Took delivery of my 2004 SL55 and put it away in my garage, the idea was to review the car and its operations before putting it on the road. Knowing the electronics would consume battery power, I attached a trickle charger to the lighter to keep the batteries charged. The SL55 had only 78 miles, and the trickle charger was purchased along with my 2004 Cayenne.
I was very surprised when I went to use the SL the next day. The keyless go would not respond, but the car started with the key in the ignition. The malfunction reading stated all power using options were off line. I drove the car to recharge the auxiliary battery and all was well.
I think what happened was the trickle charger was working only on the starting battery and maybe not connected to the auxiliary battery.

 

That could be the cause, unfortunetly I'm not the smartest person to deal with R230 electronics, I'm going to wait till blue SL comes along and tells us exactly what might've happened.

If you don't mind me asking, which Cayenne did you get? I've been staring at one for a while and am in love with the Turbo, however I can't justify a final price of 93k$ to my dad, that's after discount.

 

This whole issue is Mercedes attempt to get away from the problem of a car which is dead after two weeks in an airport car park because the battery has drained flat.

They divide the electrical "consumers" into two. The first are those which are required to be able to start the car - starter motor, ignition key, engine management, ignition, fuel pump and so on and arrange for a smaller battery in the engine compartment to run them. They are designed to place the absolute minimum standing current drain when switched off so that the ability to start the car is not compromised after an inactive period.

Everything else is run from the larger battery in the trunk but it's accepted that these items will have a larger standing current drain and will eventually flatten the battery. I've measured it on my car at 190mA which means a 100AH battery will be significantly discharged after 2 weeks. When the engine is started, you get the "electrical consumers offline" message if the battery is not properly charged while the alternator is topping it up.

The issue with keyless go is that it looks like this is run from the accessory, not the starter battery so that after a while, keyless will not let you in the car. I do not know for sure, but it seems likely that keyless is kicked into life when you pull on the door handle and will go to sleep after a while to conserve power. However, if there's no power left, it's not going to work. Mercedes solution to this problem has been to dump the card so that you're forced to use a key with metal key backup in case you cannot get into the car, instead of running keyless from the starter battery. My further guess is that keyless is integrated with other stuff and could not be simply swapped across to the starter battery without taking with it some other electronic items which were going to drain the starter battery.

In the case of Zolton's car, I do not know whether the cigar lighter is still live when the ignition is off to allow the trickle charger to work. If not, it would be worth trying the second recepticle in the trunk instead - that may be live all the time. The fact that his car had done so few miles from new probably meant the battery was not properly charged which is why it has given the offline message so quickly. The lighter is definitely run from the accessory battery, not the starter battery.

 

Regarding the Cayenne, I purchased the Cayenne S in Titanium with the 19 inch wheels and Nav.System. There was some other extras but basically off the truck. It was early in the MY and my choices were limited including any sort of discount. In short, I am very "picky"about cars. I look at quality of engineering, assembly, choice of materials, etc. I have now driven the Cayenne approx.5000 miles and can say that I would definitely purchase this vehicle again. I look at the Cayenne as a substitute for an S level MB. It has to be one of the quietest and most comfortable vehicle I have owned. Also, it seems that the used market is soft and one can purchase a 2003/2004 at a good price.

Regarding the cigar lighter I used for charging, it was hot when the ignition was off so I assumed it would charge the auxiliary battery. I will check it again.

 

And always blueSL comes throught. Thanks for the vast amount of information. Your like my number one resource for the FAQ I'm planning out for the R230.

Thanks zolton for the information, I've been eyeing a Cayenne for a long time now and I've been able to convince dealers to a 10k off on a loaded turbo. After hearing you say it's one of the quietest and most comfortable makes it reassuring because my dad loves Lexus trucks and he really wants a GX470 but hasn't driven one of those while we've drive both Cayennes.

 

blueSL was correct in his diagnosis of why the trickle charger did not work off the cigar lighter.
With the key out of the vehicle there is no power going to the cigar lighter, the cigar lighter becomes live when the ignition is activated, first touch of the start button on the keyless go system.
Although it seems obvious now why it was designed this way, blueSL has to be congratulated for bringing the obvious to attention.
Have not looked for the electrical outlet in the trunk to check yet.