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Battery AGM 5.5 years old. Charges up to 100% with charger CTEK. In a week not driving it’s at maybe 12.5v . But driving I monitor voltage with a battery monitor, it won’t charge to 100% easily and I see battery voltage jumping 12.5-14.5 over and over . Regulator is a few years old . Alternator and battery test ok. Is it just an old battery ?
Have you ever heard the expression “no one wants to know how the sausage is made or how the laws are made”
Apparently you do so let’s take a look behind the curtain and welcome to OZ.
Mercedes has picked a state of charge that they believe is the best for us and it isn’t necessarily 100%.
I can see that the voltage output of the Alternator is switching between charging and off. If the START/STOP is enabled then it will do this. Also the Alternator may be turning on and off so that it is running when decelerating or going down hill and then turning off when accelerating or climbing a hill.
To know what is really happening you need disable START/STOP and run a test with the cruise control on a flat section of highway.
The VOLTAGE DIP when starting looks good
The voltage at idle is slightly higher than the range that your app is looking for. The actual target voltage will vary depending on the temperature of the battery.
Based on your data I wouldn’t be concerned about the battery or the Alternator.
One test that I typically do beyond what you have already done is to load test the Alternator at idle.
Get a volt meter and measure the voltage at the battery and then start turning on loads: high beams, HVAC fan to high, heated seats. Rear defroster. The voltage will likely drop but it should get most of the way through the loads without dropping too much
good luck
Paul
PS thanks for sharing your data and let us know what you find out.
This data is from a post I made on a different forum 5 years ago
LR2________11.07 Volts (New battery) A4 2.0T_____10.64 Volts MFG --> April 2019 328i________10.20 Volts Original battery from 2015 Jetta 1.8T____10.53 Volts Original battery from 2015 S80_________10.52 Volts MFG --> May 2017
The A4 cabriolet got crushed in front of my house and now I have a E400 Cabriolet.
The 328i needed a replacement battery and the S80 also. I can’t remember if I changed the battery in the Jetta.
The LR2 is now in need of a new battery and I will replace it when I go to see my daughter in California in a few weeks.
When I started my E400 the current was high but it quickly came down (see the next picture). Note the battery temperature, the voltage should drop at higher temperatures. After a minute or so the current going into the battery dropped to a very reasonable low level.
6.0 Amps going into the battery
EDIT: the pictures were taken exactly 5 minutes apart.
EDIT: I took these pictures back in January when I was just learning about my Mercedes. It wasn’t hot at all.
Take care
Paul
PS I completely agree with the assessment above about changing the battery before it leaves you stranded.
it is very hot today (I live in Las Vegas) and I wanted to confirm that the voltage would be lower at higher temperatures.
If you research AGM charging you will find that the optimal voltage decreases when the temperature increases.
If possible you should verify that your Mercedes is doing this
Here is the result for my 2018 E400
13.9 Volts at 98F is consistent with what I have found searching for voltage vs temperature for AGM charging
EDIT: this picture is from today the pictures above were from January. Due to the temperature change the charging voltage also changed (this is done to protect the hot battery and to make sure that the cold battery gets a sufficient charge)
I really appreciate your post.
Now I have some data that shows my Alternator is working well
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