Mazzy

My 2015 S63 typically sits for weeks at a time, so I thought it would be best to put a battery charger on it. I bought a CTEK MUS 4.3 and have been connecting it under the hood to where you would jump start it. Is this the best place to connect it and will it keep both batteries topped up? It looks like it's working and all the lights light up to green. I've also tried the testing portion of it, but it says it fails the alternator test.

Katie22

Yes the jump portion and a bolt on the engine ect for ground. With that said my battery tender does not appear to be good for the lithium batteries for long term use. I could be wrong but it appears they now have tenders just for these batteries.

Katie

Nice Ride

Yes correct place, and no it will not keep the small trunk battery charged. They disconnect when you turn off the car.
I picked up a CTEK hard wired extension cable and permanent connection, routed the wire to come out at the front of the hood between the headlight and grill. I have it coming out under the round hole under the hood near the front.. When not used, leave it under the hood. I'll try to send photo. What is also neat about the extender cable, it will act as an emergency break-away should you drive away and forget to disconnect it. Ask me how I know...

CTEK has a unit that will do standard WET and AGM batteries. The difference is where it keeps the float charge.

Posted by another member >>
AGM batteries operate at a slightly higher voltage when charged than flooded cell. A standard battery is considered fully charged at 12.7 V. An AGM will be more like 12.9. It's just a characteristic of the AGM cell. It's one way that sensitive battery monitors can be thrown off by the wrong battery type and is one reason the aux. battery has a "Replace only with same technology" warning label. <<

AbbyKay

I would like to understand this battery and battery tender situation better. My experience with my 2015 S550 4Matic, purchased January 2019 with 28,500 miles on it follows.

The first time I was going to leave the car for 3 months, I bought a GEL compatible charger/tender (another story) and hooked it up under the hood using the jumper posts. The charger NEVER went into maintenance mode for the 3 months it sat. The yellow charging light stayed lit. The car started and ran fine when I returned. I assumed that the constant load on the battery, even with the car parked, was such that my charger/tender could not satisfy the charger’s maintenance mode requirements.

Sometime the following year, I read somewhere that there is less drain on the battery of a parked car if it is locked (vs. unlocked). I did a test with the same battery charger/tender and found that the charger went into maintenance mode in less than a day when the car was locked. So, I decided that I should lock the car during our next extended absence – I did not do that during the time described above.

During this year’s absence, I locked the car and hooked up the charger the same way as before. Six weeks into our time away, I read this thread about needing to connect the rear battery directly to the charger. I became concerned that the rear battery might be disconnected when the car is locked (vs. when the car is shut off as the thread states) and that it had a load on it and was not being charged. I decided to unlock the car via the Mercedes app to try to get the rear battery charging again – or at least re-create the situation I had the previous time. Well, that didn’t work – I sent the signal, but the car would not unlock (I have a security camera in the garage and can see the car’s side mirrors). I wasn’t sure whether that was because of a bad cell signal or because the battery was dead. (I don’t know which battery does what).

I had a friend go to the house, disconnect the charger, try to unlock the car with the key fob (successful), and try to start the car (successful). At that point, I had her reconnect the charger, leaving the car unlocked. The next day – first time I tried – I could remotely lock/unlock the car with the Mercedes app. I left it unlocked. I cannot tell whether the charger is in charge or maintain mode. I could ask her to go back, but I’ll find out when I return.

All that said, I am hopeful that once I return in another several weeks, all will be good as it was the first time I had the charger and car “set up” this way.

So, my questions are:

1. What is the impact of locking the car vs. not locking it in terms of which battery is being charged?

2. What loads are active/inactive with the car locked vs. not locked?

3. Any explanations for:At this point, I’ll search this forum for the purpose of the front and rear batteries.

Once I return home, I’ll know if the charger was able to go into maintenance mode and may have additional data and questions.

roviw1

I have used ctek charger conditioners for nearly 10 years now, also a friend has 9 cars so he uses them all the time.
My W222 2015 S500 is always connected to a ctek even if the car is standing for a week unused. However with all the cars I have always connected them directly to the main car battery using the connector lead supplied. I bolted the lead to the battery positive terminal and an earth point ( just a bolt ) nearby the battery.
Same on my Mercedes GL and two jaguars XJ as well.
The GL battery is tucked well under the seat so also has a ctek extension lead to put the actual connector just hidden away under the front of the seat.
I have found this works absolutely fine and has not let me down at all. I can see the various charging lights when I need to as the units hang down and the connector extension lead passes easily through the rubber seal of the boot ( trunk ) lid.

jnash

Not sure if this helps with those of us with the smart Lithium Ion batteries. If the battery voltage gets too low (say 11 ish volts), the battery will go open-circuit to protect itself from discharging. When this happens the car appears dead. Cannot light anything up. To solve this, use the CTEK /Mercedes supplied charger, and press the 'reset' button for 10 seconds. It sends a signal to the car battery to wake back up, and then let it charge overnight. The other option is to jump start-up and let the engine idle for 20 mins, then turn off the engine, continue to charge the battery for several hours, then the Lithium battery should be restored and working to start the car.