MPPT charge controller sizing and selection

A MPPT charge controller is rated to operate at a particular system voltage, maximum current and maximum voltage. MPPT charge controllers can charge the battery bank with any series and parallel configuration of modules that doesn't exceed the maximum voltage and maximum current, or drop below the required charging voltage of the energy storage system. Exceeding the voltage rating of an MPPT due to cold temperatures can damage it. Many charge controllers allow the current rating to be exceeded to a certain point without damage, just lost energy - it depends on the charge controller. There are several important calculations that must be performed to properly size an MPPT charge controller:

• Should be sized to work with a series and parallel configuration of PV modules that will not damage the charge controller due to high voltages resulting from low temperatures at the project location.
• Should be sized to work with a series and parallel configuration of PV modules that will still be able to properly charge the energy storage system under high temperatures and as PV modules age at the project location.

Step 1: Determine PV module power rating

60-cell and 72-cell modules are the most common module size used with MPPT charge controllers. They range in size from 250W - 400W+.

Step 2: Calculate minimum number of PV modules

This calculation will give a minimum number of modules. The final array size should always be larger than this value, thus if the the result of the calculation is a decimal, it should be rounded up. Different modules sizes and configurations can be explored to find the optimal design.

Minimum number of PV modules	= minimum PV source size ÷ PV module power rating (from step 1)

Step 3: Determine PV source power rating

This calculation will give a power rating of the PV source based upon the chosen module size and the number of modules required.

PV source power rating	= Minimum number of PV modules (from step 2) × PV module power rating (from step 1)

Step 4: Determine minimum current rating of charge controller

An MPPT charge controller is capable of of accepting varying voltages from the array and converting them into current at the proper charging voltage for the energy storage system. The maximum current charge controller could generate from the PV source can be calculated by dividing the power rating of the array by the system voltage. If the charge controller manufacturer explicitly permits it, the PV source may be oversized somewhat (typically 110-125%).

Minimum current rating of charge controller	= PV source power rating ÷ system voltage

Step 5: Determine maximum number of PV modules in series

PV module cell temperatures below 25°C will increase the voltage a PV module beyond its rating. In locations that experience low temperatures, it is necessary to determine the maximum number of modules in series that will be possible given the minimum temperature at the project location.

% change in Voc at minimum temperature	= (minimum ambient temperature - 25 °C) × PV source temperature loss parameter

Voc at minimum temperature	= PV module open circuit voltage (Voc) × ((% change in VoC at minimum temperature / 100) + 1)

Maximum number of PV modules in series	= Maximum Voc rating of charge controller ÷ Voc at minimum temperature

The final chosen charge controller should function at the system voltage and have a current rating that is larger than the minimum current rating calcualted in step 3.

Max. PV short circuit current