SM MPPT Charge Controller Design Tool

Matching Power Values for an Array and an MPPT Charge Controller:

All MPPT Charge Controller are rated by their maximum continuous power output, which is measured in watts or kilowatts. This value is the DC power output.
MPPT Charge Controller actually limit their power output, so you can use this value to figure out the maximum power input coming into the MPPT Charge Controller from the PV array.

When you look at PV module ratings, you find that the power output ratings are based on standard test conditions (STC) where the cell temperature is 25 degree Celsius and the intensity of the sun is equal to 1,000 W/m2.
PV modules rarely operate in these conditions due to the motion of the sun across the sky and increased PV operating temperatures.

• The power input coming into the MPPT Charge Controller is typically considerably less than the rated values.
• Both the MPPT Charge Controller and the array (along with all the other equipment in an Off-Grid system) have efficiency losses that must be taken into account.

You can quickly estimate all of these system inefficiencies by adopting the industry standard that any PV system will operate at approximately 80 percent of overall system efficiency.

The MPPT Charge Controller DC voltage window

MPPT Charge Controller’s voltage window consists of a maximum input voltage that you must stay under if you want to avoid damaging the MPPT Charge Controller and a minimum voltage you must stay above to keep the MPPT Charge Controller operating at the array’s maximum Power Point. As a PV system designer, you need to identify these values from a specsheet and make sure the array can operate within this window throughout the year. If you allow the array to move outside this window, you risk damaging the MPPT Charge Controller or shutting down the system charging for the day.

Calculating the modules’ maximum DC voltage contribution:

As the temperature decreases, a PV module’s voltage actually increases (voltage and temperature therefore have an inverse relationship).

To make sure an array’s voltage never exceeds the MPPT Charge Controller’s input value, you need to adjust the modules’ STC open circuit voltage value (Voc) for cold temperatures; this value is reported on spec sheets.

Applying the adjusted open circuit voltage to come up with the right number of modules in a string

Figuring out the modules’ minimum DC voltage contribution:

On the other end of the voltage window is the minimum power point tracking voltage. If the voltage from the array ever drops below this value — usually due to high heat conditions that occur on the sunniest days (as in the days system owners expect their arrays to produce the maximum amount of energy possible) — the MPPT Charge Controller won’t be able to continue operating and will shut down. Although this scenario won’t damage the MPPT Charge Controller, it may damage your reputation. If your client sees his Charge Controller shut down on a bright, sunny day because the voltage isn’t high enough.

Picking the temperature to use in your calculation:

The high temperature Record: By designing with the record high temperature in mind, you ensure that the PV array will operate in all conditions.

Accounting for mounting in your calculation:

To estimate a PV module’s temperature at the hottest time of the year, you should add one of the following temperatures to the ambient temperature:

• 35 degrees Celsius for arrays with fewer than 15 cm between the backs of the modules and the mounting surface
• 30 degrees Celsius for arrays with more than 15 cm between the backs of the array and the mounting surface
• 25 degrees Celsius for arrays that are mounted on top of a pole

By adding these values to the ambient temperature, you can arrive at a best estimate for your array’s operating temperature — the temperature you expect to see at the module level if you were to measure the temperature in the middle of the summer.

Getting you Ready to Match an MPPT Charge Controller to an Array

1. Enter your project name, & you can add your customer details to save your project to view later.
2. Choose a MPPT Charge Controller using one of these two ways:
   • Choose from a list of different existing brands & Models
   • Add your own Brand & Model of the MPPT Charge Controller.
3. Choose a PV Module using one of these two ways:
   • Choose from a list of different existing brands & Models
   • Add your own Brand & Model of PV Modules.
4. Enter your inputs:
   • Number of Modules Per string & Number of strings/MPPT.
   • Panel Max. Temp (C).
   • Panel Min. Temp. (C).
5. A result sheet will appear with the following data:
   • PV Generator Nominal Output Per MPPT (Wp).
   • Minimum MPPT Voltage at Maximum Temperature.
   • Maximum MPPT Voltage at Minimum Temperature.
   • Maximum open circuit Voltage of PV generator at Minimum Temperature.
   • Maximum short circuit Current of PV generator at Maximum Temperature.
   • Each result of above is compared with MPPT Charge Controller parameters, to show if your selection gives you the right values, it gives you GOOD, else it gives a Warning.
     - The right values are: :
     • PV Generator Nominal input to the MPPT charge controller (Wp) <= Maximum input of the charge controller.
     • Minimum MPPT Voltage at Maximum Temperature >= Minimum MPPT Voltage of MPPT Charge Controller.
     • Maximum MPPT Voltage at Minimum Temperature <= Maximum MPPT voltage of MPPT Charge Controller.
     • Maximum open circuit Voltage of PV generator at Minimum Temperature <= Maximum input voltage, open circuit of MPPT Charge Controller.
     • Maximum short circuit Current of PV generator at Maximum Temperature <= Maximum short circuit current of MPPT Charge Controller, if available.