Yep, 15V is what you might see if the battery was nearly or completely dead and the charging system was putting out more volts to charge it. MB don't usually get over ~14V. Busted is right, 15V is a tad high.

Volts are dependant on the RPM of the rotor as well as the strength of the rotor field - field strength is a function of current through the windings of the rotor which is controlled by the regulator. Basically, output voltage is somewhat variable - load and engine speed are factors.

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Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)