Alandf

My 2016 has auxiliary battery in the trunk. Not the small battery I have read about in searches that is in the left side of dash.

If I disconnect the main battery to work on car ( plug job for example just to be sure not to short anything out) does that also disconnect the battery in the trunk?
Also, along the same thought, if my float charger (pandemic -not driving much) is connected under the hood, does that keep up the battery in the trunk? or does that battery need its own float charger?
Thanks,

Left Coast Geek

as I understand it (but note I do NOT have access to accurate wiring diagrams), the 2nd battery is only disconnected when you hit the starter, otherwise they are connected together, share the load, and share the charge. its primary purpose is so the computers and stereo don't have to reset when the engine is being (re)started.

LILBENZ230

Also of note - only the early W212s have the battery in the dash.

Sunnyslope48

A little checking with a voltmeter will answer your questions. I wish I knew the answers, but I have never checked.

DFWdude

Are we sure about this? I'm open to learning new things. But I can't see where charging the main battery in the engine compartment also charges the Aux battery in the trunk. I don't think it works that way, but someone feel free to explain this if I'm wrong.

AFAIK, electricity always takes the shortest path to ground. Attaching a charger or float maintainer to the + term of the main battery, and the ground bolt next to it over the RF wheel means the electrical path is confined to the engine compartment.

bmwpowere36m3

Assuming they [main and aux] are connected together, then they would both charge. Lots of trucks and utility vehicles have two (or more) batteries. As far as how MB implements the function of the aux and main battery I'm not sure. I believe LCG premise is correct based on what I've read. However, I'd think there would be some form of switching (or isolation) so during ECO stop the aux picks up the accessory load, but when the starter engages is doesn't try to load both the main and aux.

*edit*

Here is an excerpt on energy management, up to MY 14. It appears the connection is thru the front SAM and "controlled", so connecting a charger to the main battery or jump terminals does not guarantee the aux battery will charge as the front SAM needs to determine the aux's "state of charge" before allowing charging.

Determine function sequence of status of additional battery

A simple battery state recognition function is integrated in the front SAM control unit in order to provide information about the availability of electrical power from the additional battery. This is carried out immediately after the engine has been started. If the engine is switched off while battery state recognition is in progress, the front SAM control unit aborts battery state recognition and rejects the results from the measurement up to that time.

In addition to the battery state recognition, the voltage of the additional battery is also constantly checked. To perform this check, charging must be stopped for t = 20 ms. The check is performed every t = 5 s. The battery state recognition can also be started by means of diagnosis tester.

If there is no voltage at the additional battery or if it is discharged or defective, the fault message "Backup battery fault" is shown in the multifunction display (A1p13) of the instrument cluster (A1). The rear SAM control unit transmits the data required for this to the IC via the interior CAN.

Function sequence of charge additional battery

The additional battery is permanently charged after battery state recognition when the engine is running.

The additional battery is charged via the front SAM control unit. The additional battery charging process is only interrupted until the battery state is recognized.
The charging current is limited by means of a resistor to P = 15 W. Feedback from the ECO start/stop function additional battery to the on- board electrical system is prevented by the on-board electrical system decoupling relay (K19/7) (transmission 711.6, 716.6) or the ECO start/ stop function diode (V19) (transmission 722.9, 724.2, 725).

ghlkal

Not an ignorant question at all.

I'd like to know the answer also

Alandf

No disrespect meant But, your reply does not really answer the two questions. Disconnect and charging

Alandf

YUP

Left Coast Geek

simple easy test.

disconnect main battery. measure voltage at various always-on power locations. if it stays at 12.x volts, then there'sa high likelihood the aux battery is still connected.

bmwpowere36m3

With that kind of attitude, get a copy of WIS, do some tests yourself and don't ask for help...

Alandf

There was no attitude

bmwpowere36m3

If the answer was known or common knowledge, it'd be posted already... so a little research and testing might need to be done.

First, I've haven't come across, yet, a MB procedure that asks to disconnect the aux battery when preforming electrical work. Second, based on the function description I posted earlier... I would deduce the aux battery is only "connected" when the front SAM is powered and has determined its state of charge. I know, disconnecting the main battery will reset the front SAM. To me that implies the front SAM is not powered, thus cannot perform the former state of charge check and the aux battery is not "connected". The front SAM acts as a "gateway" for the vehicle, so in a way it makes sense the aux battery is connected thru it.

All that said, for confirmation I would test the following:

• Measure main battery voltage
• Measure aux battery voltage
• Connect battery charge to main battery or jump terminal by strut tower
• Re-measure main battery voltage
• Re-measure aux battery voltage

After connecting the charger the main batteries voltage should increase. If the aux battery voltage does not, its not connected. If it does, then its connected. All very easy to test in 10 min and would have an answer. To take it one step further, you'd disconnect the main battery terminals (wait a few minutes) and see if any 12v "hot" locations are still "hot" to totally rule it out.

