# What does an alternator do? ## Quick answer The alternator is an engine-driven generator that powers the car's electrical system and recharges the battery while the engine runs. ## What is Alternator? The battery's main job is to start the car. Once the engine is running, the alternator takes over. It is turned by the serpentine belt and makes electricity for the lights, computers and accessories, while also recharging the battery. On most cars it bolts to the front of the engine, where you can see its pulley and cooling vents. Inside, a spinning rotor with an electromagnet passes through fixed copper windings called the stator, which creates alternating current. A rectifier made of diodes turns that into direct current, and a voltage regulator holds the output steady, typically about 13.5 to 14.5 volts. Many newer cars let the engine computer vary that voltage to save fuel, so readings can change as you drive. A failing alternator often shows up as a battery warning light, dim or flickering lights, or a battery that keeps going dead. A whining noise or burning smell from the front of the engine can also point to it. If it stops charging completely, the car will run on battery power until it stalls. Many alternators last 7 to 10 years. Heat, fluid leaks, heavy electrical loads and a weak battery can shorten that. A shop can test the charging system in a few minutes, which is the best way to tell a bad alternator from a bad battery. ## How it works 1. The serpentine belt spins the alternator's pulley whenever the engine runs. 2. Spinning magnets inside create alternating current. 3. A rectifier turns it into direct current for the car's 12-volt system. 4. A voltage regulator keeps output steady, typically about 13.5 to 14.5 volts while running. ## Types of Alternator | Type | What it is | | --- | --- | | Rotor and stator | The spinning electromagnet and the fixed copper windings around it that together generate alternating current when the pulley turns. | | Rectifier | A set of diodes that converts alternating current into the direct current the car's 12-volt system uses; a failed diode can cause a drain or low output. | | Voltage regulator | Controls the rotor's magnetic field to keep output in range; on many newer cars it takes commands from the engine computer. | | Pulley and bearings | The belt-driven pulley, sometimes a one-way clutch pulley, and the bearings that let the rotor spin smoothly at high speed. | ## Common Alternator problems - The battery or charging warning light comes on while driving. - Headlights and dash lights dim at idle or flicker as engine speed changes. - The battery keeps going flat even after a jump start or a new battery. - A whining or grinding noise from the alternator that rises with engine speed, often from worn bearings. - A burning rubber or hot electrical smell, which can come from a slipping belt or an overheating alternator. ## Frequently asked questions ### How do I know if my alternator is bad? Common signs are the battery light, dim or flickering lights and a battery that dies again after a jump. A whining noise or burning smell can also point to it. A charging system test will confirm it, and many parts stores and shops do one for free in a few minutes. ### Can a car run without an alternator? Only briefly. The car will run on battery power until the battery is drained, then it will stall, and electric power steering and lights may weaken before that. How long depends on the battery and electrical load, so drive straight to a shop and switch off what you do not need. ### Will a new battery fix a bad alternator? No. A new battery will be drained too if the alternator is not charging, and the car will leave you stranded again. Test the charging system before replacing parts, because a bad battery and a bad alternator can show similar signs, and sometimes both need replacing. ### What voltage should an alternator put out? With the engine running, many charging systems read about 13.5 to 14.5 volts at the battery. Some newer cars vary the voltage to save fuel, so readings outside that range are not always a fault. A reading near 12 volts with the engine running usually means it is not charging. ### How long does an alternator last? Many alternators last about 7 to 10 years, or roughly 80,000 to 150,000 miles. Some last the life of the car. Heat, oil or coolant leaks, a weak battery and heavy electrical accessories can make one fail sooner, and there is no set replacement interval. ### How much does a new alternator cost? At an independent shop, alternator replacement often costs about $450 to $1,100 with parts and labor, and dealers often charge $800 to $1,600. Price depends on the part, whether it is new or remanufactured, and how hard the alternator is to reach on your engine. ### Can a bad alternator cause a car not to start? Yes, indirectly. If the alternator stops charging, the battery runs down while you drive, and the car may not start next time. The starter itself runs on battery power, so a car that cranks slowly or clicks after a drive may have a charging problem rather than just a weak battery. ### Can a bad alternator drain the battery overnight? It can. A failed diode in the rectifier can let current leak back through the alternator when the car is off, draining the battery overnight. A shop can find it with a parasitic draw test, which measures how much current flows with the car switched off. ### What makes the battery light come on? The battery light usually means the charging system is not working properly, not that the battery itself is bad. Common causes are a failing alternator, a broken or slipping serpentine belt, a loose or corroded battery cable, or a blown fuse. Get it checked before the battery runs flat. ### Do hybrids and electric cars have an alternator? Most do not have a traditional belt-driven alternator. Hybrids and EVs usually charge their 12-volt battery from the high-voltage battery through a DC-to-DC converter. They still have a 12-volt battery that can wear out and leave the car unable to power up. ## Sources - [Car Care Council: Car care guide](https://www.carcare.org/car-care-guide/) - [Battery Council International](https://batterycouncil.org/) ## Related terms - [Serpentine belt](https://diagxcar.com/glossary/serpentine-belt) - [Hybrid car](https://diagxcar.com/glossary/hybrid-car) ## Related pages - [Battery (charging system) warning light](https://diagxcar.com/warning-lights/battery) - [Alternator replacement cost](https://diagxcar.com/cost/alternator-replacement): $450-$1,100 - [How long do alternators last?](https://diagxcar.com/how-long/alternator): 7-10 years - [Why won't my car start?](https://diagxcar.com/symptoms/car-wont-start) --- Source page: https://diagxcar.com/glossary/alternator Updated: 2026-11-03 General information, not a substitute for a qualified mechanic.