[RClementTX]

I thought this topic was worthy of its own entry since I’ve contributed to other posts in the past as we’ve all “head scratched” over how this system works.

After a bunch of research, discussions with BMW Techs and accessing a private copy of a tech guide (ST2007), I think I understand this system enough to make an intelligent post for those that are interested in its operation.

Here are my key take aways –

1. There are separate 12V and 48V systems in the mild hybrid version of the 6-cylinder 4.0 gasoline engine.
2. The vehicle only generates 48V power (via the generator/starter) which is used to power the Lithium-Ion battery mounted under the hood near the firewall on the passenger side. The battery is nominally 44V and 11 amp-hour capacity, meaning that its of relatively small capacity.
3. It uses nickel manganese cobalt (NMC) chemistry on the anode and Lithium titanium oxide (LTO) for the cathode. In battery terminology it’s referred to as a “NMC/LTO” chemistry. The battery has its own cooling system which it shares with the PCU (see below).
4. The Lithium battery also has its own battery management electronics incorporated into the battery. A switching contactor immediately adjacent to the battery disconnects the 48V system as soon as the hood is raised.
5. The 48V system will switch back on when the hood is closed, the car gets driven or when an external 12V battery charger is present.
6. A Power Control Unit (PCU) converts 48V power to 12V power to charge the AGM lead acid battery buried deep below the optional spare tire in the rear of the luggage compartment. The PCU is bi-directional, charging the 12V system from the power generated by the generator/starter (to run all the normal 12V systems). Plus, the PCU converts 12V power to charge the 48V battery/systems when/if the 48V battery is low in charge (or there’s a 12V charger operating). The PCU is often referred to as a DC/DC converter.
7. The PCU is rated at continuous 3KW / peak 3.5KW (both ways), is mounted inside the car in the front passenger footwell, is cooled by its own coolant circuit. For all practical purposes, the PCU is the “bridge” between the 2 power systems.
8. A conventional 12V starter starts the engine on the first start of the day. Thereafter the 48V generator / starter does the work. Thus since this is the “cold start” the 12V AGM terminal needs normal starting “oomph” to handle cold days.
9. External charging of the vehicle uses the 12V jump start terminal in the engine compartment and this charging is primarily for the 12-volt battery. When the 12V battery reaches at least 50% SoC, the 48-volt battery charging circuit powers up and charges the Lithium-Ion battery. The Lithium-Ion battery is also charged during normal engine operation and during coasting. External charging only functions if the hood is open.
10. If you attach a battery charger to the charger terminals with the hood closed, only the 12V system gets charged.

Of note, Lithium-Ion batteries and Lead-Acid / AGM batteries have different needs for maximum life with Lead-Acid / AGM batteries needing to be fully charged in order to get maximum possible lifetime from the battery. Lithium-Ion batteries don’t need/want full charge. (refer to https://batteryuniversity.com/learn/...ased_batteries)

My experience indicates that when I put a charger on my X7 with the hood open, I frequently get a “Power Management” error in my check control and I can hear relays opening/closing up near the Lithium-Ion battery randomly during the charging process. I suspect that the Check Control error and the relay action is related to the 48V charging and cooling circuits cycling on/off.

It seems that I really only want/need a battery tender to keep the AGM battery topped up when the car’s idle for extended periods since it’s the 12V systems that are continually stressed by systems that never completely shut off. I support this thesis since at the dealership, they have the hood closed when they use a battery tender on the showroom floor and seldom report “Power Management” errors. I’m told that “hood closed” charging only tops up the 12V battery and reduces the likelihood of a “Power Management” check control.

Hope this helps anyone that’s been befuddled with the mild hybrid electronics.

[danchee]

That's some nice info. I've seen the "battery assist" note in driving information when driving come on and off but I have not figured out the exact scenarios where it assists the engine and when it doesn't. Any one have ideas? Driving the exact same road and using the exact (well, very close) throttle, one day it comes on, but another it doesn't.

[J_PNW]

RClement, thanks for the research and excellent info! Clears up a lot about the power management and mild hybrid system.

