So after driving 1800 miles in four days I get a message that the “12 V Battery is Low”. The car is just about due to go in for it’s 40,000 mile servicing. Just does anyone have any comments as to what might be going on. Thanks

 

If you frequently drive short trips, the starter battery doesn't have a chance to charge enough, try aiming for longer trips. If your vehicle was parked for a few weeks without starting and running for a long trip, it may get drained as well. To mitigate this, turn on standby mode if you are planning to park for a long time. Alternatively, you could buy a trickle charger and plug it in.

 

Hundens, If you already tried charging the battery and still have battery light, have a mechanic check out the battery and the charging system. Besides that, you have a drain on power when car is off. Did you ever confirm the alarm is real by checking the battery voltage while alarmed.

 

1800 miles, 75 mph for 6 hours a day, for four days should charge the battery. The car and battery are three years old. I have a battery manager and will put it on. I know these cars can be quirky which is why I posed the question. Thanks.

 

when you put on the battery manager, how long does it take to come to full charge? And what size is your battery manager? How many ampere?

 

Definitely verify the voltage on the engine data screen, but something doesn’t sound right for it to be warning you with that much recent driving. I also have a 2019 wagon and it mostly sits (recently crossed 10k miles on the odometer). I’ve never seen this warning. I do have a ctek battery minder, but have rarely used it unless I knew the car was going to sit for a week or more (and remembered to go hook it up).

 

By any chance you do not have a dashcam hooked up for constant power ON?

 

I had the same issue, but with far less mileage and far less use of the car since the start of the pandemic. Initially I would drive the car for 20 minutes or so to recharge the battery and get rid of the "see manual" message on the dash. But eventually the battery died completely, and on October 4, 2021, I had to have the car transported to the dealer. According to the invoice, they replaced the "starter battery" and "on-board electrical system battery" under warranty (which was just about to expire) and I picked it up on October 7.

No problems for a couple of months, then I started getting the "see manual" message again. In January I bought a trickle charger so I didn't have to drive the car simply to charge the battery, and that did the trick — until it didn't. Managed to get the car started and took it back to the dealer on February 10, 2022. They kept it a week to check the battery over time, and I picked it up on February 17. They again replaced the "on-board electrical system battery" — according to the service documents, there was a "fault in battery/fail test." Although the car was out of warranty by then, the battery was again replaced without charge as "goodwill/parts & labor."

I've had no problems with it since then, no "see manual" messages, haven't had to use the trickle charger at all, and still driving only about 50 – 100 miles per month. But that raises a couple of questions in my mind: What would cause two batteries — one three years old and one brand new — to fail in such a short time? There's no indication on the service docs that there was some underlying problem, and no related parts were replaced — the service managers simply said the batteries died. Does MB use crappy batteries? Or was it just my bad luck?

 

Judge, thank you. I think there is a weak link in the system that you and I have experienced. I went thru the “see manual” battery manager routine six months ago. The fact you had “goodwill” coverage suggests to me it is a known problem.

 

Good point, Hundens.

OTOH they may a bit concerned about my mental stability 🤪 — I threw a hissy fit shortly after my October 2021 service because my front suspension began making loud, concerning creaking noises even though the service docs included "performed multi point inspection" and "check steering gear and linkages, check front and rear suspension system and ball joints, bushings and arms." Although the service manager persuaded me that the problem would not have been visible during such checks, they nonetheless offered to split the bill, not chagrining me for the parts (lower control arms and related), multi-point inspection (sure), car wash (which it needed), or Lyft to and from my home. Still charged me $500 for the required labor to fix the suspension and another $100 for the "quick service.," plus $79.95 for an alignment. That was about half of the original (pre-hissy-fit) total, and as a professional mediator I thought it was a reasonable compromise as I could not say for sure that the problem even existed in October 2021 when the car was still under warranty.

 

F/U. The message flashed once today for the first time in 10 days. Since the issue seemed to go away I had done nothing. I have a WeatherTech 4 Amp battery manager. It is also sold by Costco. The car gets driven about 25 miles a day every day and I lock the car because I think it goes into a deeper sleep mode quicker. I will have the local dealer check things out when it’s in for its B service next month.
PS. No dashcam

 

LIVE BATTERY DISPLAY:
These cars have a "maintenance menu" option that anyone can use to display the live battery sensor data while driving...
From there you can see for yourself exactly what is happening with your particular car: draining or charging?

> Voltage should go from [14.9... 14.4... 13.7... 12.6V] with charging current from 10Amps down to near zero when fully charged.

