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 the PV source 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 source 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 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 the minimum PV source current

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 of 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%). Larger systems often require multiple charge controllers operating in parallel.

Minimum PV source current	= Minimum PV source power rating (Step 3) ÷ System voltage

Step 5: Select a charge controller

The final chosen charge controller should:

1. Function at the system voltage.
2. Have a current rating that is larger than the minimum PV source current rating (Step 4) or multiple charge controllers will have to be used. A single charge controller is the simplest and most cost-effective option.

Charge controller maximum input voltage	= From charge controller specifications sheet

Charge controller current rating	= From charge controller specifications sheet

Step 6: 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 7: 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): 14.1–14.7 V
• Maximum system charging voltage (24V): 28.2–29.4 V
• Maximum system charging voltage (48V): 56.4–58.8 V

Step 8: 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 max power and can typically be found on module specifications sheets in -%/°C. The value from the specifications sheet of a module can be used in these calculations 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

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 7) ÷ Vmp at maximum temperature

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

Step 9: 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 these criteria then another module or charge controller should be used in the design. 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 10: 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) ÷ Minimum number of PV modules per series string (Step 8)

Step 11: Determine 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 number of strings of PV modules in parallel (Step 8).

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

Step 12: Verify excess production

During periods of poor weather or low solar resource, an off-grid PV system is designed to discharge the battery to a certain depth of discharge which can leave the energy storage system depleted. It is important that the energy storage system is brought back up to a full state of charge in short period of time or the cycle life of the batteries will be reduced. The PV array therefore must be sized to generate sufficient excess energy, while continuing to meet all of the power needs from the Load evaluation. OSSP recommends that the array be sufficiently sized to reach a full state of charge within a week or that the system incorporate a generator to ensure adequate charging.

Proposed PV source low insolation production	= Proposed PV source power rating (Step 11) × Total PV source loss parameter × Design insolation × Charge controller efficiency parameter × Energy storage efficiency parameter

Daily excess production in Ah	= (Proposed PV source low insolation production - Design daily watt-hours required ÷ System voltage parameter

Ah used at full depth of discharge	= Final Ah capacity × Depth of discharge parameter

Time to reach full state of charge	= Ah used at full depth of discharge ÷ Daily excess production in Ah

If the time to reach full state of charge is less than 7 days, then the size of the PV source should be increased until it is below 7 days.

Step 13: Verify charging current

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. 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 by increasing the PV source size. The maximum charging current for most lead acid batteries is between .13 (13%) and .2 (20%) of the C/20 rate. Most designs should have a charge rate between 5-10% - closer to 10% if the system is used heavily during the day. It is necessary 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 11) ÷ Maximum charging voltage parameter (Step 7)

Percentage of C/20 rate	= Available charging current ÷ Final Ah capacity

If the number of PV modules does not meet the recommendations outlined above, then the charge controller and PV module configuration should be reevaluated. It is recommended that the PV source should be increased in size.

Step 14: Determine 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