SL/R230: Battery tender hookup
Thread Starter
Junior Member
Joined: Aug 2025
Posts: 16
Likes: 0
From: Michigan
2007 SL550
Battery tender hookup
The last battery tender that I used, on an old Land Rover, seemed to eventually wear out the battery, so I'm apprehensive about hooking one to my 2007 SL 550. Just ordered a CTEK tender and wonder if there is a recommended hookup scheme: starter battery or consumer battery? Any tricks to this?
Thanks.
Thanks.
MBWorld Fanatic!
Joined: Oct 2019
Posts: 5,154
Likes: 1,769
From: Colorado
G550
If you have jumper terminals in your engine compartment, they are the recommended point for attaching a battery charger/maintainer. Contemporary maintainers are designed not to overcharge batteries, so you can leave them connected without concern.
Super Member
Joined: Sep 2021
Posts: 537
Likes: 116
From: Ann Arbor, MI
2004 SL600
These newer battery chargers and tenders are much more gentle on electronic modules than the old style chargers that came on wheels.
I have battery tenders on both batteries and when I'm experimenting with tunes and such with the ignition on for a long time I connect a 10 Amp charger.
The only possible hookup scheme is positive to positive, negative to negative. On these cars it is nearly always the consumer battery which drains.
Where are you in Michigan? PM me.
I have battery tenders on both batteries and when I'm experimenting with tunes and such with the ignition on for a long time I connect a 10 Amp charger.
The only possible hookup scheme is positive to positive, negative to negative. On these cars it is nearly always the consumer battery which drains.
Where are you in Michigan? PM me.
MBWorld Fanatic!
Joined: Dec 2005
Posts: 4,057
Likes: 341
From: NJ
2009 SL63 AMG
IIRC, factory procedure is just keeping the consumer battery on a tender.
During its 3 seasons of use, my SL never sits for more than a couple weeks so I don't bother with a tender.
For Winter storage, however, I disconnect both batteries and keep them on tenders.
During its 3 seasons of use, my SL never sits for more than a couple weeks so I don't bother with a tender.
For Winter storage, however, I disconnect both batteries and keep them on tenders.
Super Member
Joined: Sep 2021
Posts: 537
Likes: 116
From: Ann Arbor, MI
2004 SL600
I’ve had both batteries go dead on me. I suspect the rear went dead first and then the front connected and also drained. So yeah, only the rear technically needs a tender. When I bought the car several years ago the previous owner had wired in front and rear tenders.
I have spent many hours chasing down parasitic drain but still at about 150 mA which drain the rear in two weeks. If the car will likely sit for over a week I connect the rear tender.
I have spent many hours chasing down parasitic drain but still at about 150 mA which drain the rear in two weeks. If the car will likely sit for over a week I connect the rear tender.
