" Trickle charging happens at very low voltages" No, by definition it refers to a trickle of current, that is, low current charging. See Trickle charging - Wikipedia, the free encyclopedia
one of many references that comes up with 13.38 volts for a 12-volt battery based on 2.23V/cell. You will find that each battery maker, for each battery chemistry, supplies a slightly different number for the optimum trickle/float/bulk/acceptance charges, but the voltages they supply are simply guidelines--the charging current is often more important, but less discussed because the casual user has problems measuring or adjusting current.
"My digital readout has my solar panel putting out 14.1 to 14.6 volts most of the day, even on bright cloudy days."
What is your readout reading? A raw solar panel should be hitting 16-17V if it is unregulated and unloaded. When it is loaded up, that will plummet and you may only see 3-7V output as the sun gets lower in the sky (i.e. 7PM on a summer day with a 9PM sunset)
"Remember, during the summer the days get much longer up here than farther south. " You get more hours of daylight--but weaker sunlight because of the extra atmosphere it is crossing. I've measured charge, with an ammeter as well as a voltmeter, and as the day gets late you'll get nothing useful from solar panels. Although using JUST a meter, without an actual load, shows they are still putting out voltage. Add the real load and put the meter in the live circuit, and you'll see the difference.
Pretty much every web site (university, lab, government, industry) that has maps and charts for real solar power from panels, shows the variation by lattitude and they all use the same independently verified criteria. The rule of thumb is "divide wattage by (three, four, six
) to get useable amp hours per day" and the only reason for the variation is your lattitude or the time of year.