I just had the 12V gel cell battery in my Model S replaced (the one behind the dashboard that powers the car's lighting and accessories). My Model S was from a batch of early-2013 vehicles that had a defective 12V battery installed. The car contacted the factory when the battery began to show signs of incomplete charging. This was well before any noticeable issues appeared. Tesla then contacted me to let me know it needed replacing. After the battery was replaced by a Tesla ranger, I noticed a change that I wasn't expecting: the transient response of my sound studio audio system improved dramatically, particularly in the low end. The subwoofer, in particular, now has a much sharper, punchier sound. Makes sense.