I suspect I know the issue. A couple days ago, opened the door, climbed in to back the 2014 GLK out of the garage, I noticed the dash graphics did not display.

They did come on when I inserted the key and turn it to Run position (not start). Check the photo - they normally light up, by simply opening the door. Drove to grocery store, shopped ... same "no display" when I got in to leave. There was no hesitation at start up when cranking. I also stopped for a gas fill up, same scenario.

Got home, put the battery on a trickle charger. The dash now lights up when opening the door.

So, is this a side effect / hidden clue, that the original 11 (yea, eleven) year old factory battery is going downhill?? Or some other one-off oddity??

I'm thinking it's time to visit the MB dealership to buy another 11 year battery.

Too bad you didn’t put a volt meter on it before charging . Any auto parts store will test battery and alternator for free, they sell parts . At 11 years old yes the battery is likely going bad . It could also be the voltage regulator on the alternator, they do go bad. Get it tested ,I do not think MB AGM batteries are better. You will probably pay double at Mb . Your own battery load tester is cheap , you have a charger , keep it charged until you figure it all out. Do you have a volt meter ? or get decent load tester, around 50-60$

Well, admittedly, the original battery (Varta probably) sure has lasted longer than any aftermarket batteries. I checked mbparts.mbusa website - one battery for $319 USD, another for $275, both in stock at local dealership.

Checked local auto parts stores ... comparable pricing, but some have $400-500 batteries!! I recently bought a non-AGM battery for our 2001 Jeep, and it was $175, so the MB dealer price is reasonable for an AGM 👍

I used to have a load tester, but hurricane Harvey dumped 6 foot of water on previous property, so lost a bunch of tools (and a Jaguar + 2 motorcycles).

Not driving anywhere for 1-2 more days, so I'll let it sit and check voltage Sat morning.

I think I have a voltmeter:

Nice meter, I didn’t look to see who posted , my bad. 😮

I’m beginning to think changing batteries based on system analyzer results is replacing them too early. I’m seeing 9-10-11 year old batteries posted that are still going.

FYI — I just put an MB Varta ($313) in the ‘21 GLB and a Walmart Everstart AGM ($180) in the ‘21 GLA. Same size batteries had a big difference in price. Both performed about the same against the Ľ Ohm load (~50 Amps) after top up charge. The Everstart actually calculated lower internal resistance on the 800 Amp test, but the Varta had a better decline slope. Those SUVs will never see that load, so maybe that’s meaningless.

Appreciate the detailed replies.

So I did a quick test just now (10am CST, 34°F out).
Keep in mind, the battery has it's top cover on, so I'm probing at the "remote" terminals. Also, only have a multimeter, load tester is no more.

Also, I also compared to the Jeep, which has a new, non-AGM battery.

GLK, 1st reading 12.48, (0.12 low)
Turn key to Run position (no Start) 11.92
Turn ignition off
Wait 10 minutes, 2nd reading 12.34
Wait additional 20 mins, 3rd reading 12.34

EDIT: about 1.5 hours later, 4th read 22.42
(attached trickle charger at this point)

Jeep 1st reading 12.6, Turn key to Run position and turn on running lights to match GLK, 11.16,
turn ignition off
Wait 10 minutes, 2nd reading 12.6

I could pop the battery top cover off and check voltage directly at the battery, but I doubt there's that much of a voltage drop to the remote terminals (?).

Another test I could do (though not as accurate as a dedicated load tester) would be to attach multimeter, and do an engine start, to see the voltage drop. Rumor has it, it shouldn't drop below 9.6.

I'm pragmatic (often better translated "cheap") and my analysis has more to do with the sound of the cold engine cranking than anything. Obviously, if the car's been sitting for many days, or if the temperature is really low, adjustments to my discerning ear have to be made... But basically I don't worry at all about a battery until I hear that telltale "ugggghhh" when the engine first starts to spin under starter power. Of course, I'll still look at the battery voltage and do a test with my (cheap) battery tester, but here in Arizona batteries tend to last only 3-4 years because of the heat. It's a little easier for me since two of my three cars have three pedals, so I can always get them started that "one last time" by just rolling down my driveway and popping the clutch, then it's off to Wally World for a quick and easy swap in their parking lot (allowing me to get that instant core charge refund).

