Both a bad battery and a bad alternator show up the same way at first - slow crank, dim lights, dead in the driveway. But the test to tell them apart takes under 5 minutes with a $20 multimeter. Here is the simple decision tree and what each scenario actually costs to fix.
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If the resting battery voltage is below 12.4V after sitting overnight and a load test fails, it is the battery. Most flooded batteries last 3-5 years; AGM batteries 4-7 years. Hot climates cut life dramatically. Cost: $120 - $300 installed. DIY: Easy. Severity: Low.
Get a Free AI Diagnosis →Start the car and read voltage at the battery: a healthy alternator shows 13.8-14.6V. Under 13.5V means the alternator (or its regulator, or belt) is the problem. Battery light may or may not be on. Cost: $350 - $800. DIY: Medium. Severity: High.
Get a Free AI Diagnosis →A corroded or loose battery cable mimics both failures - the alternator cannot push current through it and the starter cannot pull current through it. Cheapest fix on this list and overlooked constantly. Cost: $0 - $80. DIY: Easy. Severity: Low.
Get a Free AI Diagnosis →A slipping or glazed belt lets the alternator spin too slowly to make full charge - especially at idle. Often paired with squealing on cold starts. The belt is cheap; replace it before it strands you. Cost: $80 - $200. DIY: Medium. Severity: Medium.
Get a Free AI Diagnosis →In most modern alternators the regulator is internal; on some European cars and older domestics it is a separate part. A failed regulator causes overcharging (over 15V) or undercharging (under 13V). Cost: $50 - $400. DIY: Hard. Severity: High.
Get a Free AI Diagnosis →If your scanner shows one of these codes along with the symptom, run a free AI diagnosis to confirm the root cause.
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12.6V or higher is fully charged. 12.4V is about 75% charged. Under 12.2V is a discharged or weak battery. Read after the car has sat at least an hour.
13.8V to 14.6V at the battery with the engine at 1500-2000 RPM. Under 13.5V is undercharging (bad alternator or belt). Over 15V is overcharging (bad regulator).
Yes. Defective batteries fail early; deeply discharged batteries (from being left dead) lose capacity permanently. A load test will catch it.
Short distance only. The alternator is no longer charging the battery, so you are running on stored charge. Most cars die within 20-60 minutes once the light comes on.
Battery: $120-$300 installed. Alternator: $350-$800 installed for most cars. Confirm which one before you spend the money - swapping the wrong part wastes hundreds.
Yes. AutoZone, O'Reilly, Advance, and Napa all test batteries and alternators free in the parking lot. Drive in with the symptom; leave with a printout that names the failed part.