I would say that your (1) and (4) are conditions, not causes.

(2) is related to load, and (2) is related to charging time. BMW will have assumed a certain load, and a certain profile during the drive to recharge the battery.

I may have accidentally led you astray, I was simply trying to say that if the alternator was too small, then all owners would have the problem. Many of us have never had a flat battery, ever, in any modern BMW. Load would vary with the number of starting cycles (the other running loads are pretty small), and the remaining variables during operation come down to how people use the car when the battery is being recharged. The last variable is what happens when the car is not being used, and whether reserves are being drawn down.

Absolutely agree. But I would have thought that with a 2 or 3 to 1 pulley ratio (just guessing, but I think that is the range) then anything above engine high idle should be contributing to the charge.

Yet another reason for manual transmissions, and regularly running the engine close to red line once it is warm.

For those interested, here is BMW's Energy Management Strategy for the E70, dating back a few years.

http://www.bmwmotorsports.org/pdf/e7...Management.pdf

Executive Summary: Penguin's Explorer, referenced above, had a voltage regulator that read battery voltage, and decided whether to charge the battery or not. Worked pretty darn well. OTOH, the above document takes 35 pages to explain when the alternator should be charging the E70, or not.

True, unless BMW chose a ratio at the bottom end of the range to improve power and fuel economy, figuring drivers would not make short trips with heavy electrical loads, such as headlights? (Or idle the engine a long time) Essentially putting a mild "underdrive" pulley on the engine as stock?


Note item #1 under the section labeled "Some Commonly Expressed Concerns about Underdrive Pulleys:"

UDP Tech

It's a Mustang in this link, but the concept still applies.

Yeah, it did. I had it for 198,000 miles and 19 years, and it only had three batteries, counting the OEM battery. And those two replacements were preventative, as I didn't want to be changing it in a parking lot in the Winter at night ( one replacement after nine years and the second replacement after eighteen years.) So it's not like an "unsophisticated" charging system hurt battery life at all.

As one of my old bosses use to say, BMW's charging system just might be "one half too smart."

The E70 shows both 180 amp and 220 amp alternators, depending on options (rear seat heating brings it up to 220 amps). It may vary further with different model years. In any case, those aren't small alternators.

I am with your old boss, I think it is a complexity problem.

This is actually a BMW training manual used in schooling techs, as good of info as it gets for most things.

Just as an aside, and certainly not even in the same 'family' of cars, but the alt in my '02 C5 is 110 amp, (& made in France :D)...the interesting point is that our C5 'cruises' at 1500 -1700 rpms, (not 'high rpms), yet that smallish alt seems to do a good job of keeping the electrical system/batt running well. No food fight, just interesting to me.

Maybe the same engineering group that does the window regs, works on the 'electrical sys' of Xs, on the weekends. ;)
GL, mD

And with that amount of complexity, just think of the job security for our beloved BMW techs!

(The training manual is posted under the Online References tab, and covers all of the E70 systems, for those interested)

One of the next things I'm going for training on is the active hybrid, which I'm sure will make this charging system look like an old 1 wire GM alternator... Just the transmission in the active hybrid is amazing enough.