Hi, I have a battery tender (CTEK) wired directly to the accessory battery in my 2003 SL55 whilst it's laid up. I watched a youtube video by 'Mercedessource' which suggests he would connect a tender to both the primary and the accessory battery. My understanding (based on a W211 car) was that they were connected inline and there was a battery management module in place. Therefore I would assume only a single connection to a charger would be required and the choice of battery is irrelevant?.

Can anyone clarify this? I'd rather not have to unplug two connections before using the car!

The starter battery is totally isolated from the rest of the car, and if fully charged, it do discharge about 3% pr. month. It is though recommended to charge every 3 month, just to extend the life of the battery.

I recommend that you install 2 connectors, one on each battery, and just move the charger 1 day a month to the starter battery.

I use two CTEKs since there is no in-line charging between the two batteries. It’s still unclear to me which battery the alternator charges. If only the starter battery, why not the accessory battery, too? How otherwise does the accessory battery get charged?

The alternator charges the rear battery directly, and it charges the starter battery through the power supply control module.

Unless a vehicle is going to sit for many months without use, there is no need to maintain the starter battery with a charger.