Using a 12V charger on the posts in the engine compartment DOES NOT charge the 48V battery. Only with the engine running (Idle or driving) is the 48V battery charged. These are my conclusions after doing a series of tests using a 6 Amp Schumacher WiFi + Maintenance Charger from AutoZone.

Even with the ignition off, I had thought that the vehicle electronics would keep the 48V battery charged when using a trickle charger or regular charger, but that is not the case. Hereaftrer, I will use the Workshop Menu to monitor the 48V battery and insure that it is adequately charged, particularly after a series of short trips or a period of vehicle storage.

I ran a series of tests.

Test 1) In the first test, the ignition and engine were off with the Schumacher charger initially applying 6.1 Amps:

Initially: 12V at 91%, 48V at 74%
After 2 hours: 12V at 99%, 48V at 73%
After 6 hours: 12V at 99%, 48V at 72% and a Text message that the 12V battery is fully charged.

The 48V battery ACTUALLY DISCHARGED while the 12V charger was attached !!!

Test 2) After two 50 mile drives with some stop and go traffic, the 48V battery went from 76% to 82% each time.

Test 3) In a third test, the ignition and engine were off with the Schumacher charger initially applying 6.1 Amps:

Initially: 12V at 93%, 48V at 76%
After 2 hours: 12V at 94%, 48V at 73%, as DISCHARGED as before !!!

Continuing, I turned on the ignition with the engine off:

After another 2 1/2 hours: 12V at 94%, 48V at 56%, down from 73% !!!

Continuing, I started the engine idling and disconnected the 12V charger:

After only another 10 minutes: 48V at 66%, up from 56%
After another 30 minutes: 48V at 70%
After another 45 minutes: 12V at 95%, 48V at 72% and a text message that the 12V battery is fully charged.

So If you followed all of that, you will reach the same conclusion that I have — a 12V charger does not charge the 48V battery even if the ignition is on. The 48V battery is only charged with the engine running.

 

Interesting research.
I put a charge on my battery after I saw a "partially charged" notation on my Mercedes Me app.
I keep forgetting how to access the Service Menu, so I haven't verified my SOC in the 48v battery.

I have a different theory about the 12v to 48v DC converter, but it's only based on logic.

​​​​​​I wouldn't expect any charge to go to the 48v battery until the 12v battery is at or near 100% SOC.
So maybe the DC-DC converter doesn't kick in until a voltage threshold is reached.
That threshold might be as high as 14.5v.
That's a guess, but it's because I see my voltage at 15.1 for hours of driving, when I'll see it at 12.9v. (I have my radar detector display set to show voltage, and that's at the 12v port in the console.}

So my next investigation will be to find that menu and check the 48v SOC, and watch it while my charger floats at full charge. If my theory is correct, the 48v will only be charging at that time.

But I have to verify charge voltage at Float, and maybe fix my trusty old Shumaker and apply a fixed high voltage to complete my test.

Interesting how much the 48v discharged, but as a Lithium battery, it ought to be OK at 20% SOC, depending on its Battery Management System.

So if my theory is correct, you'll only charge the 48v via the 12v charge points when the 12v battery is at full charge.
But it will also charge via the ISG while the engine is running.

 

I think the 48V is also charged from a "jump start" when it's sufficiently dead not enough power to start the IC motor, otherwise only off the ISG. The 12V system does not have a alternator or generator near I can tell, it's solely charged by the ISG.

 

Well, I ran another test and again, with the ignition off and the 12V battery charged at 100% and at 13.2 V, the 48V battery actually DISCHARGED from 78% to 77% in one hour while the Schumacher charger was connected.

Leaving the Schumacher connected and turning on the ignition to Position 2 with the engine and Climate Control off, the 48V battery discharged to 75% in 40 minutes. During this time, the 12V remained at 100% charge at 12.7V (Schumacher still connected.)

Later, with the 48V starting at 59%, with the engine idling, the 48V battery was charged to 70% in 20 minutes. During the time the engine was idling, the 12V system showed 14.4V and dropped immediately to 13.2V when the engine was shut odf. During all of these test, the 48V system actually showed 46V.

So I conclude again that the 48V battery is only charged by the ISG when the engine is running. It is still good to keep the 12V battery charged 'cause the ISG does not have to charge it and can concentrate on charging the 48V battery.

 

@slk55er good research. I'm heading out today (maybe) to do a similar check. This was mine last night, before connecting my maintainer. Position 1, not started.


I wonder what's discharging your 48v battery. I thought all the legacy draws would be 12v.

Maybe it's the Battery Management System, warming the battery.

Edit- All the tech info I've seen has stated the existence of two DC-DC converters between the 48 and 12v systems, 12v>48v and 48v>12v.
The 12 to 48v converter must do something, I'd think. Or maybe the info I've seen is wrong.

 

"I wonder what's discharging your 48v battery. I thought all the legacy draws would be 12v."

