I had my '16 G550 parked for about 2-3 weeks without a Battery Tender, opening the doors a few times here and there to get stuff out of the trunk.

When I eventually started it up, it started up fine, I drove 20 minutes, parked it for a quick errand. Started up again for a 5 minute drive, parked it for another quick errand. Then, when I started up again, got the message: "Auxiliary Battery Malfunction" and the auto start/stop and auto hold stopped working. I cleared the message through the menu buttons and drove about 30 minutes on the highway and the message never came back. The battery is about 2 years old, and the starter battery is less than 1 year old. It would seem having it parked for 2-3 weeks let the battery drain down.

Out of curiosity, I plugged in a 4.5A Battery Tender, and it got to 80% charge in about 10 minutes, and full charge in less than an hour. I then plugged in a cheap Chinese-made voltmeter USB plug in the 12v center console socket and kept an eye on it for a few long drives. I noticed the following:

• Right upon starting the car, the voltmeter reads 14.5+ volts for about a minute, then as I'm driving it drops down to 13.4v for a minute, then settles down to 12.4v. When I am on the throttle, it stays around 12.3v, and when I coast it creeps up to 14.9v. At a stoplight with engine running, it goes back down to 12.3v.
• Now I generally keep the auto start/stop off and never use it, but if I turn it on and let the engine shut off at a red light, it sits at 12.2v for the duration, then when the engine starts, it drops to 11.8v for about 10 seconds, then jumps up to 14.9v for maybe five minutes, then goes back to prior behavior of 12.2v while on the throttle and back up to 14.9v when coasting.
• On occasion, and I don't know why, it stays rock solid at 13.5v for several minutes regardless of whether I'm on the throttle or coasting.

I'm wondering if anyone has monitored their voltage and seen the kind of behavior I'm describing. I've driven about 100 miles in several trips since the error and have not had an issue. No error, no accessory malfunctions, nothing. Seems this is the battery management system doing what it's supposed to be doing, though having bought my first car in 1999, I've never seen voltage fluctuations like this before. Then again, I've never owned a German vehicle.

 

The voltage fluctuations you recorded don't seem unusual. When being charged from the alternator or from a charger, the battery terminal voltage will elevate to ~14.5 volts (unless severely depleted). Normal terminal voltage when fully charged is typically ~12.5 volts. Immediately following an engine start, a drop below 12 volts can be expected. 13.5 volts immediately after removing charging is normal as well. Regarding your aux battery, it might have discharged significantly if you left the vehicle parked for 3 weeks.

 

I guess I'll start using the Battery Tender if I leave it parked longer than a week. Not something I'm used to worrying about with previous cars, but I suppose it probably has five times the computers, sensors, and other consumers than any vehicle I've previously owned.

 

User "telepena" is an AI bot. Reported.

 

Get your starter battery load tested. Is it AGM as it should?

my original battery was getting weak although it always starts the car. When starter battery gets old, voltage may be fine but the eco mode will seldom work.

the aux battery should last real long. Mine is 8 years and still good.