A valid question, yes, I did this back in October. I do it every 6 months as recommended. However, it loses 2% per day. If I leave it for 4 days, I'm down over 8%, so it's a continuous drain. I'm about to open the hood and use a clamp-on DC ammeter to see what the drain from the 12V is.

 

I do it every 6 months too. But in this circumstance, I would try doing it again already now -- even if your 6 month period isn't up. (Just to continue to rule out the easy stuff)

 

Opened the hood and locked the car. Immediately upon testing, I can see 22A headed to the 12V battery. It's charging at 300W. I'm documenting with photos. Once the charging stops, hopefully in an hour or so, I will check to see what the discharge rate is. If it's low, then I got a bad replacement 12V battery, or there's some mysterious huge drain happening at weird times of the car being locked. Will post back soon!

 

Okay. Immediate update. After about 15 minutes, charging of the 12V has stopped. Quiescent current is about 40mA. That's pretty reasonable. There should be hardly any voltage drop overnight at that usage. I'll check back in about an hour or two and see if there's any difference. Wish I could datalog this, but so far, I'm under the impression that they sold me a bad replacement battery. No way it should be getting charged up this often. I may request that they order a brand new one.

 

I suppose there's always the possibility they pulled a good battery off the shelf, but failed to give it a full and proper charge before installing in your car, leaving your HV battery to do the heavy work. If that were the case, I would expect to see your phantom drains stop fairly soon.

 

Quiescent current is up to 90-100mA. That's higher than one might want, but they are connected cars. The 12V is a 70Ah battery. Let's assume 100mA consumption all day. In 24 hours, that's 2.4 Ah of consumption, or about 3.5% of the 12V's capacity. I struggle to see why the DC/DC would kick in that hard and drop the pack by a full 1.5 kWh. That seems like a completely insane conversion efficiency. So, that means to me that it will likely kick in shortly again to charge a 12V battery that's not holding a charge. Guess we'll find out more as the day progresses.

 

Yeah, I considered this, but drove the car for 2 hours and charged for 2 hours. I'd think this would be enough at 300W to fully charge, no? Mercedes-Benz app shows battery as "charged." I have the rest of the weekend to figure out whether I want them to order me a new one, I guess.

 

3 hours later, battery voltage is at 12.58V and quiescent current is at 90-100mA. Except for when I turned on the car and for 15 minutes after, I haven't seen the DC-DC converter kick in at all yet.

So. From where is the HV battery drain coming?! I've only been keeping at this for a few hours, and it's certainly possible I've missed a DC-DC charge cycle. I need to get a voltage datalogger.

 

I made an error in post 53. The 22A is consumption, *not* DC/DC charging. I wasn't paying attention to the polarity of the clamp DC ammeter. This is battery discharge! 300W when the car is unlocked. It quickly drops down to 0.1A in about 2-3 minutes. When you approach the car and the handles retract, boom, 23A again. So the car consumes 300W from the 12V battery any time it "wakes up."

When it's just consuming 90-100mA, battery voltage was 12.6V. Perfect. However, when it's consuming 22A, the battery voltage drops to 11.9V. Even when it's just consuming 1A, the voltage is significantly lower. I haven't seen *any* charging of the 12V battery yet, only consumption. I'm going to build a voltage logger so I can clearly graph when the battery is being charged. I suspect that somehow voltage is dropping too low, and quickly. Maybe they did get me a bad 12V battery, I'm starting to think.

 

I'm going to build a voltage logger tomorrow. I'm super curious to see when the charging kicks in. If it doesn't, it means I have a drain in the HV system, and I have no idea what to do if that's the case. Maybe a contactor stuck or something similar?

 

Yeah I was gonna ask if your meter had histogram, even it has I doubt it would keep data that long, thanks for doing this and taking your weekend, I have disabled only keyless-go , still got same 1% over 12 hours of period @ temps 6-15, 1% a bit more acceptable drop, I guess you will start seeing same soon as well when the weather warm up. Tesla owners only get 4-5% drain with sentry on, otherwise they say 0-1% acceptable, probably for cold weather, yeah I agree its a bit mystery, only thing we know you didnt have this issue before and it happens in cold weather (or more noticeable in temp. fluctuation). If you keep continously click on the mbux, assistance button on settings menu it shows hidden dealer menu, there are system logs export all option but I couldnt figure out how to export to disk or retrieve at least. I will be at dealer in 5 day and prob. they are following this thread, I will let you know if any update comes up from them. Tonight I am gonna try to take all weight off 12V by disabling most of thing but I doubt its right move , many other users have activated those options.

 

Also google says 12V performance itself drops in cold as well, dunno if it will help or your number checks out with this figures

"Reduced capacity: Lead-acid batteries can lose a significant portion of their capacity in cold weather. For example, at 0°C (32°F), a typical lead-acid battery may only deliver about 80% of its rated capacity, and at -20°C (-4°F), that figure can drop to around 50%."

 

What I find very interesting is that Bytemaster0 has THREE EQE's in the family and only ONE of them is exhibiting this behavior -- and it only started recently.
Add to this that so far, I've only seen you, eqe300CZ and Bytemaster0 on this forum with this significant overnight loss of charge.
So from these observations alone, it seems that such an overnight loss cannot be normal, let alone acceptable!

