The diesel generator sets on your mollusc dredger use a starting system similar to the one shown in the illustration. While the engine is running, the starting battery bank terminal voltage and specific gravity gradually fall off and the ammeter indicates a discharging battery bank, eventually resulting in a dead battery bank. Which of the following failures would most likely result in this condition? Illustration MO-0202
• Trace how battery charging current flows from the alternator, through the voltage regulator and field circuit, back to the battery bank in the illustration. • Think about what has to happen electrically once the engine is already running versus what is only needed to crank/start the engine. • Consider how an open circuit in a control or field winding affects current flow and whether the alternator can still produce an output.
• When the ammeter shows discharge with the engine running, which component in the drawing is responsible for replenishing the battery, and what must be true electrically for it to work? • Which of the listed failures would completely stop charging current from flowing, but would still allow the engine to be started normally from the battery? • Look at the roles of the starter solenoid and start switch in the diagram: are they part of the circuit used only during cranking, or part of the continuous charging path while running?
• Verify which component actually connects the alternator output (B terminal) back to the battery bank in the figure. • Check whether an open circuit in that component’s field or control side would prevent the alternator from developing any charging voltage. • Confirm that the failure you choose explains both symptoms: (1) engine can run, and (2) battery voltage and specific gravity slowly drop while the ammeter shows discharge.
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