13.6 - 14.7 is a normal range, and depends on the state of charge of the battery. The lower the charge, the higher the rate.
My solar system charges at a lower rate on cloudy days, but it still charges. In fact, I get charging as long as the panel has less than about a 1 inch snow load.
If you get only 12.5 volts on a cloudy day, it means its not charging at all and something is not working correctly.
On a side note, look at your solar charger for the data. If you are looking at a digital voltmeter stuck in a cigarette plug, forget it. The last 2 I bought were off by a lot.