Below is what I did.

start engine
wait 1 min at idle speed
2500 RPM for about 5 seconds
idle

I observed voltage across + and - battery terminals.

The reading was 12.40 V. It did not change at all when the RPM increased to 2500.

I followed the above procedure (except [start engine], since the engine was already on) with headlight and fog-light on. The reading was 12.21V. It did not respond to 2500 RPM either.

Did I do something wrong? Should I keep 2500 RPM for several minutes to see voltage increase?

The car is 1992 300D.

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1992 300D 2.5 turbo diesel. 319 k miles.
124.128 chassis, 602.962 engine, 722.418 tranny.