The Furrari has an electrical problem at the moment. This discharges the rear battery over a couple of days. The rear battery looks after everything but the starter. As tenpin and I have posted, care needs to be taken when jumping batteries in the SL. Yesterday when The Furrari was at DNA we needed the roof down but the rear battery was to flat to lower it. Remedy, start the car; the rear battery can then have another battery jumped to it to power the roof.
This works because the alternator produces 14 to 14.7 volts. The jumper battery is at 12 volts so the battery charging module sees the voltage flow in the right direction with the front battery at the higher voltage than the rear, just as it does when running normally.
Result, no potential damage to the charging module.
Kalifornia tells me there are two upgrade charge controlling modules, the latest one is designed to prevent the problem caused by this reverse voltage but we don't have details of dates or part numbers.
I'm afraid this charging requirement transfers over to the West Coast, but at least you know how to treat the Merc charging system.
Mick.