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How to maintain the 2 batteries on M850i

19K views 36 replies 7 participants last post by  eecsdude 
#1 ·
The 2019 M850i has 2 batteries. I bought a BMW battery maintainer. Does anyone know that when you connect it to the charging post under the hood which battery does it charge - the one in the trunk or the one in the engine compartment near the windshield on passenger side, Or does it charge both? When I connect it to the charging post it eventually completes the charge and goes into the float mode. Which battery is being floated? Called 3 dealer service departments and BMW customer service and no one can give me a complete answer. Thanks
 
#2 · (Edited)
You can buy a setup of cables and fuses from Ctek, just look online. This will allow you to put some permanent wiring in so when you connect your charger it is through a plug and socket, they even have traffic lights to indicate battery charge. This is cheap to do and makes life very easy and reduces any spark risk.
I think that you will find that the power side of the batteries connects to a charging point, so both batteries can charge, if I am correct my car has two batteries one for main electronics and the other for the active steering.
I use one charging point in the engine bay, yours will be the same as mine, best to use these points rather than over the batteries themselves. you will find a ground -ve and a red positive +ve I think on the right hand side (mine is left hand side) you can then hard wire to these points.
I charge my car once per week this way, have done for the last three years. You learn very quickly with these BMW's that not to look after the battery can lead to all kinds of weird issues, I leant this on my E63 and carried it over to all my cars!! I have never ever had to buy a new battery, some lasted 8 years!!!!
 
#3 ·
#4 ·
Thanks Michael and Doug. I have been using the CTEK connected to the charging post under the hood. And the charger proceeded through the lights while charging and has gone into the float mode. If the CTeK connected to the charging post is actually charging both batteries, which one is being floated at completion of charge?
The 3 dealer service departments I contacted didn***8217;t know if both batteries are being charged by the CTEK so they also didn***8217;t know which one is being floated.
After contacting customer service an area manager has contacted BMW engineers to get me an explanation for my questions. I***8217;ll post when they get back to me.
Thanks for your help.
 
#6 · (Edited)
Thanks Michael and Doug. I have been using the CTEK connected to the charging post under the hood. And the charger proceeded through the lights while charging and has gone into the float mode. If the CTeK connected to the charging post is actually charging both batteries, which one is being floated at completion of charge?
The 3 dealer service departments I contacted didn't know if both batteries are being charged by the CTEK so they also didn't know which one is being floated.
After contacting customer service an area manager has contacted BMW engineers to get me an explanation for my questions. I'll post when they get back to me.
Thanks for your help.
Not sure if you took advantage of the link Doug provided...but after going to the TIS website link Doug posted and doing some additional searching around the site and searching "recharge battery" and "dual battery" etc...I came away with a lot of information regarding charging the standard AGM battery as well as the additional Lithium battery (I'm assuming that's what you have). The multiple links the site presented doing a simple search on that website gives info about the specifics of charging each battery as well as warnings as to not overcharge using different voltage amounts.

It really is worth doing a little homework to see how the systems function and how each battery type is used (and why)...as well as how to charge each of the differnt battery types.

But you can also do as you suggested and wait for someone from BMW to get back with you. But I find reading the material myself instead of being told by someone else...it remains in my cognitive memory banks better. :D

Like this one: https://www.newtis.info/tisv2/a/en/...l-system/61-21-battery-with-holder/1VnYofmbZo
 
#5 · (Edited)
Hi, the difference for me is that I don't float the batteries, I just do the charge and if not using the car just let the battery run down, as I said I charge once very week to a full charge. There is a battery charge/condition management system on these cars, so in float mode I would surmise that each draws current as the electronics call for it, but I am not storing my car so do not need to float as my car always gets used during the week even if it is run for fun!!!
Finally it would not make sense to have individual charging cycles for each battery, and individual charging points. I am certain I read once that it is only a single charging point take a look at the WDS it will tell you !!!
 
#7 ·
Good citation.

I notice -

Warning!
Do not charge the lithium-ion battery with charging voltages of > 14.0 V! Do not use rapid-charging programs!

I know also that AGM must not be charged with more than 14.7 VDC and not with rapid-charging programs.

Note that CTEK goes to 14.7 VDC or 14.8 VDC cold temperature program. CTEK goes to 4.3 Amps not a rapid-charge program.

