If the car starts with a jump but dies soon after, suspect the alternator. If it will not hold a charge overnight or cranks slowly, suspect the battery. A meter reading with the engine off and running settles the question.
Key takeaways
- Jump then die points to the alternator.
- Slow crank points to the battery.
- Voltage readings decide it.
What each part does
The battery supplies the power to start the engine. The alternator makes electricity while the engine runs and recharges the battery. When the alternator fails, the car runs on battery power until the battery is drained.
Symptoms of a bad alternator
A battery warning light, dimming lights while driving, a whining noise, a burning smell and a car that stalls after a jump are common signs.
Symptoms of a bad battery
A slow crank, a click with no start, a car that is dead after sitting overnight and swollen or corroded terminals point to the battery.
The voltage check
With the engine off, a healthy battery reads about 12.6 volts. With the engine running, the alternator should push roughly 13.5 to 14.7 volts. A reading near battery voltage with the engine running suggests the alternator is not charging.
Why test both
Replacing the battery without testing the alternator can destroy the new battery. We test both.
Test sequence we use
First, measure resting battery voltage. Second, run a load test to see how the battery responds under demand. Third, start the engine and measure charging voltage at idle and at higher RPM. Fourth, check the drive belt and connections. This order separates a weak battery from a weak alternator and finds belt or cable problems that mimic both. It takes less time than replacing the wrong part.
What a failing alternator costs you
An alternator that undercharges slowly drains the battery until the car will not start. One that overcharges can cook a battery and damage electronics. Either way, running the car on a failing alternator risks a breakdown. If your battery light comes on or your headlights dim at idle, reduce electrical loads and get it tested the same day.
Need help in south Bakersfield?
Questions about auto electrical repair? Read our Auto Electrical Repair page, call (661) 599-9098 or request a time. Peña's Auto Electric, Smog & Repair, 303 Taft Hwy, Suite B, Bakersfield, CA 93307. Mon to Fri 9:30 AM to 5:30 PM, Sat 8 AM to 1 PM.
Frequently asked questions
Can a bad alternator kill a new battery?
Yes, by undercharging or overcharging it.
How long do alternators last?
Often many years, but heat and load shorten life.
Is the warning light always the battery?
It signals a charging system fault.
Can I drive with a bad alternator?
Only until the battery drains.