It is somewhat common for the Pre-glow relay to stay on after starting the car. Since the glow plugs take about the same amount of current as the alternator can put out the battery will go dead. Other than testing the charging system after every start or disconnecting the glow plug harness, replacement is a good bet.

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MERCEDES BENZ MASTER GUILD TECHNICIAN
ASE MASTER TECHNICIAN
27 YEARS M.B. TECH