i have the same problem with my 87 300e and its probably a weak/bad alternator, ive never had a charging problem so i ignore it (i know the little battery icon light works on my car) it could also be something with the voltage regulator but its hard to tell without swapping out for another one. i would think v belt slippage would manifest in other ways long before being able to cause a problem like this. do the belts squeal loudly even when the car has warmed up? some nice battery chargers have the option to test alternator and your local parts store will probably do the same for free (still haven't learned how to use my multi meter but im sure it would test it too) hope this helps!!

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E30 B25
126.036
123.130 with 616-Frankenbenz (sold)
124.030-Greyhound (sold)
124.030-Schnauzer (sold)

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