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.

It is worth noting that most manufacturers now provide charge controller sizing tools that greatly simplify this process - check the website for the manufacturer.

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: Determine 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 (Step 1)

Step 3: Minimum 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.

Minimum PV source power rating	= minimum number of PV modules (Step 2) × PV module power rating (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	= Minimum PV source power rating (Step 3) ÷ 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. PV module manufacturers provide a temperature coefficient for voltage that can be used to calculate increases or decreases in power based upon the environmental conditions. This coefficient is referred to as temperature coefficient of open circuit voltage (Voc) and can typically be found on module specifications sheets in -%/°C.

The maximum voltage of the module under standard test conditions - open circuit voltage (Voc) - will be used for this calculation.

% change in Voc at minimum temperature	= (minimum ambient temperature - 25 °C) × temperature coefficient of open circuit voltage (Voc)

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

This value must be rounded down to the next whole number (there are no partial modules).

Step 6: Determine maximum charging voltage parameter

The minimum number of PV modules in series must be calculated based upon the maximum required charging voltage for the energy storage system or the minimum charging voltage provided by the MPPT charge controller manufacturer in the specifications sheet. The maximum system charging voltage parameter is the value for the maximum voltage at which the energy storage system will be charged. This value depends upon the system voltage parameter and the energy storage system type. The specifications sheet or user manual for the battery that is used in the system should be consulted.

Recommended maximum charging voltage values for a Trojan AGM battery at 25°C (it is recommended that values at the upper-end of a manufacturers range are used):

• Maximum system charging voltage (12V): 14.1–14.7V
• Maximum system charging voltage (24V): 28.2–29.4V
• Maximum system charging voltage (48V): 56.4–58.8V

Step 7: Determine minimum number of PV modules in series

PV module cell temperatures above 25°C will decrease the voltage a PV module beyond its rating. PV module voltage will also decrease as the module ages. It is therefore important to make sure that the PV source is adequately sized to ensure that at high temperatures and with the passage of time that the array will still be able to provide sufficient voltage to charge the energy storage system. PV module manufacturers provide a temperature coefficient for power that can be used to calculate increases or decreases in power based upon the environmental conditions. This coefficient is referred to as temperature coefficient of open circuit voltage (Voc) and can typically be found on module specifications sheets in -%/°C. The value from the specifictions sheet of a module can be used in these calculatons if a module has been chosen, but a standard average value of (-.48%/°C) will work for both poly and monocrystalline modules.

The operating voltage of the module under standard test conditions - maximum power voltage (Vmp) - will be used for this calculation.

The mounting system will also affect the ability of the PV source to cool itself. A mounting system temperature adder should be added to the maximum temperature that is used to calculate the decrease in Voc:

• 20°C for pole mount
• 25°C for ground mount
• 30°C for roof mount

% change in Vmp at maximum temperature	= (Maximum ambient temperature + Array temperature adder - 25°C) × Temperature coefficient of max power %/°C

% change in Vmp at maximum temperature	= (Maximum ambient temperature + Array temperature adder - 25°C) × Temperature coefficient of max power %/°C

Vmp at maximum temperature	= maximum power voltage( Vmp) × ((% change in Vmp at maximum temperature ÷ 100) + 1) × module degradation parameter

Minimum number of PV modules in series	= maximum charging voltage parameter (Step 6) ÷ Vmp at maximum temperature

This value must be rounded up to the next whole number (there are no partial modules).

Step 8: Determine the number of PV modules in series per string

The proposed number of PV modules per series string must be less than the maximum number of PV modules in series (Step 5) and greater than the minimum number of PV modules in series (Step 6). If there is not a configuration that is meets this criteria another module should be tested or adjusting some of the parameters can be tested, like the module degradation parameter. As long as the voltage doesn't exceed the rating of the charge controller, more PV modules per string is generally preferrable to use a higher operating voltage and lower current.

Step 9: Determine the number of strings of PV modules in parallel

The proposed number of strings of PV modules in parallel will be determined by dividing the minimum array size by the final number of PV modules per series string. This calculation should be rounded up. The total short circuit current (Isc) of the parallel strings of modules should not exceed the current rating of the charge controller unless permitted by the manufacturer. A larger charge controller should be chosen if an appropriate series/parallel configuration cannot be found.

Minimum number of strings of modules in parallel	= Minimum number of PV modules (Step 2) ÷ final number of PV modules per series string (Step 7)

Step 10: Proposed PV source power rating

The total power rating of the PV source can be calculated by multiplying the power rating of the chosen PV module by the final number of PV modules in series (Step 7) and the final number of strings of PV modules in parallel (Step 8).

Proposed PV source power rating	= PV module power rating (Step 1) × proposed number of PV modules in series (Step 8) × proposed number of strings of PV modules in parallel (Step 9).

Step 11: Verifying the proposed design

Lead acid batteries last longer and perform better when they are regularly recharged with a current in a certain range - typically between .05-.13 (5-13%) of their C/20 rating. It is best practice to consult the manual or manufacturer for recommended maximum and minimum charging currents.

Minimum required charge current	= final Ah capacity × .05

It is necessary to check the minimum required charge current against the available charge current from the proposed PV source power rating.

Available charging current	= proposed PV source power rating (Step 10) × Step 1: Deteremine PV source loss parameters|total PV source loss parameter ÷ maximum charging voltage parameter (Step 4)

If a system uses many loads during the day, this will limit the available charging current for the energy storage system and should be taken into account. If the number of PV modules does not meet the recommendations outlined below, increasing the size of the PV source should be considered.

• A minimum charge current of .05 (5%) of the C/20 Ah rating is recommended for a system that is used infrequently or is primarily used at night.
• A charge current of .1 (10%) of the C/20 Ah rating is recommended for a system that is used regularly with significant load usage during the day.
• A maximum charge current of .13 (13%) of the C/20 Ah rating is recommended.

Percentage of C/20 rate	= available charging current ÷ final Ah capacity

Step 12: Final PV source power rating

The configuration of the PV source and MPPT charge controller must meet all of the considerations in the previous steps. It often necessary to perform this process various times to find the optimal design.

Final PV source power rating	= PV module power rating (Step 1) × final number of PV modules in series × final number of strings of PV modules in parallel