A bad battery usually causes slow cranking or a no-start, but the car runs fine once it's going. A bad alternator lets the car start but then drains the battery while driving, causing dimming lights, a battery warning light and eventually stalling. A running voltage check gives a quick hint: about 13.5 to 14.7 volts with the engine running means the alternator is charging.
Symptoms side by side
| Symptom | Battery | Alternator |
|---|---|---|
| Slow crank or no start in the morning | ✔ Common | Possible (if it hasn't been charging) |
| Starts after a jump, then runs fine | ✔ Common | Less likely |
| Starts after a jump, then dies while driving | ✔ Common | |
| Battery or charging warning light while driving | ✔ Common | |
| Headlights dim at idle, brighten with revs | ✔ Common | |
| Whining or grinding from the front of the engine | ✔ Bearing noise | |
| Burning rubber or electrical smell | ✔ Possible | |
| Battery is 4+ years old | ✔ Suspect |
The voltage check
With a basic multimeter at the battery terminals:
| Condition | Healthy reading |
|---|---|
| Engine off, sitting a while | About 12.4–12.7 V |
| Engine running | About 13.5–14.7 V |
- Engine-off voltage low (under ~12.2 V): the battery is discharged or weak.
- Running voltage close to engine-off voltage: the alternator likely isn't charging.
- Running voltage over ~15 V: overcharging, which can cook a battery.
Voltage is a hint, not a verdict. A battery can show 12.6 V and still fail under load. That's why we use a load or conductance tester.
Don't forget the belt and the wiring
The alternator is driven by the serpentine belt. A slipping belt or bad tensioner can undercharge (see serpentine belt squeal). Corroded ground straps and loose terminals can look like a bad alternator too.
Why test both
Replacing a battery when the alternator is bad gets you a few days at best. Replacing an alternator when the battery is weak doesn't fix slow starts. A full charging-system test checks the battery under load, alternator output and ripple, belt condition, and starter current draw.
Frequently asked questions
Can a bad alternator kill a new battery?
Yes. An alternator that undercharges leaves the battery repeatedly drained, which shortens its life. One that overcharges can damage it too.
How long does an alternator last?
Many last 100,000 miles or more, but heat, heavy electrical loads and accessories shorten that.
Can I drive with a bad alternator?
Only briefly. The car runs on battery power until it's drained, then stalls. Get it tested before you go far.
Can alternators be replaced on-site?
On most vehicles, yes. A few designs need a lift; we'll confirm for your vehicle.
Want a mechanic to confirm it?
We come to you in Lexington, Irmo, Chapin, Cayce, West Columbia and Columbia, and we test before we replace anything.
