Bill where do you come up with this stuff..? 13.6 fully charged? I don't know of any 12V battery that has a "resting voltage" of 13.6 for full charge? Perhaps you could point us to one? Float charging is 13.2 -13.8 volts for most brands, take away the float, and most all 12V batteries will "rest" at under 13 volts. Some will hang out at a voltage above 13 for a while when colder but all will eventually rest quite a big lower than 13.6V. Trojan wet cell batteries for example have a 100% charge when resting at 12.73V open circuit voltage. Some AGM's are slightly higher with Lifeline & Deka coming in at about 12.8V for 100% SOC at 77F..If you buy 12 volt nominal panels your batteries will not charge!!!!!!!
You need AT THE VERY LEAST 14.4 volts output with hot panels. Either 36 cells or 18 volts is considered the MINIMUM needed to effectively charge a "12 volt wet cell" battery.
Remember a "12 volt battery" does not operate at 12 volts and is almost dead when it gets to 12 volts. 13.6 is the standard resting voltage for a fully charged battery so plugging in a 12 volt nominal panel will not accomplish much of anything except to empty your wallet.
A 12V "nominal" panel is not 12V they are usually 16+ volts. Even a 25V panel can be considered a 12V panel because 25 volts is not enough to charge a 24V battery.. If you had a 12V panel, and that was the actual Vmp, then it would not charge a 12V battery and would ideally be classified as a 6V or 8V nominal panel...
Nominal: a : existing or being something in name or form only <nominal head of his party> b : of, being, or relating to a designated or theoretical size that may vary from the actual : approximate <the pipe's nominal size>
Nominal is a term very often used to describe the panels "classification", such as 12 or 24V not the actual Vmp of a panel...
Here's a link that describes "nominal" and other nomenclatures: http://www.altestore.com/howto/Sola...-Components/Solar-Panels-PV-and-Voltages/a98/