GEaston

Ok so in my intro I said almost everything works. Only issue I have relates to the Consumables battery and the 10 min wait each time I start the car.

if I leave the car 24 hrs the situation is this:

front battery - 12.7volts, no issues, car fires up first time every time.
Rear Battery - 7.4 volts (well under 12+ required)

When I start the car, rear battery immediately reads 14.7volts, but I get the reduced functions for the first tens mins. After that the errors go and roof etc all work. If I switch off the engine then immediately restart, I’m back to the reduced functions and 10 min wait.

Both batteries are new (Bosch) and changed end Feb 2021.

Is my issue rear battery related or a fault with the battery management sender unit in the trunk?

Comments gratefully received. Car has only 75km and is otherwise immaculate.

Graham

MikeJ65

It sounds like you have a current drain on the rear battery. You need to figure out what is draining it.

NHSl550

Even though it's new, it could be a bad battery.

glasgow2

Is the new consumer battery an AGM? My understanding it has to be AGM to keep enough power.

GEaston

AGM? Sorry I’m not familiar with the term. Battery is aftermarket Bosch fitted in Feb by the previous owner. Not a Mercedes supplied or fitted part.

tonylinc

Well you could look at the battery for a model number and google it to see if it is an AGM type battery, and whether it's the right battery for the car. Is it the right size, group, etc.?
Also look at the terminals and see if there is any corrosion or white powder on them. It has to be an AGM battery, for this application. Absorbed Glass Mat is a different, tougher battery technology.
Often the model number may have AGM in it. If they got a Bosch battery, it's likely it's an AGM, but who knows! There are monkeys out there.
Now, typically when it takes ten minutes to recharge the trunk battery each time you drive the car, that means the battery is ailing and it's time to replace. I buy giant Duracell AGM batteries for all my Mercedes, from Batteries Plus Bulbs, they are even black with coppertop looking motif ... and I have never had to look at the battery again.
If the battery is failing, it isn't necessarily an indicator that you have a battery drain. If you replace it with a new battery and you know it's good, and it still does this, then you might have a drain.

kbdixon

Nose around the battery and look for a date code, that should give you an idea of when it was replaced. My ‘08 had both batteries replaced prior to my purchase, since then I have replaced the consumables myself after roughly 4 years and 17k miles. Because of all the things that battery does it’s shelf life even with a proper battery tender is not great.

Keith Noon

I had exactly the same situation. Brand new consumables battery. The off line message came up after parking up 2 days. The car had random warning lights and was generally a nightmare. It turned out the battery was a none AGM type. So, I swapped it for an AGM type and have never had a problem since. The car can stand for 3 weeks without any off line message. The battery I took off is still in use on a friend's VW van.

Frederick NL

Your battery is very likely not holding its charge. To drain a charged decent battery to that low level within minutes, would take something similar to a short circuit. You should get it tested, off the car (IMPORTANT: disconnect rear battery first, and afterwards connect front battery first).

My seller had a new non-AGM Mercedes brand battery installed (probably a Varta) just before he sold the car to me. I have never had any problem with it. The car is on a trickle charger at home, not during holiday travel. I will put in an AGM, once I need to replace it.

Campa

Exactly same thing happened to me. New battery did not solve problem because it was not an AGM. Swapped it for an AGM and no problem since

GEaston

Here is the battery. As you can see it’s new…. Took the car on a 30 min motorway drive today, consumables battery fine after 10 mins then I parked and left car for 90 mins. To my surprise it was fine when I returned. So it held a charge for 1.5 hrs which is a first. I only drive the car once a weekend (every weekend) and today rear battery was a 6.5v before start. I can’t trickle charge because no power points in my condo garage park.

next2pool

That is unfortunately not the correct battery--it is not an AGM. This battery is designed for newer cars with "automatic battery management" systems that change the charge rate for improved fuel economy.

GEaston

That’s not unfortunate sir, it’s great news because that’s almost certainly the cause of my problem. Not a drain so much as wrong battery so hopefully swapping it for the correct one wil fix all my issues! Thanks - I will let you know the outcome of the switch.

GEaston

Id like to express my thanks to the experienced members of this forum for offering the advice as above. Today I switched out the relatively new but incorrect battery for the AGM battery advised by next2pool above. No issues with installation (ran the car while switching) and now have this one installed. Delighted to have no error codes, and when I park the car and return hours later simple stuff like using the push button start even opening the car with the key fob are possible!! Thank you all.

Frederick NL

Hmm - changing a battery with the engine running, I don't know if I am too enthousiastic about that. Any opinions here?

Keith Noon

Scary. You should never disconnect a battery with the engine running especially on a car like an SL that has a lot of sensitive, fragile electronic equipment. The OP is very lucky to have gotten away with it. I've no idea why anyone would do this or advise anyone else to do it.

tonylinc

I would not have the engine running while removing the battery, for fear of burning out the diode in the alternator; look at cost of another alternator. Plus Labor!!
I've heard of replacing a trunk battery while having connected with little wires and clamps, a separate 12V battery just for the duration of having the trunk battery disconnected, replaced and reconnected. And I've heard of using a CTEK connected while replacing the battery, to keep the electronics having a little power so they don't get upset...
But never heard of having the car running while removing a trunk battery. When I've replaced trunk batteries in R230s and W221s, I've simply disconnected the battery then reconnect. With no issues.
That said, my dash clock was a half hour off in my SL550 so I thought I'd come to the garage at 11:45, disconnect the battery, then re-connect at 12:00 and all would be well. I did that, went back in the house, and three days later came to drive the car, thinking the clock is gonna be right on the minute now! WRONG! It was four hours off now, and the battery was low and took 15 minutes to go back Online! Never a problem before, excellent battery. I'm suspecting that some other electronic got activated upon reconnect and what I should have done was, at Noon after reconnecting, I should have started the car and driven a few miles and then put away, and whatever got activated would have gone back to sleep. Trying to outsmart the electronics on a R230. LOL
Battery and car have been fine ever since, so there's no problem although the clock is still a few hours off. If anyone comments about the clock, I just say. "It's five o'clock somewhere!!"

Frederick NL

Some folks in the US, the UK, Germany and in NL recently noticed that all of a sudden their dash clock is totally accurate. In the original setup, that is (I posted about this a while ago). It all starts with the clock being insanely off, as yours is now. Fair chance that after you adjust the hours (or time zone, don't remember) in your Comand, you'll be good. For now.

Mendobroker

It is located under some carpet in the trunk. I sent it out to G7 computers, they repaired it for $275. No more issues since
It is Not the small black module also in the trunk

Mendobroker