The Lithium Ion BMW battery has an individually programmed charge controller built in - an additional cost of the battery. The charge controller can trip the battery off or generate check control messages.
 
#8 ·
Well you have certainly piqued my interest so I have followed the links and read as much as one can follow. Firstly I went to my own model vehicle and found that both batteries are AGM batteries one 105AH and the one for the active steering 12AH. given the one charging point recommended then I believe both batteries maybe charged, but I feel that it is the 105AH that is charged and the secondary battery 12AH takes its charge from the main battery, hence the float goes to the main battery,
Therefore there is no lithium ion on my car, so if you go to the parts catalogue you can find what types of batteries exist on your model. Again I would surmise they are the same both AGM batteries.
I have found no references to having to charge the batteries separately, only the main charging point and again no references to separate floating process!!
I have been doing charging cycles now for 3 years with no issues!!
 
#9 · (Edited)
Here's some older (for those so inclined to read this kind of stuff) info that goes back to the e32 750iL and the original e31 850 series as well as the e38 750iL which all used a dual battery set-up. As I was reading the info about the G15's dual battery set-up...it seemed to be very similar to the dual battery set-ups from the past. You can see in the info below (if you clicked the links to the G15 dual battery set-up and read through the info...as usual, BMW uses a similar design architecture. Interestingly, the 2nd (smaller) battery was used (back then) to facilitate additional electrical loads just like the new G15 8 series. There was an "isolationi switch used at that time that opened or closed which would determine when the 2nd battery would be charged. That appears to be the same on the new 8 series except now it's called a disconnect switch (based on the text I read on the TIS website).

The other major difference in regards to batteries between then and now is the use of a lithium battery to facilitate the increased electrical load vs using a lead acid or AGM battery.

{e32}



{e31 & e38}







 

Attachments

#10 ·
The M850i does have a Lithium battery under the bonnet, if you have the auxiliary heating you may well have another AGM battery, then the main AGM battery in the boot.
So if you access the online menu for the manual, you need a VIN code, you may well find in the paper manual with what it says about charging all batteries. As I don't have access I have reached a block!
 
#11 ·
Thanks QSilver7,
My car does not have any Lithium batteries. The large one in the trunk is an AGM and the smaller one under the hood beneath the passenger side windshield is also an AGM. I read all the technical material. And although it is very informative, it doesnt explain if and how connecting a ctek maintainer to the charging/jump post under the hood charges 1 or both batteries or what is being floated when the charger goes into float mode. Manual does say to always use the charging post under the hood and not connect the charger directly to the battery. My dealer service department also read all the material and they could also not determine what actually is happening if one or both batteries are being charged and what is happening in float mode. And if 1 or both batteries are being floated.
Hopefully, I***8217;ll get the answers for the engineers.
 
#12 ·
car that starts at 112K? Personally seems like the only BMW employee that could explain the process would be the design engineer in Germany.

The vehicle battery located in the luggage compartment is charged per the specifications of the battery charger. The auxiliary battery only is charged if the maintainer is connected while the vehicle is not in sleep mode. Sleep mode occurs approximately 8 minutes after the vehicle is turned off. The Hazzard switch illumination is the visual indicator of sleep mode for your vehicle. (e.g. lite on=vehicle awake, light off=vehicle in sleep mode). If the maintainer is connected when the vehicle is in sleep mode one only needs to wake the vehicle to begin the charging of the Auxiliary battery. Once the maintainer is charging the auxiliary battery it will continue to charge it even after the vehicle resumes sleep mode.
 
#13 ·
Regional manager and engineer got back with email answers They could not explain the sequential process that takes place to both batteries from bulk charge to float once BMW maintainer is connected to charging post under the hood.
Consequently, I have little confidence in BMW service departments or regional engineer knowing anything about this process. Sad support for a car that starts at 112K? Personally seems like the only BMW employee that could explain the process would be the design engineer in Germany.
Below is the only usefull info that I got. Far f a complete technical information that I requested.

