The quick answer is to use a Ctek MUS 4.3 with the full accessory kit.That is what Porsche (& many other OEMs) do.You can plug it in to the cigarette lighter socket.It will not overcharge the battery (& destroy it like the cheapie trickle chargers do) and it will reduce/remove sulphation -unlike most cheap ones.
https://www.amazon.com/CTEK-56-864-Automatic-Battery-Charger/dp/B006G14FK8