How long would a 13 watt solar panel take to charge a 12 volt car battery?
First of all, it needs to be about a 15-16v panel, since a fully-charged 12v batt is actually 13.2 volts or a little more, and you need to push in 14.5v or so to have sufficient voltage gradient for the charge to have the desired chemical effect within the battery cells. That leaves, at best, .87amps from your solar panel. Now it’s all just math. Assume a real-world efficiency of 85% from your charger, and 85% efficiency into the battery. Your results will probably be better, but at least with my guess you won’t be disappointed. All this means you can reasonably expect ~.63Ah into your battery for each hour of full sun on the panel. Checking Trojan Battery’s website (most ignition battery companies don’t talk about their products’ capacity in amp-hours, but that’s the info we need here and estimates based on Trojan’s heavy-duty products will all be very, very conservative compared to a Wal-Mart or Sears battery), we see that their smallest battery sized for a typical car installation h