The vehicle battery located in the luggage compartment is charged per the specifications of the battery charger. The auxiliary battery only is charged if the maintainer is connected while the vehicle is not in sleep mode. Sleep mode occurs approximately 8 minutes after the vehicle is turned off. The Hazzard switch illumination is the visual indicator of sleep mode for your vehicle. (e.g. lite on=vehicle awake, light off=vehicle in sleep mode). If the maintainer is connected when the vehicle is in sleep mode one only needs to wake the vehicle to begin the charging of the Auxiliary battery. Once the maintainer is charging the auxiliary battery it will continue to charge it even after the vehicle resumes sleep mode.
 
#14 ·
Here goes. The customer service manager kicked my questions up to engineering in Germany. They got a response and informed me that the response German Engineering is giving me is all they could give me. The response was useless. It only addressed charging a one battery car not a 2 battery car.
I responded to customer management that engineering is either incompetent or rude to a long time customer.

I did buy a volt meter. I followed the advice of the regional engineer of connecting the charger before car goes into sleep mode to insure the auxiliary battery is charged in addition to the main battery.
The ctek mus 4.3 bmw maintainer was attached to the charging post. As the charger went into bulk mode and the voltage at the post peaked at 14.5 and it was also 14.5 at the trunk battery per BMW Ctek Mus 4.3 spec. However, the reading at the auxiliary battery under the hood peaked at only at 12.86v. When the the charger went into float mode the reading at the charging post and st the battery in the trunk leveled off and stayed at 13.6v per the BMW CTek MUS 4.3 chargers spec. The reading at the auxiliary battery in float dropped from the 12.86v and dropped to 12.83v
So from this it looks like the reading at the post is the reading coming from the main battery only. It also looks like the BMW battery management system either limits the max volts of the auxiliary battery to 12.86v and isolates it at that point. Or maybe because the trunk battery reaches 14.5v and the charger switches to float at that point the auxiliary battery stops getting a charge even though it only got up to 12.86v.
As for the question are both floated? All I know is the trunk battery has stayed at 13.6v float charger spec for 36 hours where as the auxiliary battery never reached the 14.5v bulk charge per charger spec only rrached 12.86v and has dropped .03V in 36 hours during float.

So it looks like both batteries do get charged but the BMW battery management system does not distribute the chargers charge equally thus the auxiliary never gets to 13.6v. float spec. It also looks like it got isolated at 12.86v and it doesn***8217;t get floated since it***8217;s voltage drops while the trunk battery stays at the constant float voltage.
Maybe that***8217;s why BMW auxiliary batteries seem to fail first on BMW 2 battery cars.

It would help if BMW would have the decency to explain this to a customer who has exclusively driven BMW cars for 21 years. If anyone ever finds out how all this really works, please help.

PS Regional engineer said the auxiliary is only there as a backup to the trunk battery if it it gets depleted? not there for any other specific function. Really?
 
#15 ·
One additional fact.. Just tested the 2 batteries while engine running. Alternator at startup is charging the Auxiliary Battery to 14.5V and the Trunk Battery at only 12.9V. Interesting that at startup the battery management system is directing a heavy charge to the auxiliary battery.
I would assume at some point in time that 14.5V would be directed to Main Trunk Battery.
Sure would be interesting for BMW to explain to their customers exactly how the alternator directs the charge to the two batteries and how exactly the bmw maintainer charge is handled when connected to the charging post under the hood.
 
#16 ·
Well more testing. At this point. It seems the only time the Auxiliary battery is charged is when the engine is running.
When the maintainer is connected to the charging post only the trunk battery has its voltage rise. The auxiliary voltage drops a couple hundredths of a volt over 24 hours. So it obviously is not being charged. And when the charger goes into float mode only the trunk battery is floated. The auxiliary battery continues to lose slight voltage.
The only caveat to this theory is that I***8217;ve never had the maintainer connected for more tha 2 days continuously. So the auxiliary battery has never fallen below 12.6 volts. I***8217;d be interested to see that if it does, whether the battery management system might direct charge or float charge to it after it drops below a certain level. Won***8217;t be able to find that out until the winter, when the car will go for long time without it being driven.
 
#17 ·
Well you really have got your teeth into this issue!! When you mentioned drawing current from the auxiliary battery at startup would come as no surprise, given the amps necessary to start the engine (it is a big beast). The charging profile for the aux battery does come as surprise, that it only really charges when the engine is running and appears not to be float charged when a tender is attached. I have 4 wheel steering which operates via the second battery and not understanding if the tender will keep this battery is a concern! It does not surprise me that local BMW staff don't have a clue, but not necessarily their fault as it is BMW Corporate who controls what they know!! Have you tried looking at the WDS for your car, this may give you an indication as to how the battery and cables/connections are done through the car. It should help in ascertaining how the charging works???
 
