A week or so ago I realized I had a leaking rear air bag on my '14 GL450. Got the new bag, replaced it Sunday. Ended up leaving it in the garage for a couple of days due to needing to wait to replace it due to "doctor's orders" for a minor surgery I had. It sat for 43 hours. When I went to start it to get it situated on my lift, no dice. Battery was as dead as it could be. I've let the truck sit before for 2-3 weeks at a time, and I've never had an issue with it not starting as soon as I hit the key.

I jumped it, got the air bag replaced, and then started looking at the battery issue. Pretty sure I didn't leave any doors cracked, etc, but with two small kids, anything is possible. It has been 'fine' since I jumped it. I've monitored the battery resting voltage all morning. It started out at 12.53 volts once I finished topping it off with the battery charger. Throughout the day, over the last 6 hours, it has dropped to 12.23 volts. Right on the fringe of being ok, but I'm keeping an eye on it.

Imagine my surprise when I discovered that according to the date code, the battery is the original battery to the truck from the factory, with a February 2014 date code, which corresponds to when my GL was built. I'm really tempted to just replace the battery at this point, as 9 years ?! feels like it's on borrowed time, even though it has never given me any signs or symptoms that it was on its way out until this weekend.

Part of me says to get the battery from Mercedes, but I also know better than to expect the next battery to last 9 years. I'll probably hit up NAPA and call it a day, but I intend to watch the one I have for a few days and see if it will get back to holding steady in the 12.6 volts range. After a full discharge though, I'm hesitant and feel like that's just the beginning of the end for it.

Thoughts?

Buy it from a dealer. They are competitive on battery prices for whatever reason. Then you know it is the right part number and not what Bubba thinks it might be.

4 years is my benchmark for battery life. I change them preemptively at 4 years.

I did consider buying it from the dealer for the reason you mentioned. It will be "right". Price is competitive, $250 at NAPA vs $325 at the MB dealer nearest me.

This is the first battery in my 35 years of wrenching on cars that I've seen go beyond 5, so I'm in agreement with you as far as preemtively dealing with it. I have a 2 day trip out of town next week, and a 2 week trip to Europe in August, and it will sit in airport parking both times. I am not interested in returning at 1am or with tired kids and finding out that it "was" at the end of its life cycle.

The original battery in my 2013 GL450 lasted 7 years, and I only changed it because it was old, not because it went bad. I think that being in the passenger compartment helps it last longer, as it doesn't get exposed to the temperature swings that a battery sees in the engine compartment. Was your GL garaged most of its life? That can help too. And your profile says you life in Philly, so that means a fairly moderate climate which can help battery life as well.
I replaced mine with aftermarket, and I plan on changing it on about a 5 year schedule if it doesn't go bad before then.
Finally, is your aux battery also original? I'd change that too, it's easy and fairly cheap.

Changed original battery from 2014 year ago, just didn't like starting sequence. Aux still original.

FWIW, my 2010 GL450 is still on the original main and auxiliary batteries. I was tempted to change it a few years ago but it still load test fine and I have no issues with it so I'm going to let it go until it dies naturally.

Good stuff, glad it is working well