PV Module Configuration and MPPT Charge Controller Performance

The influence of different PV-panel configurations on behavior, conversion efficiency and thermal aspects of MPPT charge controllers.

Among the range of Photovoltaic (PV) charge controllers, Phocos offers both PWM and MPPT versions.

PWM (pulse-width modulation) charge controllers rapidly connect and disconnect the PV-input directly to the battery output without a voltage conversion.  Phocos MPPT (maximum power point tracker) charge controllers consist of highly efficient DC/DC-converters that actively convert the input voltage to the output voltage level (battery voltage) with only minimal conversion losses.

This means the MPPT charge controller searches, finds and tracks the point of maximum power output on the characteristic curve of the connected PV generator to get out the maximum possible amount of power and thus the maximum energy into your battery and connected loads. Tracking is necessary since the PV-voltage at the point of maximum power output varies over irradiation and temperature.

There are two major types of MPPT charge controllers: Buck converters and Boost converters

• A Boost (step-up) converter can only convert lower PV input voltages to the same or higher voltage output for battery charging.
• A Buck (step-down) converter can only convert higher PV input voltages to the same or lower voltage output for battery charging.
• A third kind, the Buck/Boost converter can do both, but it is usually much less efficient than a pure Boost or Buck converter.

The majority of MPPT charge controllers are Buck converters and convert a higher input (PV) voltage to a lower output (battery) voltage. In this article the concentration is on the Buck converter type. However, except for the voltage levels, the following considerations are also true for the other converter types.

PV-oversizing:

Phocos MPPT charge controllers are allowed to be connected to more PV power than they can actually convert. They have a built-in output current limitation and temperature control. The output current will therefore never exceed the maximum specified output current. In case the MPPT charge controller should not be mounted in an optimal way and/or in high ambient temperatures, it will limit its maximum internal temperature by reducing the power throughput in case it reaches a certain temperature threshold so that it will not overheat. PV oversizing will feed more energy in your batteries because most of the time, the solar irradiation is not at its maximum and the PV panels will therefore not deliver their maximum rated power. Thus, with higher nominal PV-power, you will get more energy into your batteries. You can use up to approximately 1.25 times of the maximum nominal power for the smaller (15 A) units and up to about 1.5 times the maximum nominal power for the CIS-N-MPPT 100/30.

Thermal aspects:

Even though the MPPT charge controllers have an excellent power conversion efficiency and thus only minor losses, they will still heat up. They generate more heat as more power is converted. They do limit their internal temperature to protect themselves, but higher temperature always means a higher stress to the electronic components inside. Therefore it is best to mount the MPPT controller vertically on a wall, so that natural convection causes air to flow vertically upwards around the surfaces of the MPPT so that it can dissipate its power loss (heat) into that air, which helps to keep the MPPT cooler. If available, mounting on a metal surface that will act as additional heat sink (cooling surface) will further help to keep the MPPT charge controller cool, especially if the unit does not feature heat sink ribs, like the ECO-N-MPPT.

All charge controllers should always be mounted in cool, shaded, ventilated places so direct sunlight cannot heat them up from the outside.

PV-voltage range:

The input voltage range of up to 85 Vdc or 95 Vdc (depending on the charge controller model) enables the usage of less expensive PV panels that are usually used for grid-tied purposes. For example, PV panels with 60 cells, that would not be suited for battery applications without an MPPT (with a PWM charge controller, you always have to match PV panels to the desired battery voltage). For example, this enables you to replace two 36-cell panels (130 Wp each) with one 60-cell PV panel (up to 280 Wp) at lower cost.

