MPPT charge controller sizing and selection

A MPPT charge controller is rated to operate at a particular DC 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 PV source circuit 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 PV source circuit configuration of that will still be able to properly charge the energy storage system under high temperatures and as PV modules age at the project location.

There are various tools that can greatly simplify this process:

• Open Source Solar Project Design Tool
• Many manufacturers provide tools that are specific for their products on their websites.

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 PV source by the DC 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) ÷ DC system voltage

Step 5: Select a charge controller

The final chosen charge controller should:

1. Function at the DC system voltage. If a very high current rating is required for the charge controller, increasing the DC system voltage can yield a better system design.
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. Common MPPT charge controller current ratings: 10 A, 15 A, 20 A, 25 A, 30 A, 35 A, 40 A, 45 A, 50 A, 55 A, 60 A, 65 A, 70 A, 75 A, 80 A, 85 A, 90 A, 95 A, 100 A

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 DC system voltage 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 PV source configuration

It is necessary to test various different configurations of PV modules to find the best configuration of modules connected in series per PV soruce circuit. The proposed number of PV modules per PV source circuit must be less than the calculated maximum number of PV modules in series (Step 5) and greater than the calculated minimum number of PV modules in series (Step 6).

If the minimum number of PV modules (Step 2) required by the design exceeds the maximum number of modules that the charge controller can handle in series (Step 6), then multiple PV source circuits will be required. All of the PV source circuits must have the same number of PV modules if there is a single charge controller or else it will not function properly (the number of modules in series therefore must divide evenly into the minimum number of PV modules required). If there are multiple charge controllers, then the number of modules connected in series per PV source circuit should be the same for each one.

As long as the voltage doesn't exceed the rating of the charge controller(s), more PV modules per PV source circuit is generally preferrable as it permits smaller sized wires and minimizes voltage drop.

If there is not a configuration that is meets these criteria then a different PV module or charge controller should be used in the design.

Proposed PV source power rating	= PV module power rating (Step 1) × Number of PV modules in series × Number of PV source circuits

Step 10: 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.

This step is only necessary for systems that will be used heavily for consecutive days (most systems). If a system is used lightly or infrequently then it will have ample time to recharge during days when it is inactive and this step is not necessary.

For a system that is used heavily for consecutive days, it is recommended that the array be sufficiently sized to reach a full state of charge within 7 days or that the system incorporate a generator to ensure adequate charging.

Proposed PV source low insolation production	= Proposed PV source power rating (Step 9) × Total PV source loss parameter × Design daily 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 ) ÷ DC system voltage

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

Step 11: Verify charging current

Lead acid batteries last longer and perform better when they are regularly recharged with a current in a certain range that depends upon battery type. Flooded lead acid and gel batteries should be charged with current that is between .05-.13 (5-13%) of their C/20 rating. AGM batteries can should be charged with a current that is between .05-.2 (5-20%) 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. 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.

These calculations are performed with the Ah rating of the total energy storage system.

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 9) ÷ 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 12: Verify PV source and charge controller size

MPPT charge controllers have a maximum PV source power rating in watts that limits the size of the PV source. Verify that the maximum PV source power rating is greater than the final PV source power rating. If it is not, the charge controller size needs to be increased.

PV source and charge controller compatability	= Final PV source power rating must be less than the maximum PV source power rating of the charge controller

Step 13: Final configuration

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 number of PV modules in series	=

Final number of PV source circuits	=

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

Final charge controller current rating	=