By Pablora Parts LLC parts specialists · Updated September 8, 2026 · Part of Batteries & electrical parts
Alternators do not usually die all at once. They fade: dimming lights at idle, a battery warning that flickers on wet mornings, a whine that rises with engine speed, then a car that will not start after a short trip. Because the first symptom looks like a bad battery, a lot of alternators get diagnosed with a new battery that dies a month later. Here is how to tell them apart.
Battery or alternator, in one test
With the engine running, measure voltage at the battery terminals. Roughly 13.8 to 14.6 volts means the alternator is charging. Below 13 volts with the engine running, or a reading that drops when you turn on the headlights and blower, means the alternator is not keeping up. Engine off, a healthy battery reads about 12.6 volts; below 12.2 after sitting overnight, the battery is weak or being drained. Most parts stores run both tests free.
Symptoms in the usual order
- Headlights dim at idle and brighten when you rev.
- Battery or charging warning on the dash, sometimes only in the rain or with the AC on.
- A whining or grinding noise from the front of the engine that changes with engine speed, from the alternator bearing.
- Electrical accessories acting up: slow windows, flickering dash, a radio that resets.
- A burning rubber or electrical smell, from a slipping belt or an overheating alternator.
- Finally, a dead battery that a new battery does not fix.
Check the cheap things first
A loose or glazed serpentine belt, a weak tensioner, a corroded battery terminal or a bad ground strap all give charging symptoms without a failed alternator. Look at the belt, wiggle the terminals, and clean the grounds before buying an alternator.
Remanufactured or new
A quality remanufactured alternator has new bearings, brushes and voltage regulator, carries a warranty and costs less than new; it is the right choice for most vehicles. New makes sense on late-model vehicles with computer-controlled charging, where the regulator electronics matter, and on high-output or dual-alternator trucks. Match the amperage rating and the pulley type to the original; the rating is on the alternator's tag.
Tell us the symptoms and the VIN. We tell you whether it sounds like the battery or the alternator, and quote a remanufactured and a new unit matched to your amperage.
Questions we get about this
Yes. An alternator that undercharges leaves the battery discharged, which sulfates the plates, and one that overcharges cooks it. Diagnose the alternator before replacing a battery that died young.
Commonly 100,000 to 150,000 miles. Heat, a slipping belt and heavy accessory loads shorten it.
At least the same. A higher-output unit that fits the mount and pulley is fine and helps on vehicles with added lighting or audio; a lower-output one will undercharge.