This reminded me of my experiment before finalizing my solar installation.
Some of my solar panels have the specs of a Vmp of 30V. Vmp is the voltage for max power. Now, my bank of lithium phosphate cells cannot be charged above 28.8V to avoid damage. That leaves 1.2V for voltage drop, which should be plenty. A charge controller for this set up can be very simple and cheap.
Test results were terrible. What happened was that the above Vmp was spec'ed for the standard solar cell temperature of 25C (77F). When the ambient temperature reaches 110 to 120F here in the Southwest, what is the temperature of the solar panel in direct sunlight? 160F easily.
Vmp of photovoltaic cells drops with rising temperature! The panels cannot put out a voltage higher than that of the battery in order to charge it, no matter what wire size.
My solution was to wire panels in series in pairs to double up the voltage, then use MPP controllers at the battery to step down the voltage and to regulate the charging at the same time. The DC/DC converter of the controller also doubles up the current into the battery, and no power is lost because of the voltage mismatch. The wires used between each pair of panels and the controller are of AWG12. The smaller wire size is good, because the run is 100'x2 long, for the farthest panels. Bigger wires would cost too much.
My calculations show only a few percent of power loss due to wiring. And I used more than 1,500' of wires for the whole panel array.
Now, if wiring 2 panels in series is good, then why not 3 or 4? The problem is that higher DC voltages have a higher potential for arcing which can have catastrophic consequences, and also require more expensive controllers. So, I had to make a compromise.
