How do I know if it's the battery or the alternator that's failing?

Jump-start the car and drive for 20–30 minutes. If the car starts fine after sitting overnight the next day: probably the battery. If the car dies again within the hour or won't restart: likely the alternator (it's not recharging the battery while running). Free tests at AutoZone can confirm both.

What does the battery warning light mean?

The battery light (looks like a battery symbol) means the charging system is underperforming — typically the alternator is not putting out enough voltage. This is NOT the same as the oil pressure light. When the battery light is on while driving, the car is draining its battery and will eventually stall.

What voltage should a healthy alternator output?

13.5–14.8 volts at the battery terminals with the engine running. Below 13V means the alternator is not keeping up with demand. Above 15V means the voltage regulator is failing. A $15 multimeter or a free test at an auto parts store will give you this number immediately.

Alternator

moderate DIY~2h job

What it does

Your alternator is the car's generator — it converts engine rotation into electricity to power everything in the car while also charging the battery. When it's working, your battery stays topped off and everything runs fine. When the alternator fails, the car runs on battery power alone — and depending on how much load is running (headlights, AC, fans), the battery can drain in 20–60 minutes. Warning signs appear on the dashboard (battery light) before the car dies completely.

Fix-vs-Skip Money Panel

DIY Cost

~$120

parts only

Shop Cost

~$450

parts + labor

Moderate DIY~2h job

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DIY tutorial

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Frequently asked questions

car maintenance — complete guide