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Car Auto Repairs of Texas

Guide · 6 min read

How to Know If Your Alternator or Battery Is Bad

The short answer

If your car won't start but runs fine after a jump, the battery is likely bad. If it starts, then dies while driving, or the battery light comes on and headlights dim, the alternator is the problem. A free charging-system test confirms which in minutes.

A simple way to tell a dead battery from a failing alternator. This guide covers Signs of a bad car battery; Signs of a failing alternator; How we test the charging system so you can describe the problem and decide what to check next.

Car Battery & Electrical Repair: a closer look

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Illustration of a battery tester connected to car battery terminals

Battery testing

Battery testing is one part of investigating starting and charging concerns.

Illustration of a technician using a diagnostic scanner

Computer diagnostics

A fault code identifies a system to investigate, not necessarily a part to replace.

Illustration of a technician inspecting a vehicle on a lift

Hands-on inspection

A hands-on inspection helps connect the driver's symptoms with the vehicle's condition.

Illustrative images, not customer repair photographs.

Signs of a bad car battery

Slow cranking, clicking with no start, corrosion on terminals, a swollen case, and a battery older than 3 years in Texas heat.

Signs of a failing alternator

Battery warning light while driving, flickering or dimming lights, whining noise, electrical accessories acting up, and a car that dies shortly after a jump.

How we test the charging system

A healthy battery reads about 12.6 volts off; a running engine should show 13.8–14.5 volts. We load-test the battery and check alternator output and ripple.

Need car battery & electrical repair?

Batteries, alternators, starters, wiring, and lights. Typical cost: $149 – $299 battery · $350 – $850 alternator/starter.

See our car battery & electrical repair service →

Frequently asked questions

Yes — undercharging or overcharging shortens battery life.

$350–$850 installed for most vehicles.

It may start but won't stay charged.

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Car trouble? We're open until 11:45 PM, every day.

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