A typical car alternator produces between 13.5 and 14.5 volts when the engine is running. This voltage range is deliberately higher than a battery's nominal 12 volts to effectively charge it. The alternator's output isn't fixed; it's regulated by the vehicle's voltage regulator based on electrical demand and battery condition.
The primary job of the alternator is to power the car's electrical systems (lights, infotainment, A/C) and replenish the battery, which is drained to start the engine. The exact voltage can vary slightly depending on several factors:
- Engine RPM: Output is generally lower at idle and increases as you accelerate.
- Electrical Load: Turning on high-demand accessories like headlights and rear defoggers increases the load, which the regulator compensates for.
- Battery Temperature: The charging voltage is often adjusted based on ambient temperature for optimal battery health.
If the voltage consistently reads below 13.5 volts, it indicates a potential undercharging issue, which can lead to a dead battery. Conversely, a reading consistently above 15 volts suggests overcharging, which can damage the battery and sensitive electronic components.
| Vehicle Type / Condition | Typical Alternator Output Voltage (Volts) |
|---|
| Standard Passenger Car (Idle) | 13.8 - 14.2 |
| Standard Passenger Car (Under Load) | 14.0 - 14.5 |
| Heavy-Duty Truck/SUV | 13.8 - 14.8 |
| With Significant Electrical Accessories | 14.2 - 14.7 |
| Optimal Battery Charging Voltage | 14.2 - 14.7 |
| Potential Undercharging Warning | Below 13.5 |
| Potential Overcharging Warning | Above 15.0 |
