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What is MPPT?

An MPPT (Maximum Power Point Tracking) charge controller optimizes the connection between solar panels and your battery or the utility grid. The maximum power point (MPP) is the ideal point on an IV curve where current and voltage multiply to provide the maximum power that the given PV system can produce at a given moment.


Should I get a PWM or MPPT charge controller?

PWM (pulse width modulation) and MPPT (maximum power point tracking) charge controllers are DC to DC converters that take the voltage output from solar panels and convert them to a more suitable voltage to charge a battery bank. PWM charge controllers are typically older, cheaper, simpler, and meant for smaller systems. MPPT charge controllers are newer, more efficient, offer greater flexibility of design, can be built for larger systems, and typically offer longer warranties from the manufacturer.

The main difference between MPPT and PWM charge controllers is that MPPT charge controllers allow the PV array output voltage to be higher than that of the battery bank without losing power. A typical PWM charge controller will only be able to regulate the output voltage of a PV array, but not the current. A PWM charge controller will maintain about the same current as the PV array. This means that the output power (current x voltage) will be reduced. An MPPT charge controller will monitor the PV array for the maximum power point, and attempt to use this information to not only regulate the output voltage of the PV array, but also regulate the current. Typically this means that it will lower the voltage, while increasing the current at the same time and maintaining most of the overall output power.

The MPPT ability is essential for larger PV systems, as it allows for greater flexibility in system design and performance. A typical battery voltage range is a multiple of 12V, typically going up to 60V as an upper limit (12V, 24V, 36V, 48V, and 60V). A typical output voltage of a PV array using 60 cell modules, is a multiple of about 30V (30V, 60V, and 90V). So, if we have a system with 3 PV modules at about a 90V Vmp output, connected to a 48V battery bank, there is a significant voltage difference that could result in power loss if current cannot be adjusted on the output.



Example of PWM vs. MPPT difference in design flexibility:

Scenario A : PWM

If we design a PV system with a PWM charge controller. We will choose to use the SOLARIA PowerXT 330W solar panel, with a Vmp of 36.6 volts and a Imp of 9.02 amps. 

Note: Vmp x Imp = Maximum Power Point in STC 

36.6V x 9.02A = 330W 

We can also choose use the Trojan Solar AGM SAGM 12V battery as the storage system. Ignoring charge characteristics of the battery, we can roughly assume that if we find a PWM charge controller that can lower the output voltage of the Solaria module to 12V to charge the battery, but not regulate the output current, we will have the following result. 

9.02A (Imp from PV module) x 12V (battery voltage) = 108W 

The output of 108W is significantly less than the maximum power output of the Panel, 330W. Therefore, it is best to try and match the panel’s expected power output, to that of the battery system in this example. Perhaps by having three batteries in series.

Scenario B : MPPT

Now we will design a PV system with a MPPT charge controller. We will use the same Solaria module. The PV output voltage will have to be decreased to match the charging voltage of the battery, about 12V. This time however, the output current will increase to ensure that the overall power transfer is maximized.

To find out what the ideal current rating of the output will be we will need do the following.

Maximum Output Power / Battery Charging Voltage x Temperature Correction Factor 

330W / 12V * 1.25 = 34.4 A 

Note: The temperature correction factor comes from Table 690.7 in the NEC code. It is a correction factor for the local ambient temperature when sizing current carrying conductors.

Now we just have to make sure that we choose a charge controller capable of outputting 35A or more to a 12V battery. One charge controller that will work for this combination is the Outback Flexmax 60A Charge Controller from outback. This charge controller supports an input voltage of 150V and an output current of 60A.