Is it not because the 48V is keeping the 12V charged as the 12V discharges from the legacy draws. I think all of the DC Convertors go from the 48V to the 12V with the exception of an outside charging source attached to the 12V charging posts. It is my understanding the 12V is only charged by the 48V ISG and does not have a separate 12V generator/ alternator.

 

That's true, as I understand it, and it does through the 48vDC to 12vDC converter. It has to be stepped down.

So, to follow on, if the 12v AGM has been depleted in your garage, listening for keys and downloads, does the 48v immediately (or periodically) send some juice to the 12v?
I doubt that, since my ME app showed "partially charged" when I checked last week, indicating that it wanted some juice that the 48v wasn't providing.
I didn't get a notification - I was checking my car status.

 

I am on the interstate driving from Tucson Arizona to Albuquerque New Mexico mostly at a speed of 80 mph. When I started this trip battery was at 70% charge. It is now at 58% charge. While driving at a constant speed of 80 mph, the workshop display shows zero amps and there is no charging of the 48 volt battery while maintaining a constant speed. In fact the battery gradually discharges while climbing even the slightest incline. Display shows a discharge of 5 to 20 amps on a regular basis and the battery will gradually decline in charge while driving at a constant speed on the interstate.

Braking for the exit briefly supplies 250 amps and brings the charge up to 84% in a matter of 15 seconds or so so I conclude that the ONLY WAY to charge the 48 volt battery it is under braking.

I intend to do another test driving around several blocks in circles in my neighborhood, accelerating braking, accelerating braking, etc to see if I can bring the battery voltage up to 90% or more and have it stayed there.

 

This document has some good information on the system too:

https://www.google.com/url?sa=t&sour...uua3bvr1QbJySh

It's wild that the E-ABC system can also recharge the 48v system too.

 

As a follow up to my 430 mi trip from Tucson to Albuquerque, after stopping and the hotel 48 volt charge was at 71% which was essentially the charge at the start of travel. During the time on the interstate a charge was almost always steady at 58%. Never did I see any charge current applied except under braking.

I are an engineer with a masters in Electrical Engineering and PhD in Civil but it's been 40 years or more since I've done calculations to answer this question - I need help. Is it reasonable for a 48 volt charge at 280 amps for 10 seconds to charge our lithium battery by 25%? That's what I observed when exiting and braking on my travels. I do not know the capacity rating of our lithium battery.

Today I did see a 5 second charging current of 200 amps while coasting downhill. The charge has increased from 69% to 81%.

 

re: capacity
The document I linked above states a 0.9kWh 48 volt Li Ion battery. That was in 2021 if we go by the date of the article - might be different today - not sure.

 

The thing here to keep in mind is that a lithium battery is a very different animal from a lead acid or AGM battery. Lithium batteries are not supposed to be fully charged. They are most happy if kept between 40-60% for long term storage and shouldn't be charged over 80% very frequently. I think what you are seeing is simply the system managing the lithium battery charge. It leaves room so that when you are driving, the ISG can use it to recuperate energy. If the 48V battery would get fully charged by a trickle charger it would a) shorten the battery's life and b) it would defeat its purpose of having room to store recuperated energy while driving.

Managing the SoC of a lithium battery is very important for the life of the battery. There's nothing really unusual I see from your tests. For example I use my laptop mostly plugged in and there is a setting in the BIOS that limits the battery charge to 50% so that the batteries don't go bad from being kept at 100% for an extended period, or my iPhone has something called optimized battery charging. It charges the battery to 80%, then stops and only charges it to 100% shortly before it's expected to be disconnected from the charger. It does this by learning one's charging habits at specific locations. It's the same with EVs, the advice there is to only charge the battery to 100% shortly before a long trip and otherwise not charge it higher than 80% and for longer term storage the recommendation is to keep it around 50% charged

This is just a theory, but the battery management could have logic that when it detects a trickle charger it prepares the 48V battery for long term storage, assuming that the reason you hooked up a trickle charger in the first place is to leave the car parked for an extended period, so it puts the 48V battery charge at a sustainably level to protect the life of the battery. That could be why you are seeing the 48V discharging while the trickle charger is connected. Remember, lithium batteries are happiest if kept at 40-60% charge.

 

I did a drive while watching the workshop menu. A couple of observations.

1. Ignition in position 1, 12v resting voltage at 12.2v. I had removed my maintainer 6 hours prior. Lots of lights and systems alive in position 1, screen showed 17A draw, held 12.2v for a couple minutes, as it should.
Engine running, 12.7v, which is a slow charging voltage. So there is a charge going to the 12v, it seems.

2. I saw the highest 48v charge rate at 280w, during regenerative braking, AND for one second after I hit the throttle.
That makes sense according to a 3-yr-old video about EQBoost (now gone, drat). The video explained that the EQBoost is designed for throttle response for the first two seconds, until the turbo spools up. A one second negative current flow, immediately after a one second draw, fits that concept.