On Tue, when I got in GLK for grocery store - when I noticed no lit dash (though came on at key Run), it cranked right up. At store, it cranked right up , then at gas station, cranked right up ... no hesitation.

Yes, the Jeep did the uggghh start thing not long ago, why I got a new battery (load tested at auto parts to verify).

They've both been sitting in garage since Tue, and we went to freezing Wed and Thur (never rose above both days). However, out of habit, on Wed & Thu I put trickle charger on both until charged - took about 20 minutes each. No charger today though. (now 36°F)

Okay, the latest news from GLK battery statistics (and compared to Jeep new battery).

Here's what I did - hooked up the multimeter to battery, then placed the multimeter face down on the windshield and used painter's tape to secure the meter in place.

Then I sat inside the vehicle and aimed my smartphone at the meter and turned on video mode. Then slowly turned the key (in ignition) to each position, so can hear each step of the startup process ... and to see how voltage drops at each key turn.

Then started up the engine ... let it get to idle and when the meter "settled down" to the final voltage, turned off vehicle, and stopped video recording.

This allowed me to review (frame by frame in video) where the battery is before startup, thru the key stages, then I can see how far the voltage drops during crank-up, then the final voltage at idle. Here's the stats:

Jeep, at rest 12.6, startup down to 10.93, up to 14.33 idle.

GLK, at rest 12.6, startup down to 9.70, up to 14.90 idle

I forgot to turn on the running lights on the Jeep before startup, because that's the behavior of the GLK, so the Jeep's drop isn't as much.

Admittedly, before I did this, I put the GLK on the trickle charger BEFORE the test (about 30 minutes), because it's voltage was at 12.42 , so got it to 12.6 ... then the test

I'm gonna keep a daily watch on the GLK battery, and I think I'm gonna order one from MB to be prepared. Don't want to be caught off-guard.

You're way (!) more proactive than I am... My GLK battery (only a couple years old) rests lower than yours and drops lower than yours on start-up, but I'm not worried about it at all (yet...). I'd be worried that your new battery might be half expired by the time your current battery really needs to be swapped.

Why not just invest in one of the handy little engine jump start / inflator boxes. They're small, light, and really not too spendy (I just got one for my Mini for about $50). If I misjudge the end of my battery's life, I get at least one do-over that way! ;-)

I used to have one ... if I remember, it was a Cen-Tech from Harbor Freight. It eventually died - wouldn't keep a charge.

Curious, I did a search to see what's up nowadays. I've never heard of a "super capacitor" jump starter. If I understand, they use your car's battery (even a dead one) to juice the charger up, then it turns around and provides the jump start. They are small and lightweight and don't require to be charged up. Interesting!! Might consider that.

BTW, if I do get a new battery, it's going in the GLK - no sitting on a shelf until required 👍

Haven't seen a super-capacitor jump box, but the idea is interesting. I've had good luck with the jump boxes I've used - they're certainly a lot (!) smaller and lighter than the big lead-acid style box I used not long ago...

My apologies if I’m telling you stuff you already know:
The GLK battery dropping to 9.7 V on start up is too low. Mercedes calls a battery replaceable at 10.8 V in the starting cycle, but they are kind of conservative. Your GLK might be showing no instrument panel lights when opening the door because BCM senses that the battery is going bad. There is a component in the pre-fuse box called a quiescent current relay. As the BCM senses the battery voltage dropping, it will begin turning off non-essential components (consumers, in MB-speak) to preserve enough battery to start the GLK. The quiescent current relay is the last line of defense that turns off everything that’s not in the starting circuit if there is parasitic current draw or low battery voltage. The tests you have done are more than adequate. I think you need a new battery. You already know where MBOTW is but I wouldn’t be afraid of a Walmart Everstart to save ~$140.

