The short answer is in most cases depending on the size of the battery it will take around 5 8 hours to fully charge a 12 volt car battery with a solar panel that can produce 1 amp of current.
How long to charge 12v battery with 100w solar panel.
How long to charge 12v battery with 100 watt solar panel.
10 watt 18 volt solar panel charging a 12v 10 amp hour lead acid battery 120 wh from 50 full to full time 60wh x 2 10 watts 12 hours.
In most cases a solar panel that produces 1 amp of current takes 5 8 hours or more depending on the size of a battery.
Some quick rules for estimation.
The solar charge times above assume a 25 degree celsius day with the panel pointed directly at the sun.
For an effective charge you need to make sure that the panel is directly facing the sun and no obstacles in between.
Let s say you want to compute for the average charging time of a system with a 12v battery with 50 ah capacity and a 15 watts solar panel.