The battery dampens and filters the alternator output.

One thing that most modern vehicles do is the DME/ECU can and will kill spark and fuel if the input Voltage gets too high to protect the DME/ECU and other modules in the car in the case of a high Voltage swing. Usually 17-19 Volts it the trip point for most vehicle. No idea the exact value for BMW and I am not interesting in finding out on any of my cars.

You can arc weld with 120-200 Amps and as mentioned it is not a wise test to disconnect the battery on a modern vehicle for testing. Without the battery as a static load and filter, the alternator can do some funny things. Even filtering out AC ripple that can high peak to peak Voltages.

A $20 Voltmeter or using the Hidden OBC Menu to access the Voltage and display it on the dashboard is all that is needed. Charging and starting systems are really easy to trouble shoot as long as the EWS does not get in the way.

Learn how to use a Multimeter/Voltmeter and learn how to perform Voltage Drop testing and you can correct the majority of problems that creep up. I also use a spare pair of jumper cables to connect temporary grounds to identify and find problems.

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2008 4.8i Black with Tabacco. Black Y-spoke wheels and shadowline trim.