Have you tried a battery charger with a “restore mode”. My Ctek and Noco have them. On my 7 yr old mb agm it load tested about 25-50 amp higher after load tests before and after. Its just a cheap 50$ Solo load tester . Instructions say dont use the Restore feature to often, and yes i took a friends dead battery, it would not even take a charge on a new charger until i jumped it with a regular battery. The short is I kept doing the Restore feature then it went down in amps. So doing it once or twice works. I did replace the 7 yr old and do use the restore feature once to twice a year now.

That “dead” battery, was at like 0.3v. After repeatedy doing charge and restore cycles at 3-4 amps for 3 days. It tested maybe 10% of new with a load tester. It went back in service in an old 1/2 ton pickup. But after about 5 restore cycles amps went down, so i stopped the Restore function.

Something that is probably not of issue to you is Phantom Load. i noticed my battery lower every day,not wanting to do a cumbersome test i tried my IR thermometer on all my accessories first, sure enough the first thing i hit with the IR was 20 degrees warmer than the rest of the interior. It was my old dash cam. I reset it and no more parasitic draw . I think this way of testing would even work on fuses to identify draining circuits.

How many battery manufacturers are there making AGM ? Not many as ive heard.
The Ctek in my opinion does a better job at the “Restore “ function , i noticed more amp gain

No restore mode charger, just a nice automatic trickle-only charger. My neighbor's Porsche Cayenne sat for about four months and would barely click during startup. Hooked up the trickle charger, and well over 24 hours later, the "finished" light was blinking. Porsche fired right up, no issues after.

Yea, one day, I might buy another load tester - I'll lazily shop around.

Phantom load. There are no "aftermarket" devices on the GLK - all bone stock. And if there was a P.load, the battery issue would be more constant, I think.

Ouch ... I'd read if it drops below 9.6, then replace, but yea, I can see MB spec a higher value.

Okay, the "BCM sensor" info makes sense to me - I had a suspicion that a low battery had caused the no dash on entry. Good for MB for doing that - it's an obvious visual hint at a prob. Too bad that info is hidden, Internet wise.

And yep, MB-OTW is about a 30 minute drive, each way ( 🫤 ). There's a Wally World about 12 minutes away, plus an Oreilly (where I bought Jeep battery), Advanced Auto, AutoZone, Napa, and a Batteries Plus ... all within a couple of miles of each other (and Wally).

I just did a Part# search at mbparts for the original battery, and yea, it's in stock for $319 at MB-W. Save $140 and get an Everstart (?). How long might it last? I'll shop around.

Thanks for this great info!
====
Okay, here's a morning update. About 30 minutes ago, I did a battery voltage test (37°F out), no startup test. No charge since yesterday's testing.
Jeep: 12.58 (12.6 yesterday),
GLK 12.46. Dash lights come on.

I planned on picking up a battery today, while I'm out for groceries, but I think I'll spend the weekend shopping around. Plus, it'll be about 70°F on Monday 👍

Sounds a lot like perpetual motion to me.

So there is a free lunch...

@MBKLUE and @John CC ... I know seems farfetched, right . I asked AI in the browser ... here's a boatload of info:

A supercapacitor jump starter works by using a large bank of supercapacitors to rapidly store a burst of electrical energy from a car battery, even if it's partially drained, and then quickly release that stored energy to the starter motor, allowing the car to jump start; essentially, it harvests the remaining small charge in a dead battery and delivers it in a powerful pulse to jump start the vehicle, all without needing to be charged itself for long periods as supercapacitors can charge very quickly.

Key points about supercapacitor jump starters:

Fast charging:
Supercapacitors can be charged very quickly, meaning you only need to connect them to a slightly charged battery for a short time to gather enough energy to jump start a car.

High power discharge:
Once charged, supercapacitors can release their stored energy rapidly, providing a high current needed to crank a car engine.

No battery needed:
Unlike traditional jump starters, supercapacitor jump starters don't require a separate battery to store energy, making them lighter and potentially more reliable in extreme temperatures.

