[Sgooter]

Unless there's something unusual in your SL's history, your car is covered by the M-B warranty. So call a dealer and get it fixed. If it won't start, then call M-B's roadside assist and they will dispatch the local dealer to come to your location to address things like a dead battery or you can also have the car towed to the dealer.

WRT SL batteries, here's some useful info I found at another site posted in 2004 by BlueSL (still pretty much valid today):

"There are two batteries in the car to address the problem of not being able to start the car after a period of standing idle.

The smaller battery in the engine compartment is used to start the car, not just the starter motor, but the ignition, fuel pump and engine management and is designed to have the absolute minimum of static current drain to allow you to start the car even after several weeks. The alternator charges both batteries.

The larger battery in the trunk powers everything else and can also start the car if the small battery is dead. However, the larger battery powers everything else in the car, including those items which are never switched off and this battery quickly looses its charge when the car is not used. I estimate that after 10 days, it's lost 50% of its charge. If the battery is not sufficiently charged when you start the car, some items will not work, for example the roof and you get the "Electrical Consumers Offline" message. When the battery is charged up by the alternator, the message goes away and in theory, everything starts working. It does look like there's a problem with some part of the car when this happens though. My air-con goes hay-wire until I stop the car, switch off and switch back on.

If you put an ammeter in the circuit, you see the current drain from this battery decrease over a period of about a minute after the car is locked as the various systems shut down. When all is quiet, the car is consuming a few watts of power only but this is enough to drain the battery over a couple of weeks.

If the battery is flat, you cannot get into the car with anything other than the mechanical key because the central locking is dead. This is why they've dumped the card for keyless go because you are now forced to carry the real key with the mechanical insert to allow you to get into the car if the battery is flat.

So what is being run when the car is shut down? The interior motion sensor and alarm system, the central locking controller plus the internal network will be active.

As pointed out, both batteries will loose their charge over time anyway due to "internal discharge". Trickle charging the car should not be a problem, especially if its in a reasonably well ventilated space."