The rear trunk battery is called the "convenience" battery. It runs everything in the car, except
crank the starter armature, which is the sole job of the engine compartment battery.
With regards to coming home and car won't crank, and the battery could not have been frozen,
it should charge up with a normal battery charger. If the convenience battery then shows it is also
discharged (which is likely because this battery keeps all the computers going when the car sits),
it can be charged enough with a main battery charger to get everything going. Then get yourself a
battery maintenance charger (shuts off when fully charged and comes back on when needed).
I live in central Texas, so the freeze thing is not a problem, but the excessive summer heat is.
When the car idles around in traffic with AC at full blast, radio on, running lights on, etc, the convenience
battery does not hold a full charge just by driving. In addition to that, the convenience battery is always supplying
power to the car computers even when the car sits not running in the garage.
I have a Harbor Freight battery maintenance charger ($20 and it shuts off when fully charged) to maintain the trunk battery
throughout the summer. Have used it over 5 years on the 05 SL500 and now have wired it into the 15 SL550. No problems.
The trunk battery is under the trunk floor. Just pull up the floor, and there it is.
The one question I do not know however, is if the front battery is charged with a maintenance charger on the trunk battery. I have
never had an engine battery fail, except when they just wore out over time. Does anybody know the answer to that?
For you guys up north, or owners who hardly ever drive the car, a battery maintenance charger is an SL necessity, and should have come
with the car. But what do you want for $115,000.00? I don't even have a spare tire on the 15.