3. The 48v SOC seems to return to around 70-75%, or centers around that.

So, with the @slk55er observations and @superswiss input, I'm leaning toward calling the 48v battery an "electron sink," with room for regenerative braking as well as EQBoost.

I think the 48v battery capacity increased to 1.0 kwH in '22.
The main battery is probably 150+ kwH, so entirely different purposes. And design, probably, focused on large current flows, instead of storage.

Interesting thread.

 

Yes, that is largely its purpose. The whole point of mild hybrid is to recuperate energy that then later can be used to power systems that used to be belt driven by the engine such as power steering, oil pumps etc, and to decrease the load on the alternator and consequently the engine to save fuel. Charging the 48V battery with the engine is not very efficient for the most part. There is a scenario where it is more efficient. Internal combustion engines are most efficient at a certain rpm and load, so when cruising steady speed on the highway it may not run most efficiently, because the power needed to maintain speed is not that big. So increasing the load on the engine will actually make it run more efficient and the extra power that is not needed to maintain a steady speed can then be stored in the battery for later use. This is how the original Prius works for example. Using the CVT transmission it puts the engine at its most efficient rpm and load and simply uses the extra power to charge the battery and then turns off the engine whenever there's enough charge to drive it purely on electric. This is a complicated process, because of the fact that an internal combustion engine doesn't have a constant efficiency.

 

Excellent dialogue and analysis. Thank you all for your input and insights.

 

That's a good summary of reasons for the 48v system, plus smaller wire sizes.

Based on this thread (or other knowledge) how do you think the 12v gets juice?

 

From the engine. I'm not sure if there's still a traditionally alternator, or if the juice is coming from the ISG. However, there's still a traditional 12V starter motor for the initial engine start. The ISG is only used to restart the engine during auto start/stop cycles. The 12V battery is still responsible for starting the car and if it's flat, then the car won't start. Also the case in EVs. So really you don't have to worry about the 48V battery, but you need to worry about the 12V battery.

 

Yes, agreed. The Workshop Display shows % charge of the 48V battery but not of the 12V battery, unfortunately. On my cell phone with the Shumacher app, I can see the % charge for the 12V sent from my Shumacher Wifi charger. On my less sophisticated chargers I can see green LED lights indicating full charge, but it would be nice to see % for the 12V on the Workshop Display.

 

Here are three photos of my Workshop Display:

1) Cruising at 80mph with a steady 58% charge and zero amps
2) Braking from 80mph with 285 Amps charging
3) Stopped with 84% charge and a slight discharge of 5 Amps

 

The 12V battery is monitored by the telematics and its status can be seen in the Mercedes me connect app under Service. In addition if the 12V battery charge goes critical you get an alert on your phone from the app as well as an email letting you know that you need to charge the battery. I've gotten this alert a few times while my car was in for service and the battery started to run down as they were working on it, and I could alert my SA to let the technician know to hook it up to a charger. I guess with the 48V system it's not shown in the workshop menu, but I do see the 12V battery status in the workshop menu for my C63 along with the current draw.

 

Yes, thank you for the reminder of the charge showing in the service menu. Mine shows "fully charged." Will it show a percentage?

On my SL550 which of course does not have the 48 volt system, charge level of the battery is not available from the Mercedes me app. At least I have not found it.

 

No, it will show Charged, Partially Charged and Critical. Mine shows Partially Charged most of the time, because my car sits in the garage on most days and is not being driven much at the moment. There's also a standby mode you can activate for longer term storage. The nice thing about it is that it tells you exactly how long the battery lasts on standby. I use this when I go on vacation. Below is my C63 after trickle charging it overnight, so with a full battery it's good for 21 weeks. Now that I know that I can gauge the SoC based on how many weeks it says I can leave it on standby. I've seen it as low as 7 weeks with my current driving, so that would mean my battery is generally at around 30%.

 

On the Mercedes Me app where it shows the 12v status (fully charged, partially charged and critical), any idea what it showed in the event that you had your 12v at 94% and 48v at 56%?
Not sure if the 12v that the app shows really meant the 12v on our hybrid setup or whether it may actually reference to the 48v. Its a pity the app doesnt show both batteries status....

 

On the M256 Hybrid, there is no more 12v alternator and starter. No belts and the only starter/charger part i checked in EPC is the 48v ISG itself.

 

Yes, it means the 12V battery only. This was clarified with the most recent app update by changing the wording. It used to say Starter battery, which was confusing for the EVs and MHEVs. Now they explicitly changed it to 12V battery to make it clear, and also change the explanation to a more generic one encompassing all vehicle types. The 12V battery still is required to start the car, because it powers the auxiliaries, but it doesn't technically power a starter motor anymore depending on the powertrain technology. Still, if the 12V battery is flat you can't start the car whether it's a traditional ICE, MHEV or EV, so it's still a starter battery to start the car, but now it's clear that it is the 12V battery status, and not the 48V battery status.