#18 ·
Thanks Michael. Just to clarify. I***8217;m not attaching the maintainer directly to the Auxiliary Battery. I***8217;m attaching it to the charging post under the hood as directed by the engineers and the manual. Was told by the regional engineer that it would charge both batteries. They also said never to attach the maintainer directly to either battery as it may damage the battery management electronics.
And being an former technical IT guy, the lack of anyone at 3 dealers or regional engineering or Corporate engineer, really is frustrating.
I***8217;ll look at the WDS, to see if can shed some light.
Thanks again,
Stan
 
#19 ·
Just an update. Have left the BMW maintainer on the post under the hood for 1 week. When I parked the car after a long drive and before putting the maintainer on the post, the trunk battery was at 13.02 volts, the auxiliary battery under the hood was at 12.84 volts. After a week with the maintainer still attached the trunk battery is being floated at 13.63 as the maintainer manual said it would be. The auxiliary battery voltage has dropped to 12.79
So it seems as if the Auxiliary is only charged while driving and that once the engine is shut off it is isolated. Hence it is losing about .01 volt per day.
Which means there will be an issue with the auxiliary battery if the car would be stored for a long period.
This is maybe why I couldn***8217;t get a straigth answer from anyone at regional or corporate about how the auxiliary is effected by the maintainer. And maybe this is why their auxiliary batteries fail so quickly.
I guess BMW engineers are not very sharp.
They put you in a Catch 22 situation when they tell you never to connect your maintainer directly to either battery as it would damage the car***8217;s battery management; and then they don***8217;t charge the auxiliary battery when you connect the maintainer at the charging post under the hood as they direct you to do.
At least on a Mercedes with 2 batteries, you can put a maintainer on both batteries.
I still wonder if at some point, as the auxiliary battery continues to lose voltage, it hits a point when the battery management system would start directing a charge to it. But at this point no one I***8217;ve contacted at 3 dealer service departments, at regional, and at corporate can give me a competent explanation
 
#20 ·
Hi! It could be that the auxiliary battery has zero drain cct wise and only comes into use when driven, the main battery of course has a continuous drain to Alarm etc. I agree with you about people storing these cars and not realising the aux battery is not being charged, I am a believer that if a battery is not correctly charged or kept at capacity then it leads to component failure, I look at the old 6 series E63/64 which has major battery problems and component failures, especially people who did not use their car as a daily driver or very low usage. I do also share your concern that no one from BMW Technical can give you a suitable answer!!!
 
#21 ·
Thanks 645-333. I also had a 645ci and I had 2 batteries die prematurely in 3 years causing all kind of weird messages and problems I then put a maintainer on the battery when not driving it. That battery lasted 9 years. Given that experience, that***8217;s why have been so anal about maintaining the 2 batteries on the new M850xi. But with BMW insisting not to connect a maintainer directly to either battery to prevent damage to the battery management system, and given the auxiliary battery doesn***8217;t get charged when the maintainer is connected to the charging post battery under the hood, the auxiliary battery is doomed to a short life. This is total incompetence and malfeasance on the part of BMW. Thanks 645-333
 
#24 ·
Even though the auxiliary is a 12V AGM, it is not getting charged from the underhood post that obviously is just charging the main battery. Bottom line by my monitoring of both batteries for over a month, they are isolating the auxiliary once the engine is turned off. Unacceptable egineering.
 
#26 ·
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#27 ·
I read that certain 5 Series models with an auxiliary battery require 24v for their steering at times, so there is switch that puts the two 12 volt AGM batteries in a series to provide the 24v when needed. Anyway, I fault BMW with not providing their service departments with knowledge that they can use to explain to their customers how to maintain both batteries witen the car is stored. The service departments have no clue. It is now obvious that the auxiliay is isolated when the engine is turned off; and it is not charged when a battery maintainer is connected to the charging post under the hood. That only charges the main battery. And they say not to connect the maintainer directly to either battery. BMW needs to get their act together. It should not be a mystery to their service departments, on how to maintain both batteries. Totally unacceptable.
 
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