I was wondering whether leaving my ‘40i unused for a month might lead to battery problems and considered leaving it plugged into a tender.
After a couple of trial runs (granted, not more than 24 hours plugged into a relatively weak 2-amp tender) I decided against it and simply left it standing in the garage for a little over a month.

No issues whatsoever, started perfectly and no battery level light.

I’m sure that for longer periods, a tender may be necessary. However, the complexity of the two systems is such that I would worry about a 3rd party tender charger potentially causing issues.

So for folks who are looking at ~1 month of no usage, no worries about battery not holding based on my experience.

[thenew3]

Is the BMW 48V mild hybrid system similar to MB where the 48v starter/generator is sandwiched between the engine and transmission? Or is it like the VW/Audi system where there's a rubber belt connecting the 48v starter/generator to the engine crankshaft?

[RClementTX]

Originally Posted by thenew3

Is the BMW 48V mild hybrid system similar to MB where the 48v starter/generator is sandwiched between the engine and transmission? Or is it like the VW/Audi system where there's a rubber belt connecting the 48v starter/generator to the engine crankshaft?

BMW system has the starter/generator sitting in the position of the old alternator and belt driven. The belt is larger than that on my wife's non-hybrid X5 S4.0 presumably in order to transfer more horsepower into / out of the starter generator. Plus the starter/generator is liquid cooled by a leg off the main coolant loop for the engine.

[RClementTX]

Originally Posted by J_PNW

RClement, thanks for the research and excellent info! Clears up a lot about the power management and mild hybrid system.

I was wondering whether leaving my ‘40i unused for a month might lead to battery problems and considered leaving it plugged into a tender.
After a couple of trial runs (granted, not more than 24 hours plugged into a relatively weak 2-amp tender) I decided against it and simply left it standing in the garage for a little over a month.

No issues whatsoever, started perfectly and no battery level light.

I’m sure that for longer periods, a tender may be necessary. However, the complexity of the two systems is such that I would worry about a 3rd party tender charger potentially causing issues.

So for folks who are looking at ~1 month of no usage, no worries about battery not holding based on my experience.

Your experience of a one month hiatus is what started me on this quest for information since I have periods where I'm out of town for a few weeks in a row. From what I've learned, I still believe that keeping an AGM (or conventional lead acid) battery at full charge meaningfully extends its life compared to letting it sit in a discharged state. So for me, knowing that I can still start the car after a month is nice to know, but a month of slow discharge for the systems that don't shut down leave the AGM battery sufficiently discharged that I believe (from my reading) that the battery life is affected. I intend to float my X7 on a smart charger (CTEK or BMW branded version of CTEK) set to AGM charging profile when I'm gone for extended periods simply as a means of extending battery life to a maximum. But when I charge, I'll have the hood closed before powering up the charger so that the 48V system never kicks in.

[RClementTX]

Originally Posted by danchee

That's some nice info. I've seen the "battery assist" note in driving information when driving come on and off but I have not figured out the exact scenarios where it assists the engine and when it doesn't. Any one have ideas? Driving the exact same road and using the exact (well, very close) throttle, one day it comes on, but another it doesn't.

The BMW technical guides define 2 conditions where the battery is “helping” out with some added horsepower to give the gasoline engine an assist.

ASSIST mode
“The combustion engine is mechanically supported via the electrical machine of the starter motor generator with up to 8 kW (10 hp) in order to achieve an optimization of the load point. Thanks to the additional torque of up to 120 Nm (89 lb-ft) the torque in the combustion engine can be lowered, which in turn results in reduced consumption and emissions.” To me this means that the starter / generator reduces the load on the combustion engine with the objective of improving mileage. But remember the Li-Ion battery is relatively small, so the Assist mode is necessarily limited.


eBOOST mode
“During the eBOOST function the starter motor generator briefly makes available additional mechanical power of 8 kW (10 hp). This additional power supports the combustion engine during acceleration, thereby improving the vehicle dynamics. The eBOOST function generally only lasts a few seconds and is intended to support the combustion engine in situations in which the exhaust turbocharger cannot yet generate sufficient charging pressure…” I think the operative word here is “briefly” which means that when you look for incremental horsepower compared to what the combustion engine can deliver, the starter/generator kicks in briefly (momentarily?) to provide a “kick”. That probably means that unless you are concentrating on the instrument display, you'll miss the notice indicating a boost took place. I've never seen it on my X7 yet, but I have less than 1000 miles under my belt in the vehicle.

