Even though an alternator is rated as 170amp or 180amp unit, I believe it is a "per hour" rating. In other words, when the alternator is running at the proper RPM, it is capable of charging at a rate of 180 amps per hour while maintaining 14.7 volts (typically the voltage implied for overcoming internal resistance in the battery). The vehicle's battery supplies any additional power required above that hourly amperage draw.

This would be especially true in a diesel when the glow plugs themselves place a huge demand on the battery, coupled with the load of the starter turning over a higher compression engine with "stiff" oil. I can remember years ago with my GM 5.7 liter diesel that the startup at 32 degrees F could produce a 360 amp draw between the glow plugs and starter. And the old rule of thumb with gasoline engines was it took twenty minutes of normal driving to restore the energy to the battery that was used to start the engine.

It's quite possible, that depending on what electrical items were on at the time (A/C fans, electric radiator fan, headlights, stereo, heated seats, rear window defogger, etc), the instantaneous amperage output of the alternator was exceeded, dropping the voltage below the point at which the ignition system would work.

2002 x5 3.0 248,125 miles
2004 325i 110,000 miles

Historically: 1981 Pontiac Bonneville 2Dr Coupe 5.7L Diesel 265,000 miles!

from my school days, i think A/hr is a unit of charge used in batteries (similar to CCA rating) and is not used in generator type electricity, as it assumes that there a finite amount of energy in a vessel (battery) which can be depleted over certain period of time, while a generator has infinite amount of energy while the generator is spinning - the moment it stops spinning, there is 0 energy (not accounting for minute amounts stored in winds of wires and possible capacitors of the regulators)

but... i could be wrong... too lazy to look in the old books to see how alternators are rated...

from reading the PDFs of system read-out it seems that the systems just went off vertical cliff, and there were no bumps prior to that - that still does not mean that there was not any power surge or demand for power...

I don't know how you have your battery cutoff switch wired but I just started my X and then disconnected the battery using the cutoff switch and the engine kept running.

I have not tried to see how long or under what circumstances a X5 can run on the alternator but I don't recall it a problem on any vehicle I have owned. Not good to do and I got to a place to deal with the problem as quickly as practical but none of them died.

Don't worry about any transmission problems from losing electrical and then just coasting to a stop.

that was my take on the electrical system - you use battery to get started, and then the car runs off the alt... even if the battery is cut off... in my case, it is wired as simple as they get - the negative terminal of the battery wired to the switch, and the other end of the switch - to the negative cable that is grounded to the car...

The switch was tripped, there is no question about it... it was physically turned, and i had to turn to re-engage the battery...

demand in power is a very feasible explanation for a car that kept running on alt with switch already turned, and then losing the power when demand happened...

well, i am going to get one of those switch covers (like on THE RED BUTTON!!)...

i think i have a casualty in the LCM - it seems that one of the transistors kicked a bucket... not sure which one, as all the lights are working (all the ones that matter on the outside, all the internal ones that i had already checked).. but the INPA is still pointing to a burned leg 3 of a transistor ( i think, it is a collector leg... could be emitter... does not matter - transistors are not repairable, they are replaceable)... the actual transistor is numbered, and i did not figure out yet to which light that one corresponds to...

:iagree: with Terminator--check the high amperage connections and fuses in the spare tire well in front of the battery.