We do not have conventional flooded lead acid car batteries that are not at all tolerant of being even slightly discharged. They are specifically designed to provide a lot of energy for a very short period of time (seconds) to start the engine and then be immediately recharged in a very short period of time. The plates used in conventional flooded lead acid batteries soften (not good in a moving platform) and warp (and maybe short to adjacent plates) as they become discharged. Also, the sulfate deposits harden effectively removing surface area. The capacity associated with that surface area is now no longer available. The construction also makes them vulnerable to mechanical shock which, of course, is part of their existence in a moving car.
However, we do not have conventional flooded lead acid batteries. We have AGM batteries. AGM batteries are still lead acid chemistry but they are built very differently. Every car I have had for the past 12 years has had AGM batteries. These batteries are not flooded batteries, structurally very robust, tolerate deep discharge much better than flooded lead acid batteries, have a much lower self discharge rate than flooded lead acid batteries, and are maintenance free. I have heard that 50% discharge is max recommended but I am not sure of that. In any case the threads on this board and a BMW X3 board I used to visit frequently pop up with concerns for trickle chargers etc. In my opinion, there appears to be a lot of undeserved paranoia about this subject. The advice in the posts above is mostly all very good advice. Just leave the thing alone unless the car will not be used for a very long stretch (many months).
Also, I frequently see a concern about driving only a few miles to work and not fully charging the battery. Dealers use this as an excuse to not fix an electrical problem. This is also an problem founded on only paranoia. If you look at the amount of energy needed to start the car and the rate the big ass alternators we now have pump that energy back in, the battery fully recharges from starting in a very short period of time. I also noticed, starting with my 2011 X3, the electrical systems maintain proper voltage even at idle. For a few years I had a client in unreasonably cold/snowy/miserable Upstate New York. In the morning in near or sub-zero temps, I would start my X3, turn on headlights, window defroster full fan, seat heaters, rear window heater, and steering wheel heater and then remove the ton of disgusting snow on the car. I noticed, without exception, with this large electrical load, the electrical system maintained 14.6 volts or so at idle meaning the alternatior was not only carrying 100% of the load it was also providing 100% of the battery charging needs. My trip to the plant was only 7 miles and I never had a problem keeping the battery charged. I am certain the electrical system in our Boxsters is equally robust. As a side note, it is a misconception that a battery cannot be charged when the car is idling. Simply controlling alternator output voltage via the electronic voltage regulator and the computer maintaining idle RPM is all that is needed.
BTW, while on this subject, there is only a a few tenths of a volt difference in charging voltage for the two types of conventional lead acid batteries, regular ones and maintenance free. Installing a maintenance free battery in a car designed for a non-maintenance free lead acid will result in it failing in about a year (my experience) solely because the required charging voltage for a maintenance free battery is a few tenths of a volt higher than the car with a non-maintenance free battery. That few tenth of a volt undercharged takes a toll over time in lost capacity due to hardened sulfate deposits.
