is it possible to have a malfunctioning alternater on this car without the warning light coming on?ive been caught with a dead battery twice in the last several days despite a battery change.car starts fine then dies after a 1/2 hr or so of driving.very inconvenient.any suggestions for a fast easy alternater check

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michael cole 1980 300SD gone now but not forgotten,87tdt,90 300te 4matic 95 e320 wagon