Stores electrical energy to start the engine and run accessories when the alternator isn't producing.
Lead-acid (or AGM/lithium) cells store energy chemically. Cranking voltage drops as cells age and sulfate.
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Replacement runs $130–$350 including parts and 0.3-0.6 hours labor. Exact cost depends on vehicle, parts tier (OEM vs aftermarket), and local labor rate.
Expected lifespan is 3-5 years (2-4 in hot climates). Driving conditions, climate, and maintenance affect real-world life.
Slow crank; Clicking on start; Battery warning light; Bloated case; Corroded terminals; 3+ years old.
Most modern vehicle batterys are sealed units that get replaced rather than rebuilt. A shop will diagnose, confirm with a load or output test, then swap the unit and verify operation.