Examples:

• One 60-cell panel with a Phocos CIS-N-MPPT 85/15 in a 12Vdc system instead of two significantly more expensive 36-cell panels
• Two 60-cell panels in series with a Phocos CIS-N-MPPT 85/15 in a 24 Vdc system instead of two or three significantly more expensive 36-cell panels
• Two 60-cell or 72-cell panels with a Phocos CIS-N-MPPT 100/30 in a 12 Vdc system instead of four significantly more expensive 36-cell panels

The maximum open-circuit voltage of the connected PV panels must never significantly exceed the maximum input voltage rating of the MPPT charge controller. The Phocos MPPT products feature a PV overvoltage protection that will switch off the power conversion in case of an excessive PV voltage. Even if the PV input voltage would rise to a slightly higher value than allowed, it will still do no damage to the MPPT, but will keep it inactive while that higher voltage is present if the overvoltage is not too extreme. However, exceeding the PV voltage significantly will likely damage the charge controller. Check the user manual for specific voltage recommendations for each Phocos MPPT charge controller.

Conversion efficiency:

Even though the Phocos MPPT charge controllers have input voltage ranges of up to 85 V or 95 V, it is better to use a lower PV voltage for best conversion efficiency, especially in combination with 12 Vdc systems: The conversion efficiency of the DC to DC converter in the MPPT charge controller is dependent on the input to output voltage ratio. The smaller that ratio, the better the conversion efficiency. The result is a higher output power and lower thermal stress to the unit. At best choose a PV array configuration to achieve an mpp-voltage (Vmp) that is well above the maximum battery voltage, but at best not very much higher. The closer the Vmp is to the maximum battery voltage, the higher the charge controller efficiency will be. For this, the connection of two strings of panels in parallel might be necessary.

Expert Tip: Connecting two photovoltaic panels or two strings in parallel is no problem and does not require extra wiring effort (or a PV combiner box) if you use readily available Y-connectors for that purpose.

The wide input voltage range together with the PV oversizing capability of the Phocos MPPT charge controllers, allows for many different PV panels and configurations to choose from.

Solar Module Configuration Examples:

1. 12 Vdc system with Phocos, CIS-N-MPPT 85/15, or CIS-N-MPPT-LED
   Maximum power conversion capability: 15 A x 15 V = 225 W. With the PV oversizing capability you can connect up to about 280 Wp.
   Options:  
   1. Two 36-cell 130 Wp panels in series or in parallel or
   2. One (up to) 280 Wp 60-cell or 72-cell panel
      
2. 24 Vdc system with Phocos, CIS-N-MPPT 85/15, or CIS-N-MPPT-LED
   Maximum power conversion capability: 15 A x 30 V = 450 W. With the PV oversizing capability you can connect up to about 560 Wp.
   Options:
   1. Four 36-cell 130 Wp panels in 2s2p configuration (two in series by two in parallel) or
   2. Two 280 Wp 60-cell panels in series or
   3. Two 280 Wp 72-cell panels in parallel for better conversion efficiency and lower thermal stress
      
3. 12 Vdc system with Phocos CIS-N-MPPT 100/30
   Maximum power conversion capability: 30 A x 15 V = 450 W. With the PV oversizing capability you can connect up to about 675 Wp.
   Options: 
   1. Four 36-cell 130 Wp panels in 2s2p configuration (two in series by two in parallel) or all in parallel or
   2. Either use two 60-cell or 72-cell panels with up to over 300 Wp in series, or in parallel for better conversion efficiency and lower thermal stress.
      
4. 24 Vdc system with Phocos CIS-N-MPPT 100/30
   Maximum power conversion capability: 30 A x 30 V = 900 W. With the PV oversizing capability you can connect up to about 1350 Wp.
   Options:
   1. Use 8 pcs. 36-cell 130 Wp panels in 4s2p configuration (four in series by two in parallel) or for better conversion efficiency and lower thermal stress in 2s4p configuration (two in series by four in parallel) or
   2. Use four 60-cell or 72-cell panels with up to over 300 Wp in in 2s2p configuration (two in series by two in parallel)
      

This detailed review and technical description should help to optimize your next solar project design and installation. For 20 years, Phocos has supplied proven, robust PWM and MPPT charge controllers. Click here for a simple summary of models and features (page 5 and 6).