There is a lot of good to bad info out there, and answers on this forum are opinions often and not fact.
So to start with why not get a little more information. Start with this 2 part article: http://www.marxrv.com/12volt/12volt.htm
There is a lot of info here, some is still not absolutely correct. In fact many solar panel sales companies may have bad info.
Handy Bob's Charging puzzle is another good article: https://handybobsolar.wordpress.com/the-rv-battery-charging-puzzle-2/
Here is an article about shading and angle: https://pdf.wholesalesolar.com/Download%20folder/solar-panels.pdf
There are many discussion about this subject but not as much as monitoring how your batteries are doing. I love this product by Bogart(TM-2030-RV) that lets you know exactly what is going on with your batteries. http://www.bogartengineering.com/products/trimetrics.html
So for those who want answers and don't want to read much. Solar panels provide about 17.5 volts to push the watts to your 12.7 volt batteries. The smaller and longer the cables the cables the voltage could drop low enough to provide no push. But if you keep putting too much current you will boil or ruin the batteries. So that is what a charge controller is for, it provides the batteries the voltage the batteries want depending on their state of charge.