I'm pretty sure you have an alternator that uses an "exciter" circuit to make the alternator charge at lower RPMs when you first start up. If that circuit isn't intact, the engine will have to be revved-up to RPMs one would normally see while driving to initiate charging. This might explain why you see normal charging voltage only after driving for a time. I also think you have a weak battery.

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-Louis

'87 300D Anthracite