I recently purchased a new (to me) 2020 SL450 and am looking for a recommendation for a battery tender as well where to plug it in - trunk or somewhere inside the car. Thanks, Ron

THere are connection points (jump start) under the hood/bonnet that MB recommend you use (in the owners manual).

Many people use a CTEK charger/maintainer(the unit sold by MB, Porsche, Bentley and some others are all rebranded CTEK) . My SL is currently on one.

I use a Deltran Battery Tender. Probably the most popular brand. Picked mine up at Costco.

Have been replacing the older Battery Tender and CTEK units when the fail with Noco Genius1 units. We have a mix of cars/motorcycles in our storage with various battery types, and the flexibility of the Noco to support Lead Acid, AGM, Lithium battery types make them very convenient to use.

I have mine connected to the battery in the truck, and just added one to my Aux battery when I replaced it last month,

I have one charger for each battery on an R231. I'm not sure about the R230 but on the R231 the batteries are separated from the charging post in the front. The trunk battery is directly connected to the jump post. The aux battery is not. The r230 setup is different.
Simple test is to hook up a charger to the jump post or the trunk battery (same circuit) then read the voltage at the trunk battery. It will be anywhere from 13.8 to 14.6 volts which is the output voltage from the charger. Then test the voltage at the terminals of the aux battery behind the passenger seat. It will read no more than 12.6 volts which indicates it's not on the charging circuit.
Remember, this applies to an R231.

I’ve had my car since new & it’s now on it’s 6th winter (done 19,000 miles) on a CTEK connected to the main battery as per the owners manual. Never charged the auxiliary and last time I checked (about a year ago) it was still holding it’s voltage ~13.8v.

Thank you all for your replies. I ended up attaching a CTEK maintainer to the main battery posts in the trunk and will test the auxiliary battery’s voltage next week to see if it gets charged.

Ron

Hi all,

is there a particular CTEK charger you guys use? There are a bunch of different ones on Amazon. I actually used the cigarette lighter version (Porsche brand, but a CTEK) on my ‘15 911…anyone gone that route? I probably need to check and see if the battery is an AGM.

Any advice is appreciated, as the SL is now in stall 3 and won’t see daylight until probably April.

JHop

Mine is a ~14 year old MSX7 (i.e. 7 Amp) which I connect to the jump start points (as recommend by MB for charging).
If it is purely for maintenance you can easily use a much lower Amepere ratting.
I don’t believe the 12v socket route will work as they are supposedly isolated when the engine is off.

I am using this one:
It comes with both battery post connectors as well as alligator clips.

Ron