Until the battery goes flat, and that's usually 25 to 30 minutes with a good battery, or a few minutes with a weak one. The alternator makes all the electricity once the engine's running. When it stops, the ignition, fuel pump and injectors drain the battery until there isn't enough left to keep the engine alive, and it stalls in traffic.
So the honest answer is: far enough to reach a shop if one is close, with every accessory off. Otherwise, a tow.
What's going on
The battery starts the car. The alternator runs it. With the engine on, the alternator should hold the system between about 13.5 and 14.8 volts (O'Reilly), which charges the battery and powers everything else.
When the alternator fails, the battery is the only source. A battery at rest reads about 12.4 to 12.7 volts (O'Reilly). Below roughly 12 volts the fuel pump and ignition coils start to struggle, and below that the engine computer gives up. You don't get a warning beyond the lights getting dim.
The symptoms line up with that. O'Reilly's list runs: headlights that dim at idle and brighten with revs, a battery or ALT warning light, slow windows and a stereo that resets, squealing or grinding from the belt area, a battery that keeps dying after a jump, and finally stalling.
Why it failed, in order of likelihood
Worn brushes or a dead voltage regulator inside the alternator are the usual cause on a high-mileage car. Bearings go next, and you'll hear a whine or grind that follows engine speed. A slipping or snapped serpentine belt gives the same symptoms as a dead alternator, and it's a far cheaper fix, so look at the belt first. Corroded battery terminals or a bad ground can also mimic a charging fault.
Blown alternator fuses and chewed wiring are rarer but worth a look on older cars and anything that lives outside.
How far you can go
Minutes, not days. Rerev's figure is about 25 to 30 minutes with a healthy battery, with a real-world spread from five minutes to two hours depending on the battery's condition and how much you're drawing from it.
Situation | Realistic range | What to do |
|---|---|---|
Battery light just came on, battery is newish, daytime, accessories off | 20–30 minutes, maybe 15–20 miles | Drive straight to the nearest shop |
Battery light on, battery over 4 years old | 5–15 minutes | Nearest safe place to stop, then tow |
Night, rain, headlights and wipers needed | A few minutes | Pull over somewhere safe, call a tow |
Car already stalled once and was jump-started | Nearly zero | Tow it, the battery is empty |
Dash lights flickering, gauges dropping | Seconds to minutes | Pull over now |
The electrical load decides it. Headlights, heated seats, rear defroster, a blower on high and a phone charger can double what the car draws compared with a bare daytime run. O'Reilly's advice is the right one: turn off everything you don't need, and don't shut the engine off until you've arrived, because it probably won't restart.
Don't try to "charge it at home and limp to work for a week." Every deep discharge takes life out of the battery, and a battery that's been run flat a few times is often the second thing you end up buying.
What it costs
A new alternator fitted runs about $750 to $1,100 at a shop. RepairPal's national range is $809 to $1,088, split as $196 to $288 labor and $613 to $800 for the part. SoFi cites Kelley Blue Book at $747 to $842 average.
Those are OEM-grade part prices. A remanufactured alternator from a parts store is a lot less, and on an easy-access engine the swap is about an hour of labor. Where the price climbs is on cars where the alternator sits under the intake or behind the subframe, and RepairPal notes some models need the cooling fan, suspension parts or even the bumper removed to reach it.
Two add-ons are worth doing at the same time. Replace the serpentine belt, because it has to come off anyway (RepairPal). And test the battery, because a battery that's been drained flat by a dead alternator may not take a full charge again.
If the belt is the actual fault, you're looking at a belt and maybe a tensioner, well under the alternator price.
How to confirm it's the alternator
A multimeter at the battery, two readings. Engine off: 12.4 to 12.7 volts means a charged battery. Engine running: 13.5 to 14.8 volts means the alternator is working. TrueCar gives a slightly wider window of 13.0 to 14.5 volts running, and calls anything under 13 or over 14.5 a failing alternator.
If the running voltage is the same as or lower than the resting voltage, the alternator isn't charging. If it's over 15 volts, the regulator has failed high, which cooks the battery and bulbs.
No meter? O'Reilly, AutoZone and Advance test alternators and batteries for free in the parking lot. Drive there with everything off, and don't shut the engine off until they're ready.
What to do today
- Switch off the headlights, AC, radio, heated seats and any chargers. Leave the engine running.
- Decide now: is there a shop or parts store within about 15 minutes? Go straight there. If not, pull over somewhere safe and call a tow before the car chooses the spot for you.
- Pop the hood and look at the serpentine belt. Missing, shredded or loose means belt, not alternator.
- Get the free charging test at O'Reilly, AutoZone or Advance, or check voltage yourself: about 12.5 volts off, 13.5 to 14.8 volts running.
- If it's the alternator, get quotes for the job plus a belt, and have the battery load-tested after the repair. Expect $809 to $1,088 at a shop (RepairPal).
Sources
- RepairPal: Alternator Replacement Cost
- O'Reilly Auto Parts: Signs of a Bad Alternator
- TrueCar: How to Test an Alternator
- Rerev: Can You Drive With a Bad Alternator?
- SoFi: Guide to Alternator Repair and Replacement Costs
Featured image: Car alternator on a workbench surrounded by various auto parts in a garage setting by Shixart1985, CC BY 2.0. Source
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