The first thing you have to do it to compute for the ampere rating of the solar panel by following the equation below.
How long to charge 12v battery with 100w solar panel.
For an effective charge you need to make sure that the panel is directly facing the sun and no obstacles in between.
Some quick rules for estimation.
The solar charge times above assume a 25 degree celsius day with the panel pointed directly at the sun.
To get the most effective charging by a solar panel you have to maximize the amount of current by keeping the board pointed directly to the sun.
How long to charge 12v battery with 100 watt solar panel.
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.
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.