E70 Increased Battery Discharge while Stationary

[Penguin]

Originally Posted by JCL

It isn't a peak charge problem, it is an average charge problem.

But Alternator output is dependent upon RPM. So you are right, it isn't a peak charge problem, but still may be an undersized alternator problem in that the alternator cannot provide enough output at lower RPM to fully-charge the battery on short trips, e.g., the software may be asking for max charge from the alternator, but the alternator cannot comply due to low RPM. I suspect that there would be no problems with batteries going dead from short trips if people put the transmission in manual mode on short trips and kept the engine RPM's at 4,000, even while stopped at stoplights.

Here's the curve for a couple of alternators, and you can see that at 2000 - 3000 RPM (shaft RPM, not Engine) the output is much less than Max:

http://image.superchevy.com/f/902272...c_alt_03_z.jpg

http://www.vortexbuicks-etc.com/CS144.jpg

I don't have the X5 pulley ratios, so I can't translate alternator output to engine RPM, but the effect is there, it is only a question of to what extent.

To help avoid the battery issue, I tend to put the transmission into Sport mode if I know I am doing short trips.