SL/R230: Battery Tender connection, main or accessory battery?
Thread Starter
Junior Member
Joined: May 2017
Posts: 61
Likes: 11
From: South West
2003 SL55, 2004 E320 CDI Estate
Battery Tender connection, main or accessory battery?
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!
Can anyone clarify this? I'd rather not have to unplug two connections before using the car!
Senior Member
Joined: Aug 2019
Posts: 340
Likes: 189
From: Europe
09 SL500, 03 E320 CDI Wagon
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 recommend that you install 2 connectors, one on each battery, and just move the charger 1 day a month to the starter battery.
Senior Member
Joined: May 2019
Posts: 280
Likes: 37
From: Lincolnshire, IL
2005 SL55 AMG
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?
Banned
Joined: Jan 2006
Posts: 1,982
Likes: 393
From: Atlanta, unfortunately
'09 S600, (2) R129 300SLs, '03 SL500, '03 SL55
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.
Unless a vehicle is going to sit for many months without use, there is no need to maintain the starter battery with a charger.