 

Thanks Scott for the heads up, good to have confirmation

 

Okay. I built a voltage logger. Extremely simple, using an Arduino Uno R4 (the Renesas MCU has a nice ADC built in), a voltage divider, and a computer with a very basic Python script polling every 5 seconds. It'll save the data to a CSV that I'll share as a plot here later. I noticed that my 12V battery voltage was at 12.45 volts this morning - virtually unchanged from last night despite a continuous 100-130mA draw. The HV battery voltage was about one percent lower. I can't confirm that it charged the 12V last night, and it may in fact never do so unless the car is on or being charged. I took samples for a minute, then plugged in the car to charge. Voltage goes up to 14.83 or so, and current into the 12V battery (I double-checked polarity this time, lol), is 22.7A, so it's charging at about 335W into the battery while the charge cable is connected. I'll leave it for about 24 hours to see if the voltage ever increases again - that'd be a sign that the DC-DC converter is kicking in overnight to charge the battery. I'm guessing total DC/DC converter capacity is about 700W, as the car draws about 300W to operate and is charging the battery at 335W.

If I don't see a voltage increase in the plot during the 24-hour period, it means the HV battery is draining due to a different reason. If it doesn't charge the 12V battery overnight and I don't see a further drain, it may just be that the new 12V battery need a bit of a break-in period. We'll see. Pics below of the voltage logger and 12V charging current.


Please don't make fun of my ad-hoc setup, I'm just trying to avoid anything +12V from touching anything else, lol.

22.7A charging of the 12V battery while plugged in.

Output from the data logger, which will be stored to a CSV. You can see when I plugged the car in, as it went from 12.45V to 14.83V.

More details to follow!

 

Okay, some by-the-minute further intel. The moment the HV battery stops charging, the 12V battery voltage drops. This indicates that the DC-DC converter has turned off. And, I went to measure current, and this is the case. For about a minute, 1.75A was coming from the 12V battery, meaning it's now being used to power the car's systems. That current quickly dropped then to 380mA, and then will continue to drop to idle state of about 100-130mA continuous. The 12V battery voltage is now 12.91V, indicating a full charge. Which happened fast! I guess if you dump 300W into the 12V battery, it tops off pretty quickly. I'm starting to suspect that the DC-DC converter is not coming on at night, and not charging the 12V battery. I'll confirm with the logger in 24 hours, but if this is the case, then the HV battery drain comes from somewhere else. We'll see!

 

Not fun to think about what this might be...

 

Woah you turned into nice weekend DIY project! Can I ask do you have anything connected to usb-c port? I have ssd drive that I have removed today , brought to home now and in suspended state it draws 0.5w from PC, I will go check at the car now when dashcam active vs car shutdown. Still I doubt because probably before than this issue happens you had same setup. Also can I ask when you finish sampling if you can draw some power from usb-c ports by charging phone or connecting some usb and monitor 12V ? I am just curious to know that if main board connects to 12V.

 

Nope , when car lockdown its nicely cut the power on usb-c port after a minute , during the drive I had around 1w draw

 

Thanks! Worth looking into for sure, and lets me rule out anything 12V-related. Also means if it isn't the 12V, then I have a big issue somewhere else. Every thread I've read about an EQ HV battery drain has been resolved by replacing a 12V battery. Wish mine were that simple!

I'll connect a USB-C peripheral, but I'm sure it'll just show up as current coming from the 12V. It's hard to distinguish this for a while, because of the 22A required to power the car's systems as soon as you unlock. But, I'll plug in a phone and then let it run for 10 minutes before sampling the current, I'm sure it'll show up there.

Oh, and I do have a USB flash drive always plugged in, but that's been there for over 2 years. Shouldn't be affecting anything but the 12V battery. MBUX doesn't touch the HV battery.

 

OK , thanks, then I got my answer, mbux connects to 12 V as well as usb ports too, please dont bother with testing usb then , you are right when I asked I didnt know ports will not work when car locked down.

 

TBH the more i read about 12V issues I couldnt agree more

 

One thought that has occurred to me is that the air suspension has developed a slow leak. The 12V draw could be the air pump recharging the system. I had an issue with my 2021 AMG wagon where one of the connections for the air tank from the compressor developed a pinhole leak. The car would turn on periodically to run the pump.

 

Man, I wish that were true, would have loved the air suspension! Alas, I didn't option it when I built the car. Great guess, though I suspect it also runs on 12V.

12 hour update, the 12V battery is at 12.70 volts and has been for a long time. We'll see the plot tomorrow, but I think the battery is holding. If my HV battery is discharged 2% tomorrow, that means it's going to be tough to track this down.

 

Okay. 24-hour update. The 12V battery is fine. It is at about 12.64 volts this morning, which is basically still fully-charged. However, the HV battery is down to 78% again after 24 hours. The graph shows that absolutely no DC-DC charging is going on during the 24-hour period. Just the one charge cycle when I plugged it in, and then a very flat discharge curve for 24 hours. The 12V is never charged on its own, and at this point given the discharge curve, I'm confident that the 12V battery is not the issue at all now.

This brings me to an uncomfortable thought, which @ScottC2 has already raised, that it could be an issue internal to the HV battery. Specifically, a cell that is failing or not staying up to pack voltage, that the BMS is trying to balance over and over. In addition to the annoyance of losing a lot of charge, it raises significant safety concerns. There were two EQE fires, after all, and it's entirely possible this is a reason for it. Perhaps the affected cell(s) aren't really that below spec, but losing 1.5 kWh in a balancing attempt every day is far too much. I'm going to escalate this now with my dealership, and will bring them the information showing them that the 12V battery is totally fine and not part of the equation. Hopefully it's just something silly that's not shutting off and is currently sapping power from the HV battery, but I am very concerned it's loss due to balancing, which isn't a good place to be in. I'll update once I've had a chance to talk to the shop foreman, who is 100% on the ball here in Louisville.