[smp]

Does the A/C compressor run off the 48V battery in the 2021 or is it still belt driven? The biggest problem with Auto start - stop systems is the A/C system quits working when the engine shuts off.

[thenew3]

Originally Posted by smp

Does the A/C compressor run off the 48V battery in the 2021 or is it still belt driven? The biggest problem with Auto start - stop systems is the A/C system quits working when the engine shuts off.

My guess is no since RClementTX says the 48V starter/generator replaces the old 12V alternator and is still belt driven. My guess is the AC compressor is still belt driven and won't work when engine isn't running.

the MB system jams the starter/generator in between the engine and transmission and has no belts at all. The AC compressor and everything else is run off the 48v system so AC continues to run when engine is off.

Also the battery in the BMW seems to be about 1/2 the size of the battery in the MB. 44v 11ah is around 0.5kwh, the MB mild hybrid has a 0.9kwh battery. Maybe it's bigger to support the AC and other devices that run off the 48v system.

[danchee]

I can confirm A/C goes into lower power mode when start/stop is active. When the engine turns off, I can hear the A/C fan decrease and cooling reduced.

RClementTX - I have seen the eBOOST function kick in, primarily when going up hill. Long steep hill and if you step on the accelerator hard, you feel this slight shove then hesitation (very very short blip) and then turbo kicks in and keeps pulling.

[RClementTX]

Originally Posted by smp

Does the A/C compressor run off the 48V battery in the 2021 or is it still belt driven? The biggest problem with Auto start - stop systems is the A/C system quits working when the engine shuts off.

A/C runs off the same belt drive as the Starter / Generator.

From what I can tell, mechanical systems in the mild hybrid version of the 4.0i mimic what was in the non-hybrid 4.0 I-6 engine EXCEPT for the Starter/Generator replacing the alternator, a new belt tensioner system associated with the S/G (since sometimes the rotational thrust on the S/G changes from CW to CCW), and the size of the belt/pulleys.

[freshxdough]

Charging the 12v battery via the jump points under the hood is possible. Once the 12v battery is seen as full, it will then start charging the 48v system.

When the hood is open the 48v system is naturally deactivated. So I would assume that it still somehow gets charged, but perhaps does not enable 48v circuit in the engine compartment near the SGR and through the visual purple lines seen in the engine compartment.

[streborx]

If the 48 volt battery is rated at 11 amp-hours, I don't understand why a 20 amp charger needs to be used. The SIB refers to chargers less than 20 amps as "trickle chargers"? I think not. Charging any battery at 2x its capacity is an endeavor I wish to be nowhere near while executing.

BMW engineers designed in a battery mutual suicide pact. Certainly explains the vehicle sudden death syndrome reported by several owners.

[X7 Recruit]

Thanks for the helpful overview. I’m in a 40d with the 48v system. I’ve read a lot of mild hybrid bashing on the forum and, IMO, it’s a good system that is almost invisibly integrated, especially the start/stop feature.

I tried charging the battery using a CTEK charger with the hood open. I didn’t get any error messages, but the charger box itself was very warm to the touch after running overnight, something I’ve never experienced with normal operation. Needless to say, I haven’t repeated the exercise. I haven’t tried charging with the hood closed, but the car doesn’t get parked for extended periods.

[julioccuk]

Hi All,

I have just taken delivery of my LCI X6 which comes with the BMW MHEV system.

This may be a silly question from a newbie, but I would like to disconnect the battery to do some work on the steering wheel which requires the airbag to be removed. Usually I'd disconnect the negative terminal in the trunk so the horn/airbag don't work as you carry out the job.

I tried that, however the horn was still going off so I believe it's related to the front battery that I didn't touch, do you have any idea how I would be able to get the job done by safely disconnecting the batteries and if so, what order/how do I go about it?

It would be greatly appreciated.

Thanks
Julio