cyberay

I have asked this question of 3 different techs at 2 different Mercedes Service departments and none knew the answer. I have been trying to come up with an explanation for more than a year and am out of ideas.

Upon entering the workshop menu on my w217 S550 coupe and selecting the "vehicle data" sub-menu the first screen to come up contains the following battery parameters: UB-battery voltage in volts, IB-battery current in amps, TB-battery temperature in degrees F, and BN which for the life of me I cannot figure out. There are no units associated with the numbers for BN, and which alternately flash between the high 60's and 110. These numbers increase in value the longer the car is running and as the battery charges. The first number changes sequentially and has never gotten higher than 83, even when the battery is fully charged, and displays for about 3 seconds. It then toggles to the second number which displays for about a second and a half and always begins as 100 when the car is first started, and then jumps to 110 when the battery is fully charged. Anybody out there that can shed some light on what this parameter is reporting will earn my eternal gratitude.

TommiM

iirc, they are German acronyms. BN, iirc is internal resistance of the battery. Either way, that is one factor being monitored. Its basically displaying the battery current and discharge rates. You can use those to assist in diagnosis of battery charging or discharging rates, for example. It is sometimes more useful with the hybrid cars, since it can help diagnose whether you need to look at the 12v system or the high voltage side.

JeremyH

I think I can answer this - on my 2016 C63s, BN appears to correlate to the level of charge in the battery.

If I read the battery status with a third-party tool, then 80% charge appears to give me BN 80/100 on the display - and at 76% charged, I get BN 74/100. This number goes up when I'm driving and drops if the car is parked for a couple of days without being used.

I would guess the first number is the estimated Ah the battery has charged, the second number is likely the size of the battery in Ah.

fire3280

I know it's been a few years but just to let other people know when they google what BN means.

BN has 4 meanings.

- when it's between 50-90, it means the charge of the battery in percentage (or related to percentgage). 12.2V is roughly 70% on a car with 80Ah capacity. The car probably won't start with anything lower than 11.5V. I have tested my car and drain the battery to 11.6v and BN is 50, and once it reaches 50, your AC blower will be throttled, and your audio will be turn off (screen is still fully functional, just the audio is off, probably the amp being shutdown)

Otherwise when it switches to other numbers.

- 0, it means your auto start/stop function is disabled due to low charge. Regardless of your setting status.

- 100, it means your auto start/stop function has enough charge to activate it, still won't activate unless your setting is enabled.

- 110 means that in addition to "100", your battery charge is high enough (when BN is 86+ in my case, depending on your actual battery design capacity) that the car will charge the rest of the capacity only with regen charge. That is, when you release gas paddle or cruising (not cruise control). It is only possible in Normal mode, in another mode, it won't show 110. During this time, if you measure the car voltage from OBD, it will only show 12.6-12.8v instead of 14V+. Basically your alternator stop engaging.


I am having problem after replacing my battery to a non-Mercedes brand and somehow the system don't want to charge to more than 78, causing the car to never get "110", which means the alternator never stops engaging and my battery voltage never reach more than 12.2v with engine stop. I tried resetting the battery using OBD scan tool it works until next engine start. I was told that it could be because the new battery does not meet the standard for internal resistance comparing to a OE battery. Basically the non-OE battery has way too much resistance when at brand new, making the car thinks that it is an old battery and won't try to charge too much. (Yes, the car monitor the internal resistance of the battery very closely, for safety)

anyway, I may try to buy a Mercedes battery from dealer for $400 (comparing to the Costco interstate one for $180).

to be fair, there is absolutely no problem keep running this way unless you have OCD and can't tolerate the "issue"(like I am) . Well, the only possible issue I can think of is that my car would never have more run 70% of charge, and more likely to die if your discharge it for too long, like you keep it on but engine off when waiting for hours.

JeremyH

Before you buy another battery, you may want to check whether your car needs calibrating to the new battery. This shouldn't be needed on a like-for-like replacement, but I seem to recall seeing someone hint at this being necessary when changing batteries over.

Since my previous post (which I'd almost completely forgotten about posting!), I came to the same conclusions as you, although my experience on the C205 differs very slightly:

• The first number on my car shows the % charge of the battery, up to 99%. When driving, I never saw this go above 93%, however an external charger will take this to 99% with a newish battery. My three/four-year old battery now won't go above 87%
• For the second number, 0 is the same (charge is insufficient for start/stop), 100 means sufficient charge (above about 60% from memory) but not enabled, and 110 means battery has sufficient charge and start/stop is enabled
• I seem to recall at one point I also had a 101 showing when the battery was above a certain level which meant the alternator was disabled and the car was running off the battery only (there is no regen I'm aware of on the C63s). This started happening after a service and stopped happening after the next, so I can only imagine was implemented by mistake on a firmware update - and was later corrected.

As an aside, I was perhaps fortunate enough to have my car start when showing charge of 35% and still had the radio running, but with a warning not to switch off the engine. A second warning then said it might shut down the radio if the charge level falls further. I would imagine this is car and year of manufacture dependent. I also got an email to say the battery was extremely low and needed charging, but it didn't send this until I started the engine.

fire3280

Thanks for the add.

Funny thing is that I was draining the battery earlier to see when will the low voltage warning pops up. voltage stays at around 11.8v while BN drops from 64-50, I think the computer recalibrate itself in the process.

