I have read all of Handy Bobs report on solar panel
and his reccomendations for large wire and the voltage required to fully charge a flooded wet cell battery.
I have a pair of 40 watt panels wired in parallel.
In my case I use 16 gauge wire for all my wiring and my solar controller never produces more than 13.9 volts as an output.
My battery fully charges usually by around noon time in this AZ sun. I measure it with a digital voltmeter that during sunlight hours measures around 13.6 volts.
Since Handy Bob said that a hydrometer is needed to measure the state of a battery's charge I bought one. The hydrometer shows it's fully charged.
I believe the 16 gauge wiring works in our case because 80 watt panels are relatively low current devices compared to Handy Bobs 375 watt panels.
On a seperate topic we have now completed over 5000 miles of towing and almost a half year with our taped down flexible solar panels without any issues.