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A
It starts the engine
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B
Generates electrical power while the engine is running, charging the battery and providing power for all vehicle electrical systems (lights, radio, computers, ignition, etc.)
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C
Stores fuel
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D
Cools the engine
Why this is the answer
Automotive electrical system: BATTERY provides power to START the engine and run electronics when engine is off; ALTERNATOR generates power once engine is running, recharging the battery and supplying all electrical loads. Alternator (modern AC generator) components: (1) ROTOR — electromagnet driven by accessory belt from engine crankshaft; (2) STATOR — stationary three-phase windings; rotor's magnetic field induces AC voltage in stator; (3) DIODE BRIDGE/RECTIFIER — converts AC output to DC (vehicle systems are DC); (4) VOLTAGE REGULATOR — controls rotor field strength to maintain output voltage at ~13.5-14.5V (above battery voltage so battery charges, but not so high as to damage components); modern: built into alternator. Output: typically 60-200+ amps depending on vehicle's electrical demands; modern vehicles with many electronics need higher capacity. Belt: driven by accessory drive belt (serpentine belt) from crankshaft pulley; same belt typically drives water pump, power steering pump, A/C compressor, idler/tensioner pulleys. BATTERY: lead-acid in most vehicles (some newer: AGM/absorbed glass mat or lithium). Voltage: nominally 12V (six 2V cells in series); typical fully charged: 12.6-12.8V at rest; while charging from alternator: 13.5-14.5V. Capacity rated in: COLD CRANKING AMPS (CCA — current available at 0°F for 30 seconds, important for cold-weather starting), RESERVE CAPACITY (minutes battery can supply load if alternator fails). Battery dies: (1) Old age (typical lifespan 3-5 years in moderate climates, shorter in extreme); (2) Parasitic drain (lights left on, electronics drawing current with engine off); (3) Faulty alternator not charging; (4) Cold weather (chemistry slows in cold, but cold-weather demands highest — cars hardest to start when battery weakest); (5) Loose/corroded connections. Symptoms of dead battery: slow cranking, clicking when starting, no lights/electronics, jump start gets car running. Symptoms of bad alternator: battery doesn't hold charge after jump, dim lights at idle, dashboard battery warning light, electronics misbehaving. STARTER MOTOR: high-current DC motor; engages flywheel ring gear via SOLENOID; turns engine until it starts; very high current draw (typical 100-300+ amps for brief seconds); reason batteries need high CCA. Starting circuit: ignition key/button → starter relay/solenoid → starter motor (drawing power from battery). Without functional battery, starter can't crank engine. CHARGING SYSTEM testing: check battery voltage at rest (should be >12.6V healthy); check voltage with engine running (should be 13.5-14.5V — too low = alternator weak; too high = regulator failure); check 'voltage drop' across cables; load test battery to see if it holds voltage under high current. MODERN VEHICLES: many sophisticated electronics — engine ECU, transmission ECU, ABS module, airbag module, body control module, infotainment, sensors, lights, heated seats, power locks/windows/mirrors, etc. Total electrical draw on modern luxury vehicles can exceed 100 amps. EVs/HYBRIDS use larger battery systems, DC-DC converters supplying conventional 12V system from main high-voltage battery, regenerative braking partially recharges main battery.
Source: ASVAB AI, Alternator and Charging System