Mixing different panels in parallel

The reason that Photowhit says that if the array voltage is "too low" (Vmp-array) is because:

1. Solar panel labels are based on standard test conditions. A "flash" simulated sun for a few seconds, and 25C/77F. In real life, solar panels run much hotter in full sun. And Voc/Vmp falls as panel temperatures rise. Full sun, hot climate--Vmp falls to 0.81x Vmp-std. Example 18 volt Vmp * 0.81 hot panel derating = 14.58 (rough number) volts Vmp.
2. Electricity flows "down hill" "doe wn hill". I.e., your solar array has bwie to be higher than the wiring voltage drop+controller voltage drop+controller to battery bank wiring drop... Vmp=14.68 volts in hot weather, and you want to charge the battery bank to 14.75 volts (FLA example). You will get less than full Imp-array current. Vmp is a "soft peak"--I.e., current does not drop to zero amps. But current will not be at full Imp either.

A bit more about solar panel electrical properties.

https://forum.solar-electric.com/discussion/5458/two-strings-in-parallel-with-unequal-string-voltages

MPPT controllers are (almost always) a Buck Type switch mode power supply configuration:t

https://en.wikipedia.org/wiki/Buck_converter

And they need several volts drop (at least) the MPPT controller (panel in to battery out). If you have panel Vmp tha falls to near 15 volts, and you want to charge the battery bank to near 15 volts, the MPPT controller does not have the "head room" to do its magic. It becomes a PWM On/Off controller (no "MPPT Gains").

More or less, the Vpanel input to the controller should be ~1.3x Vbatt voltage to operate in MPPT "mode".

14.75 volts battery charging * 1.3 minimum MPPT headroom = 19.175 volts Vmp (ideal--But remember Vmp falls as panels get heat from full sun)

The optimum (simplified) is 2x battery charging voltage for MPPT. I.e., 15 volt charging * 2 optimum = Vmp-array ~ 30 volts.

A PWM controller is nothing more than a computer controlled on/off switch (in basic function). The power to the battery bank is:

Array Current * Battery voltage = Battery Watts in

You can have an array at 15 volts, 18 volts, 30 volts, and a PWM controller will simply deliver the available solar current to the battery bank. I.e., a 180 Watt panel, Vmp=18 volts, Imp=10 amps (made up numbers):

180 Watts from panel (Standard test conditions)

10 amps * 14.75 volts Vbatt = 147.5 Watts best case power to battery charging

For an MPPT controller, because it has an inductor to store energy, it can act like a DC version of a transformer and Watts in to the MPPT controller equals Watts out to the battery bank (excluding any losses). Say we have a 180 Watt and 36 volt Vmp panel with Imp=5 amps.

5 amps Imp * 36 Volts Vmp = 180 Watts from "ideal panel"
180 Watts / 14.75 volts battery charging = 12.2 amps into battery bank

Vs the 10 amps from an 18 Vmp panel into a 12 volt battery bank with PWM controller

12.2 Amps MPPT / 10 amps PWM = 1.22 "improved" harvest with MPPT controller and "higher voltage array"

Remember that a solar panel can run at 81% of STD ratings vs Running "Hot" in full sun (panels are always running "hot' in full sun).

Yes, Vmp-array-cold in (typically) sub freezing weather can have significant power transfer gain for MPPT contorller over PWM controller--But for the average person in > freezing conditions--I would not suggest purchasing an MPPT controller for this function.

MPPT controllers are great because they they can run the Vmp-array at upwards of 100 Volts (or even 400 Watts for "high voltage" MPPT controllers and GT inverters)... This allows us to run solar arrays a much larger distance from the MPPT controller with reasonable size cooper cables. It makes a huge difference in system design (array in full sun, home in shaded glad for cooler summers as an example).

-Bill

Midnite Solar has a couple handy solar array calculators for their MPPT Solar Chargers. I use it because it does give results that can address the voltage/current/etc. data needed for other brands of controllers too:

https://www.midnitesolar.com/sizingTool/index.php

If you want to do the math directly (or in a spread sheet), the default/generic math would be:

Voc-at-temperature = Voc-std + [Voc-std * (0.0033 per C * (25C ambient - Actual array temperature)]
Vmp-at-temperature = Vmp-std +[Vmp-std * (0.0045 per C * (25C ambient - Actual array temperature)]

Note that for Voc it is -0.33% drop in in output voltage for every degree C rise in panel temperature (25C is STD condition). Vmp is -0.45% per C drop for every +1 C over 25C.

-Bill "I am forgetting my basic Algebra" B. 

Fixed a couple of typos in #2 (drifting touch pad cursor):
-Bill