Then I start to idle to charge the battery, surprisingly, it went all the way from 50 to 86, and now battery voltage can stay at 12.76v (measure with multimeter) without engine running. Meaning that it now can charge pass 12.2v and after 86, it can only be charged by regen. So basically my problem is fixed. I think like you said, it did a recalibration.

my car is 2018 w205 c43 sedan, battery is Costco interstate H7 (Costco website says C43 uses H6, no, it uses H7, same as C63).

and yes, C63 is different from C43 in terms of charging process. C43 has torque converter instead of clutch on C63. torque converter allows C43 to have gliding mode as well as regen charge. Doing it with a clutch can introduce excessive/unnecessary wear to the clutch.

JeremyH

Sounds like you fixed it :-)

The C63s also has what I call 'sailing mode' (because of the icon on the dash), but I've often wondered about the wear on the clutch. This makes me ponder whether regen also works on this car as the battery charges significantly quicker than the IB numbers suggest.

Quick q: I tend to find, each time I take the car out on anything less than a 20 minute journey, the battery loses 2% of its power when cooling down, which means charging it every few weeks. I don't think starting it is the issue, but the fans cooling the engine down afterwards.

fire3280

Interesting to know that C63s would continue running the fan after shutting down. My mini does that, but c43 does Not, despite it also being a hot engine. (Normally take about 5 hours to cool down from 210F to ambient temp after engine off)

and yes, the fan definitely would consume a lot of power. As I was draining the battery earlier, I found that the quickest way was to turn the AC blower to the highest, it would drain about 40amp total, and would drain 1 BN every minute or two. Imagine the radiator fan being a lot bigger, it could be the same or even faster.

I drove my car again just now, and it seems that it wasn't drain enough to complete recalibrate the battery, because it just won't charge more than 87 no matter how long I do the regen, went on freeway down hill cruise/regen over 3 minute at high speed still won't go more than 87, and charge amperage was only 1A. Measure the battery afterwards, battery is already at 12.8A, which usually means 100% full already. Basically the car now thinks the battery isn't full when it is, completely opposite of what it was. Regardless I am down chasing the problem, it's good enough for now.

you probably don't need to worry about the battery drain caused by the fan, as long as the computer is properly calibrated, it will charge it very quickly if it starts with low BN. When I was charging it from 50 to 85, the charge amperage starts from 110A, and gradually reduced to 5A as it gets to 85.

JeremyH

Interesting to hear about how much power the fans draw.

On the topic of recalibration, while the car might be trying to adjust, you may find you need to go somewhere with the relevant gear to change the calibration settings fully. Then again, a Bosch battery might work out cheaper!

JeremyH

Just to add to one of my earlier posts, I double checked the numbers I quoted above and it seems, on my car, Start/Stop becomes available at 68% charged and no longer displays a different code to indicate that the alternator has been disabled. I suspect the wrong firmware update ended up on some C63's, disabling the alternator at higher battery charges, but with no regen available to charge it up.

I agree that the car charges the battery quite quickly when 50% charged, however if I do a 4 or 5 mile journey, the battery depletion when switched off (around 2%) is usually higher than the charge gained during the journey, especially during the winter. This may have changed with the last software upgrade, though, so I'm going to keep an eye on it and see what happens...

I appreciate the additional info :-)

IanBa

A bit more detail for this long running thread!
I just replaced the battery on my c350e. (BN was zero, hybrid motor was disabled)

All looks fine. But after driving for a few minutes, BN goes to 91/110, charging stops, UB drops to 12.2 V and IB sits at around -40A. BN drops to 85/110 and then to 81/111

Once hybrid battery falls below 9%, petrol engine starts and alternator starts charging . UB goes to 14.7V, IB goes to +50A. BN reverts to /100

Is this normal? It appears that the system prevents charging the auxiliary battery when in EV mode and there is adequate charge in the 12V system. Wonder what 111 means...

Note that I haven't coded the new battery to the car. There are no fault codes. Vehicle drives fine

CaliBenzDriver

I read this thread for lunch.... there's an unfortunate mix that prevents understanding.

Hybrid vehicle traction battery and conventional combustion engine system battery ARE TWO ENTIRELY DIFFERENT TOPICS.

The original MB engineering documents are available in WIS software.

Late non-hybrid cars use a variable charge rate. The ECU directly controls the ALT output based on a Batt sensor.
These are the raw data displayed on the iCluster menu.

Great!
MB is using the Bosch "Smart Charge" save lots of gasoline on twin-turbo V8... this is helping tremendously with "save the planet" struggle, one droplet at the time.


What could go wrong with this perfect system?
The crazy alternator control drains the battery below 12 Volts and punches it with 90Amp current when hot already.

Low voltage helps 3rd party modules that don't work well near 11 Volts go randomly crazy under heavy A/C loads.

The longest way possible:
The battery sensor at the front is hosted all the way at the rear by the R-SAM in charge of posting its data over CAN-B for the ECU to read it through a busy Gateway module.
The outcome is not particularly reliable voltage control.

EV's battery are a totally unrelated game. The one thing all batteries have in common is HEAT is enemy of their longevity. Hybrid batts are carefully cooled to work high amps motors.

Phileep

you said in your post that tha BN number was fluctuating between high 60’s and110. I have been trying to find out why the stop/start isn’t working and the BN reading I have is 0 to 50 by I think the fault I have is the battery as it should read what you are having