I thought it wise to change both batteries since they were the original 2015 ones. The strange thing is, I’m suspecting the new starter battery has a problem in holding a charge. Before, with the old one, I could trickle charge with a tiny 0.8 amp battery tender and it would turn green after about 24 hours. Now, it will not turn green. I bought a new trickle charger (1 amp) same problem. Will not charge to float. After a 30 minute ride today I tested the voltage on the starter battery at 12.5 v and the little one in the trunk at 13.05v. I have a blue driver obd2 plugged in and it tells me I’m getting about 14.5-14.7v while driving so it’s not the alternator. Is this normal, or is one of the new batteries deficient? And how do I tell which one since I assume they are both being charged if I charge the main one? One thing I notice is that the start/stop is working again in eco mode. It hasn’t worked since I bought the car last year. Although it does take 10 minutes of driving before the start/stop starts working. I assume because there is not enough voltage. Even after being in the trickle charger all night!

 

I have a 2016 e400. Maybe a similar issue. When I installed a pigtail to make it easier to put a battery maintainer (not a trickle charger) I started getting “aux battery malfunction “ messages about 5-10 seconds after starting.
No drivability issues.
I checked both batteries and voltages were within spec. After not using the car for a couple of months, I started it and the error message was gone only to reappear in about a week of normal use.
I suspect the aux battery may be on the weaker side of “ok”.
Similar to you, my battery maintainer takes about 24 hours to go “green”. I am using a NOCO device.
I intend to replace the aux battery shortly but have been reluctant to do so, since I am not certain that is the culprit.
Depending on the wisdom of the list to see what / if it could be something else. I prefer not to easter egg this issue.

 

You might have the same or similar problems I had. I had two issues really. One is the car would suck a lot of power while it was off, so a 1A charger would not cut it. Plus I had an issue where it would often just stop charging while I'm driving. Apparently this is somewhat normal, which is not doubt an emissions thing, but sometimes they won't start charging again. So mine would charge normally most of the time, but now and then it would basically just stop. My point being you have to monitor the voltage over time, not just a quick check. Mine would also often be up at 14.7 because it was working hard to charge the half dead batt thanks to all the other bs that was killing it.

The power mine would suck when the car is off is just one of many problems you can expect with an electronic car. Want to take something that works fine and make it problematic? Add electronics. Apparently I have 48 computers onboard, and I don't believe there's ever been a single day they all worked, because how could they. That's like having 48 co-workers and nobody calls in sick, it just doesn't happen.

So I suggest getting a more powerful charger. I bought one, which has a display showing charge rate. I also have it connected to a Wifi outlet that records power usage live so I can not only see it remotely, it records power usage over time.
This is the one I bought, which works fine and is only $20.
I extended the 12V cables so I don't need to drag the charger over to the car. I also cut off the clamps and put a plug on it, and matching plug on the car.
Now I don't need to open the hood or anything, I just plug the car in like it was a giant phone.

Optionally you can see the current flow when the car is off with a Multimeter or something to physically measure it. Another is Xentry will tell you, kinda, more or less.
Once I realized I had a power drain, and Xentry was completely useless to troubleshoot it, I used a Flir to look for heat which told me exactly what was using the power. I still didn't know why, only what was using power.
I no longer have these problems, but if I don't plan on driving it for three or more days I'll plug the charger in just in case.
I'm sure more expensive chargers work fine, but being electronic, they will fail. I'd rather buy, or replace, a $20 charger than $100+ charger. Mine is now 2.5 years old, always plugged in, and so far so good. Plus, for that price, I bought a spare. As a bonus, when troubleshooting, the car can draw more like 15A, so I'll plug both of them in and I'm good to go.

Also, I have never had any kind of battery warning, no matter how dead either of the batteries were. The either car starts, or it does not, the end.
Another thing, that you may or may not care about, is the turbo needs voltage. The turbo solenoid to be exact. If the car is not charging, or the charge is low, you may not get full boost. Plus, as a bonus, when you floor it the car will reduce alternator output that again can hurt boost. It depends on some factors but low voltage absolutely hurts boost in my car.

 

Usually some aftermarket batteries and alternators drop you down about.5 volts go for a factory Mercedes Benz battery

 

Wow that is crazy cheap for a 10 amp trickle charger. I did notice an increase in boost with the new batteries. Also, the NOCO 1 amp charge is finally doing the job in fully recharging in about 24 hrs. Maybe the battery was not fully charged when bought. So hopefully, all is well now. Although I’m still getting the low voltage fuel pump code 🙁 I also unplugged the OBD2 sensor that I had permanently plugged in. Maybe that made the difference 🤷

 

Maybe your battery was too shot out to get that little charger to go green. I'm surprised it lasted as long as it did anyway. I had an Optima last pretty frikkin long once, but normal batts I generally I get 2-3yrs.
My Start-Stop stopped working long before I replaced the batt, and like you it worked with the new batt. I hate that function.

Not sure about the BlueDriver, plus it's against my religion to use any Apple product :p
I use an extension cable for the OBD port because anything plugged in would hit my leg. The extension makes an immediate 90 towards the pass side and only sticks out the port like 1/2", if that. Fyi.

 

It’s actually quite the opposite. The charger would go green with the old battery, not the new one. But now that it has, I can sleep better. I didn’t know blue driver was an apple product? Not to my knowledge. But I wonder if it was still pulling power from the battery after being powered off. My phone wouldn’t connect to it after I turned the car off, so…🤷

 

I may be dated, or just wrong, but from what I recall in the past was people with a Bluedriver used Apple. At least one co-worker told me, some five or so years or so ago, is it was the only thing he could find that worked on his iphone. He was also bummed because it was >10x as much $ as my Android dongle. Ironic because his phone and laptop also cost 10x as much as mine so you'd think he'd be used to it.
Way way back in the day non-Apple users would tease Apple users with; "Play any good games lately?" Not sure if it's still the case, but when Mac users were finally able to play some games they had to use a Windows emulator to do it. As if Apple is simply incapable of gaming at all?
One thing I still tease Apple users with, and one reason why I would never use Apple, is the 1984 commercial that shows Apple freeing the Orwellian people from Big Brother.

I too would assume if it won't connect that it's off, but I would also check to see if it's warm. If not warm at all then I wouldn't worry about it. Otherwise you'd have to actually measure the power to know for sure, which few are interested in doing. I simply unplug the dongle when not in use. That's part of why I use that extension cable, because I do use OBD all the time and that extension makes it substantially easier to plug into.

 

youre not entirely wrong since I do have an iPhone, but I think they make them for Mac and android. And it was one of the cheapest out there 😜