On my '11 the aux battery is in the dash and a little harder to get at. Later its in under the trunk (very easy to access). Maybe I'll do it myself at some point...

pierrejoliat

Well IMHO it did. The auxiliary battery is regulated by the SAM, if there is no power to the SAM, there is nothing to tell it to connect, so it is disconnected, therefore not able to be charged through the main battery or able to take on it's primary chore of providing power to accessories. ??????

I do not think your question was ignorant at all, just the opposite, very pertinent.

Mud

Interesting thread as I just changed out the original aux (dash) battery in my non-ECO, non keyless-go 2011. 12v, 1.2 amp-hours so less capacity than a newer model with more features.

I think this relates to the pdf that bmwpower posted. When I looked at the WIS wiring diagram I didn't see anything that led me to believe that the aux battery is disconnected from the system when the main battery is disconnected. Plus I remember awhile back when I replaced the main battery in the car I was warned by my local indy to not allow the positive lead to accidentally touch ground as there was still voltage in the system.

This is for my 2011 but I think that there is some similarity to later years. Logically, the aux battery has to remain connected even if the main battery is disconnected or dead as one of its purposes is to enable the car to be shifted into park or neutral if the main battery dies, along with ECO control and maybe some accessories. Neutral to roll the car onto a flatbed if needed as I understand.

The WIS diagram for my car shows the aux battery G1/7 connected to left footwell ground W15/5 and to fuse F1/1f1 at the front SAM N10/1 which also includes a relay module and voltage sensor. Voltage is routed from the SAM to (1) the ignition switch module N73 and (2) transmission control module U490. Makes sense if it's needed for column shifter operation when the main battery is dead - allows ignition to unlock and for shifter to be operated. Charge voltage determination was as mentioned through the SAM module that senses voltage from the aux battery and increases charge if needed. The WIS shows the aux battery as a separate circuit independent of the main battery circuit, and it would appear that charge voltage (engine running) would be sent back to the aux battery via the SAM as it's very low amperage (small ga wiring) involved.

One difference that I saw was that for ECO/hybrid there was a circuit to the F32 pre-fuse unit, so it would seem that there are additional electrical loads for the later trunk mounted battery setups, especially with ECO. It may be that the charging voltage is routed through F32 since the amperage required for the larger trunk battery is higher than the mini-me dash battery.

The quickest way to tell is with a voltmeter, but this my interpretation of the wiring diagrams and thinking about the operation of the shift mechanism.

ygmn

Either test main battery leads when disconnected for voltage or
Better safe then sorry....and also disconnect aux battery.

bmwpowere36m3

Here's the ECO Start-Stop diagram...

It shows the aux battery has four (positive) connections:

• Front SAM via F96 (small wire, 7.5A circuit)
• Eco start/stop relay (controlled via front sam)
• Rear SAM (from Eco start/stop relay)
• Front pre-fuse box (built-in relay controlled by front sam to couple alternator to both batteries)

So a lot of the show is controlled by the front sam. There is that single connection via F96 which would remain between the aux battery even if the main battery was disconnected. Now what that does I don't know. The "main" connection of the aux battery is thru the rear sam (first thru eco start/stop relay) and then must be distributed to all the required modules during an ECO stop.

Alandf

From OP
Thanks .. If I understand correctly the main battery cannot charge the aux battery whether it is on a float charger or not
So is it a good idea to put a separate float charger on aux battery. And if on charger then disconnect main battery?

pierrejoliat

Well, if it has four positive connections, only made when the sam's are powered up and telling the relays to open or close, then logic would dictate they would be closed when powered down and so there would be no need to disconnect anything while charging either battery. IMHO

up_too_late

This thread has been weighing on my mind so I tested this today. There does NOT seem to be any connection between the main and aux battery, at least from this test. If that was the case I would expect both batteries to show the same voltage at all times, but they showed different voltages before, during and after charging of the main battery. For what it is worth I charge the two batteries independently. Approximately monthly I cycle around the family fleet and put the CTEK on them, because I'm a nerd. Sometimes more frequently, like this past weekend when my son left the dome light on when it was -10F.

Sunnyslope48

Great that someone actually did this test. You show as having 2 cars - which of them did you use? And I presume you tested with key off? Just wondering if there would be a different answer if it were key on - engine off.

up_too_late

I tested it on my 2015 E350 Wagon with the key off. I would not charge the battery with the key on as a normal practice. The 2018 GLE350 appears to have only one battery, that I have found anyway.

ghlkal

All right. This is good information to file away

Alandf

So now with all the information, especially how the SAM is not powered up when the car is off. the new question is "should I have a float charger on the main battery AND a second float charger on the battery in the trunk"
Obviously, the car is sitting still because it is winter and covid.

up_too_late

My opinion is yes. I have one charger and I alternate batteries.