How it works in practice:
1. Connecting to the battery
You connect the jump starter cables to the dead car battery.
2. Charging from the battery:
Even though the car battery is considered "dead", it still has a small amount of charge left which the supercapacitors quickly absorb.
3. Storing energy:
The supercapacitors store this collected energy within their internal structure.
4. Jump start initiation:
Once the supercapacitors are sufficiently charged, you initiate the jump start by pressing a button, which rapidly discharges the stored energy to the car's starter motor.

Advantages of supercapacitor jump starters:
Fast charging time:
Can be ready to jump start a car within a few minutes.
Long lifespan:
Supercapacitors have a very high cycle life, meaning they can be used for many jump starts before wearing out.
Lightweight and compact:
Due to not needing a large battery, they are typically lighter and more portable.

Must have an inverter in it. Otherwise, if the battery was at, say, 9.0 volts, there's no way to charge the capacitor beyond 9 volts.

I wonder why the "rapid discharge" doesn't just go back into the battery instead of the starter.

Yea, what I posted is from an AI response. They aren't always intelligent... it's possible it misinterpreted a source it found and rewrote it incorrectly. Each of those paragraphs are associated to a different source of Internet links.

(side note: retired software engineer here with 40+ years experience. I'm still hesitant to trust AI 👍 )

This post is more about information some may not be aware of. And I forgot about it until just now shopping for a battery at O'Reilly. (Brain has gone mushy since I retired 4 years ago).

Anyway, most all US parts stores offer a discount for military veterans. I know for sure O'Reilly and Advanced does a 10% discount, as I've shopped at both. So the $250 Super Start Platinum AGM I'm looking at will be $25.00 less. I think there might be an Oreilly customer appreciation discount, but that might be a wishful thought.

I'll be checking Napa, Advanced, Batteries Plus, Wally World, and AutoZone next ... but I suspect the batteries are competitively priced. I checked if MB offers a vet discount, but don't find one.

Side note to this thread...when I activate the "secret menu" to monitor voltage and current, it always climbs up to 15.0 volts with about 1 amp of charge current after a few minutes after starting. Is this normal?

The reason I ask is because I had to replace both battery and alternator at Christmas - on the drive home from work the other morning, the big nasty red battery symbol displayed on screen. The car began shutting down non essentials (heater fan slowed and increased repetitively, radio went off, 120V socket kept powering up/down that my cell was plugged into, etc.). I went to the shop that replaced the alternator, they tested for an hour running the engine with no faults found. I've been monitoring on the secret menu for a couple of days now and no problems.

Is 15 volts normal on the dash display, or should the alternator be replaced again?

I think 15 is normal as mine goes there. But something isn’t right . Either the battery or alternator probably is bad . Possibly the Regulator on the. Alternator. Do you have any test equipment? Volt meter ? Battery charger ? Any auto parts store should white you wait test the battery and alternator for free. Any mechanic That replaces both the battery and alternator might be selling more than Necessary, and suspicious is he can’t find the issue . Get it tested , check battery voltage with a meter , charge it . My charger also tests the alternator . Is it an AGM battery ? It should be , you need a charger with an AGM cycle .

Many thanks, both battery (I personally installed an 800 CCA AGM battery with 5 yrs warranty from Canadian Tire - they found a bad cell in the old one) and alternator (new, rebuilt, 1 year warranty sourced by trusted mechanic, from trusted source. Old was tested and found bad - no output) were replaced at Christmas time and have worked well. No issues before or after this instance.

I'm going to look at it this week when temp goes up to 4° or 5°C from -20° (-4°F). Going to make sure all wires and connections are tight.

Mechanic already has a replacement alternator in hand if needed. He has been great.

All the best!

Your mechanic is not good, you don’t use “rebuilt “ alternators , and him saying he has another one ! He is a hack. I’ll bet his “rebuilt” is the cause . So he will offer another Used alternator if his first failure fails . FCP euro is the only place I get stuff and other people that I know . They have a Lifetime warranty they honor on everything they sell. I’ll bet your regulator is bad. But that’s more work.