Sk8

I was mistaken. I see no evidence in the vehicle that I have a Mild Hybrid system. I was under the impression I did but there's no additional battery in the trunk/cargo area.

Mrclopec

Fas-uh-ney-ting! Thanks again for the confirmation.

@ThomasWShea @retom This whole debacle is emissions driven and the sole purpose of the belt starter generator is for the coasting function and braking...thats all...

angrypengu

I'm going to posit a hypothesis, as this is no longer a guess.

As our cars age, a very sure thing to start failing first, especially for daily drivers that aren't garaged (due to extreme heat and cold), is the 12V battery. As mine has aged over the years, I find the DC DC converter in step down mode (15.4V) for longer and longer during each drive. When the car was new, step down mode was only engaged for 5 or so minutes every half hour or so. Now, it's on all the time.

Today I almost lost my mind. I drove my car out of my garage to get it to operating temperature since I barely drive it in the winter. Sure enough, at one point when I was OBD11'ing before starting it, I get a battery error (12V). It was a false error as I was reading the voltage of the battery using a multimeter. Anyway, the saga begins.

Car starts right up, is held to 2000rpm in my driveway while I check out the coolant and oil temperature to get it 'hot' (to prevent fuel dilution in the crankcase). The BSG is doing its thing - slowly charging the 48V battery from 50% to 60%, and the car remains in step down mode at 15.4.

Once I'm done, I park the car, turn her down, and at this point what usually happens is once the car is locked, the car should immediately to go 12.8V and slowly settle down. But this time it doesn't.

It's stuck in this 15.4V state which means the 48V battery is actively being drained to charge the 12V battery. No matter what I did, the car remains in this state.

Now I'm thinking - oh crap, the 48V battery is going to drop to like 20% SOC which will kill itself.

Fortunately after 20 or so minutes, the voltage has settled back down to 12.8, then 12.7, etc, which means the DC DC converter is now offline.

So here's my theory.

I always keep an eye to my voltage level on my 12V system throughout the history of owning this car.

I wonder if what is happening now is that the 12V batteries are aging and as such people's 48V batteries are draining themselves to charge the 12V battery. As people start their car, the BSG is madly charging the 48V battery, and the DC DC converter continues to try and top up the 12V battery which it can't do after a specific point b/c the battery is on its way out.

Could this be it? I don't know, but it seems plausible.

I bring my car in for service next month. Maybe I'll just swap out the 12V battery just in case.

I've since learned how to sever the 48V and 12V systems. See Page 10/14 (of this PDF)

https://www.audi.de/content/dam/nemo...Audi_A8_EN.pdf

TIL. If I need to just disconnect this system, I will. What craziness.

retom

Of course + sway stabilization + electric supercharger (4.0TDI) which also need 48V.

ThomasWShea

You have spent a lot of time on this do you have any data collected or are you summarizing your observations? My response is partly my own questions and partly to help figure out what holds water.

I do not understand how the BSG is madly charging - doesn’t it always output 48 volts, all the time? Then that power goes to the BMS that’s integrated with the battery and the BMS decides what to do with the power coming in from the BSG? Honest questions here, I assumed the BSG was always putting out that 48 volts, so long as it is spinning.

The component that fails is the BSG and the software update audi does in the TSB is the software for the BSG. Though we know this software update isn’t a complete fix because some cars break again.

Although I think the behaviors ur seeing in the dc/dc converter makes sense to me…i.e. as the batteries age the converter will respond to those conditions and behave differently - I don’t think they would be the sole root cause of the exploding BSG issue.

If what you outline was the root cause, the system would be quite frail. I assume they would want to abstract the functionality out so that each conponent was independent of the others. The BSG doesn’t care about the dc/dc converter or the batts, and vice versa. Obviously there are designed interactions but you get the idea. They would need to design for symmetrical or asymmetrical battery wear. At some point the batts will need to be replaced, but that should not cause failure or stress on the dc/dc converter or the BSG because those components are designed for perpetual operation.

Thoughts?

angrypengu

Both. I'll comment in the rest later, just busy at work right now. But yes, I'm a data driven person so am going off data collected, only.

ThomasWShea

Look forward to seeing this. @retom and @Mrclopec might have good insights as well.

I've read and watch a bunch of videos on various 48 volt mild hybrid systems. The precise flavor of what audi is doing is still something we don't know all the details on, we are basically deducting what specific flavor of 48 volt mhev they are using case by case. One of the ideas mrclopec put forward was that it was all emissions driven and the sole purpose of the belt starter generator is for the coasting function and braking.

I agree, the emission driver is a primary reason for the advancement of mild hybrid - the technology would have never even hatched if it weren't for the emission drivers, HOWEVER, it does have one plus: It allows vehicle makers to electrically power higher power items (i.e. an A/C compressor) that were previous powered mechanically with the engine. You remove that drain from the engine and you can save fuel (i.e. emissions) or just dump it out as more useable horsepower . Either way you get to call it an mhev.

With all of this mess, if given a choice, I'd prefer the set up in mrclopec's car (14 volt air cooled alternator, feeding a 12 volt battery w a dc/dc converter keeping the 48 volt capacitor charged). With this approach, it seems like there is the option to engineer the car so you'd keep the car running even if the 48 volt side dies. With a 48 volt BSG to 48 volt battery w a dc/dc converter keeping the 12 volt system charged, the car won't move even one inch if you have a failure in one of those modern 48 volt components - and the 48 volt components are clearly not fool proof yet. Of course audi could still have the system in mrclopec's car set up so one failure brings it to its knees, but as we've seen in practice, the air cooled alternator cars are not subject to this disaster BSG issue. So it kind of proves the point that until they really know what they are doing keeping the primary system 12 volt makes more sense.

a75

Unfortunately, in our cars, the air conditioning compressor in the old fashioned way rotates mechanically with a belt and is not electric.

ThomasWShea

Is this the case with the BSG cars too?

a75

As far as I know, only the hybrid model (not mild hybrid) has an electric compressor.
All others are mechanical.

V8 TFSI

all others

Hybrid