Hoping someone can help! Brand new battery is losing charge almost immediately after driving - battery checks out, alternator checks out, and we can’t figure out what the issue is. We’ve seen the battery jump from 5v to 15v continuously, and are seeing strange errors of too much current and not enough (pictured below).

My first instinct is voltage regulator, but it’s not super obvious.

car is a 2014 E350 sedan - 65k miles.

 

What sort of symptoms does this MY'14 car comes with ?

Reading 5V on a new car battery sounds like skewed measurements, exactly what all the painted GND posts can deliver: drop voltage.

I don't believe these cars can run a "real 5V"... so it's got to be a "false 5V" reading, right?

Have you observed voltage on the iCluster while driving .... What does that show ???

As far as doing a "Battery Reset" we can asume it was done recently during the battery swap. It's usually good at helping crazy cars that drain batteries from "keep-awake".

Are you familiar with the electrical diagram between Prefuse and R-SAM... pull-it.
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As of now there are no symptoms other than when the battery drains completely - seemingly only if the car sits for a few days.

it’s been at the indie shop since Monday AM - we’ve tested the battery and alternator which both check out. We tried clearing those error codes to see if they’d come back but last 2 days it has started up fine after the mechanic fully charged the battery back up and no errors or symptoms.

 

When you say you've seen the battery read between 5-15V, where are you measuring that? Need to verify at the battery and then work backwards as you may have a faulty ground or relay downstream which is leading to voltage drop.

Also losing charge immediately, what does that mean? Do you suspect a parasitic draw? Car does not want or struggles to restart?

 

it might’ve just been fluctuating because the battery charge had been run completely down. This fluctuation occurred when the car was running the day I jumped it and brought it to the shop. When we put the meter on the battery was jumping immediately back and forth from 5 to 14ish.

Since then the mechanic has charged the battery and validated the battery to be okay, as well as the alternator. It has been in the shop for 3 days now and the mechanic said it has started right up the last 2 days. We have also verified there at least isn’t an obvious parasitic draw.

I have some aftermarket “ghost lights” installed - essentially a light that goes in the door to shine the logo onto the ground when the door is open. Any chance those lights could be malfunctioning in a wat that would be intermittent or would it usually be an easy draw to spot?

Otherwise maybe its a firmware update and some weird software issue going on? I’ve heard the mbrace system being disabled causes issues after you do quicktests. Maybe the Rear-SAM has an issue? Maybe the fusebox itself or relay?

Incredibly frustrating to have this many question marks at this point. Otherwise been a great car, just hoping to get it back to being reliable again.

 

J,
I hope you have a DC clamp meter and I am certain you have a digital voltmeter and a proper battery charger, to define your finding of OVERNIGHT BATTERY DRAINING.
Poor charging can be misread as OVERNIGHT BATTERY DRAINING, depends how and where you measure amps & voltage.

If you have the tools, I can point you where to start to first confirm OVERNIGHT BATTERY DRAINING , aside from potential poor/screw-up charging from your alternator.

 

If this started following a battery change, I recommend inspecting the battery tray and all wires in the area. On my C-Class, a battery change resulted in parasitic drain that I eventually traced to a small, frayed wire in the battery tray under the battery itself. I had no idea what this wire is for, but the exposed brass was grounding on the battery tray. I repaired the wire, routed it outside the battery tray, and no further issues.

 

..... I bet they replaced the battery guessing it had to be bad to act up

I think these cars are like Japanese ORIGAMI folded papers... the complexity is hidden by the simplistic look.

Jokes asside, XENTRY has a provision to help troubleshoot "overnight drain" issues. These car computers can report their power consuption and sleep timings. This is made possible by a power shunt on the major modules... I know 'cause I resolder them when I see them.