> If you witness high currents greater than 20Amps [30... 40... 60... 90Amps!] and low voltages below [12.3v... 12.0...11.7...] you have a dangerous discharging issue that's wasting your electricals (battery, prefuse, alternator...).

Extended drive cycles:
By experience the car charges normally for about 40mn each time "keyless is cycled ON/OFF" then after driving a while the ECU & SAM modules goe into a soft-crash when voltage swings BELOW 12.0V....

Fasten your seatbelt and take a look at your maintenance display to see what sort of Volt/Amps data you're getting: High/Low?

 

my E300 hybrid (6 months / 12.00 kms) is behaving weird as well:

at start the voltage as given on the display, is often around 12.0/12.5
then after 30 minutes or more it might go up.

or, if i set the ventilator to high, voltage will go up too (13+)

it doesn't matter if i drive electric or petrol

It's a bit of a problem as i have batteries for my dashcam, which won't charge under 12.5v

 

Can somebody clarify for me :

in the morning my battery is at around 12.2v for about 30 mns
the last 10/15 mns of my drive it will be at 13v

But when i dirive back in the evening the battery goes straight to 12.8/13v and wil stay there for the duration of my trip

 

You've got it! A VARIABLE charge is to be expected as car voltage is managed. The SAM's use battery sensor plus consumers information to signal ECU for a desired voltage. The target voltage is then requested from the alternator. Most cars and trucks use this Bosch design for over 10 years.
Seing alternator voltage around 13.5V helps supply the power of heavy loads like: engine fan, A/C or headlights.

> The deep battery discharge below 12.6V is dangerously abnormal. Alternator is supposed to hold supply voltage at 12.6V. Right now alternator is getting turned off and letting the car voltage drift down low with 100% load coming out of system battery!
This condition crashes CAN modules (random weird errors), abuses prefuse and battery with abnormally HIGH currents.

> You are missing the AMPs numbers to understand the charge/discharge current of system battery when voltage changes.

> Until you can get this fixed, the safe work around is to turn the "Headlights ON" to request 13.5V.

> The normally working condition is when car is able to supply a stable 12.6V or above.

Now you know, more power to you!

 

thanks, will have it looked at during service

 

The headlight trick doesn't work..
But like clockwork, after 30 minutes the battery shoots up to 12.8/13v

 

...meaning your battery voltage stays low around 12.2V regardless of headlights being ON?
Below 12.6V the battery is being discharged.

Did you read any battery Amperage value ?

 

yes correct . this is what the car itself reads + i also have a cigar plug voltage meter

And Like i mentioned before: It doesn't matter if i drive electric or petrol / ventilator on max does raise the voltage to 13v

how/where can i read read the battery Amperage value ?

 

> Display menu:
-- To display your battery current, you have to call the "Maintenance" menu before starting the engine.

-- This will show you effectively the Power (Voltage x Current) drained or charged from system battery.

-- The current measures how discharged or charged the battery really is at a given time.


> Unregulated disfunction:
-- The disfunction is about (the Rear-SAM) not being able to control 12.6V out of Alternator. As you noted whenever a voltage greater than 12.6V is desired the ECU mostly gets it.

-- The fancy Alternator control is a distributed system that lacks fail safe control. Consequently it's UP TO YOU TO AVOID this unsafe operating condition: 12.2, 11.7...


> Ignore decimals:
​​​​​-- Measuring voltage from cig-lighter is inaccurate by way of the heavy current path through PreFuse distribution.

-- Regardless it can be a good indication (+/- 0.7V) of what voltage range is powering all the computer modules.

 

How do I access the service/maintenance menu?
"home 5secs / ok 5secs / home / swipe all the way left" does not seem to work

 

lt can be a little bit challenging at first.

Key press sequence needs to be executed specifically before starting...

Check this topic on YouTube for a tutorial that match your M/Y Display menu.

This will show you the exact numbers from the battery sensor.

 

unfortunately i can't get into the service menu .
But like clockwork, every morning after exactly 29 minutes the voltage shoots up from 12.2v to 13v
Weird.

 

that sounds like this may be your R-SAM requesting the ECU to kick up the ALT voltage from 12.6V to 13.7v to compensate power output for 100Amp engine fan.

You are not seing exact voltages because of the inline voltage drop, R-SAM is helping offset.

Simply put: this voltage range is ok.
What you never want to see is 12.3, 12.0, 11.7... while Hwy driving or anytime for that matter.

 

to clarify: i'm on full electric when this happens (though when on petrol, the exact same behaviour happen)