. Additional power harvested from the modules is then made available as increased battery charge current. MPPT can be used in conjunction with a mechanical tracking system, but the two systems are completely different.

 

 To understand how MPPT works, let’s first consider the operation of a conventional PWM or Relay Type (non-MPPT) charge controller. When a conventional controller is charging a discharged battery, it simply connects the modules directly to the battery. This forces the modules to operate at battery voltage, typically not the ideal operating voltage at which the modules are able to produce their maximum available power. The PV Module Power/Voltage/Current Graph (at right) shows the traditional Current/Voltage curve for a typical 75W module at standard test conditions of 25C cell temperature and 1000W/m2 of insolation. This graph also shows PV module power delivered vs module voltage. For the example shown, the conventional controller simply connects the module to the battery and forces the module to operate at 12V. By forcing the 75W module to operate at 12V the conventional controller artificially limits power production to 53W.  what is mppt


  Rather than simply connecting the module to the battery, an MPPT charge controller calculates the voltage at which the module is able to produce the most power and connects to the panel at that voltage. In this example the maximum power voltage of the module (Vmp) is 17V. The MPPT system then operates the modules at 17V to extract the full 75W, regardless of present battery voltage. A high efficiency DC-to-DC power converter converts the 17V module voltage at the controller input to battery voltage at the output. If the whole system was 100% efficient, battery charge current in this example would be VMODULE  VBATTERY x IMODULE, or 17V 12V x 4.45A = 6.30A. A charge current increase of 1.85A or 42% would be achieved by harvesting module power that would have been left behind by a conventional controller and turning it into useable charge current. But, nothing is 100% efficient and actual charge current increase will be some what lower as some power is lost in wiring, fuses, circuit breakers, and in the MPPT controller.

mppt charge controller



 

Actual charge current increase varies with operating conditions. As shown above,the greater the difference between PV module maximum power voltage Vmp and battery voltage, the greater the charge current increase will be. Cooler PV module cell temperatures tend to produce higher Vmp and therefore greater charge current increase.This is because Vmp and available power increase as module cell temperature decreases as shown in the PV Module Temperature Performance Graph (at right). Modules with a 25C Vmp rating higher than 17V will also tend to produce more charge current increase because the difference between actual Vmp and battery voltage will be greater. A highly discharged battery will also increase charge current since battery voltage is lower,and output to the battery during MPPT could be thought of as being “constant power”.

What most people see in cool comfortable temperatures with typical battery conditions is a charge current increase of between 10 – 25%. Cooler temperatures and highly discharged batteries can produce increases in excess of 30%. Customers in cold climates have reported charge current increases in excess of 40%. What this means is that current increase tends to be greatest when it is needed most(during the winter); in cooler conditions when days are short, sun is low on the horizon, and batteries may be more highly discharged. In conditions where extra power is not available (highly charged battery and hot PV modules) most MPPT charge controllers will perform as a conventional PWM type controller.

 what is mppt


 


Another way to think of an MPPT charge controller is to think of it like the transmission of a car. If the transmission of your car is in the wrong gear the wheels do not receive the maximum power available at the best efficiency possible. This is because the engine is either operating at a higher or lower rpm than its ideal. The transmission is there to transfer the power to the wheels in a way that allows the engine to operate at the most efficient speed whether driving on the highway at 110 km per hour or a bumpy country road at 20 km per hour. Without the transmission you would be very limited to what speed you could drive your car.

For the analogy let's make the solar module the car engine. The solar module, like the car engine, likes to operate at a certain voltage (or speed) to make the most power or operate at the best efficiency. This is called the Vmp (maximum power point voltage) or the Vpp (the peak power voltage). The Vmp of a solar module changes with changes in temperature and sunlight intensity. The brighter the sun the higher the Vmp. The colder the module, the higher the Vmp.

The battery voltage is similar to the speed of the car's wheels. It varies according to battery type, battery temperature, battery state of charge and any loads that are applied to the battery. Large loads will temporarily decrease the battery voltage. For a 12 volt nominal system, the battery voltage could vary between 11.0 volts and 16.0 volts.

To charge a battery, we need to apply a voltage that is higher than the battery voltage. If the solar module has a Vmp lower than the battery voltage the current will not flow. This is similar to your engine operating slower than the wheels of the car. That is why solar modules must have a Vmp significantly higher than the battery voltage. If it was not, the module would not charge the battery when the module is warm or the battery is almost full. To make sure the module will charge on a hot day, manufacturers make the Vmp of the voltage somewhere around 17-18 volts. On a hot day this may go down to 15 volts, and on a cold day, as high as 19 volts .

If the Vmp is higher than the voltage of a battery (without an MPPT charge controller), the module voltage is dropped down to the battery voltage which is generally less than ideal. PWM and relay type charge controllers connect the solar module directly to the battery giving you no additional power. MPPT charge controllers connect to the solar module at the Vmp (maximum power point voltage) and then reduce the output voltage to get all the power available into the battery. Most MPPT charge controllers have to adjust every few seconds to minutes as the sun intensity, module temperature and battery voltage changes.

For a list of MPPT charge controllers please click here...

If you still have questions like

"What is an MPPT charge controller?"

"Do I need an MPPT charge controller?"

Please do not hesitate to email us or call us at 1-888-485-2249.

Outback Flexmax 80 MPPT Charge Controller

  

 
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