What is the output voltage of a Subaru alternator?
Typically, Subaru alternators regulate output to about 13.8–14.4 volts when the engine is running, regardless of the amperage rating. That means the charging system maintains the 12-volt battery at roughly 14 volts under normal operation.
However, the exact reading can vary with engine speed, electrical load, battery condition, and temperature. This article explains how the voltage is produced and what you should expect when diagnosing charging performance in a Subaru.
How the voltage is regulated
The alternator relies on an internal voltage regulator to hold the system voltage at a target level. The regulator adjusts the field current in the rotor based on feedback from a sense line connected to the battery, so the voltage stays in the ~13.8–14.4 V range even as loads change. If the battery is low or the load increases, the regulator increases excitation; if the system is full or the battery is charged, it reduces excitation.
Key components of the regulation system
The core parts are the regulator, the sense line, and the field winding inside the alternator. These work together to maintain stable charging voltage across a range of engine speeds and electrical demands.
Typical voltage range and what affects it
Several factors determine the voltage you read at the battery while the engine runs.
- Engine speed (RPM) — higher RPM helps the alternator reach and hold the regulator’s target voltage.
- Electrical load — lights, HVAC, audio, and other devices draw current that the alternator must supply while maintaining voltage.
- Battery health and state of charge — a weak or sulfated battery can influence the regulator’s behavior.
- Temperature — extreme temperatures can affect battery chemistry and regulator performance.
- Alternator condition — belt tension, wear, or a failing regulator can cause voltage drift.
In normal operation, you should see roughly 13.8–14.4 volts across the battery with the engine running and loads typical for your vehicle. Readings outside this range may signal issues with the regulator, belt, wiring, or the battery.
Diagnosing with a voltmeter
To verify the alternator output, measure the battery voltage with the engine off and then with the engine running. A healthy system usually shows about 12.6 V when off, and about 13.8–14.4 V when running under normal load. If you consistently read below ~13.5 V or above ~14.8–15 V, inspect the drive belt, connections, and regulator, and consider professional testing.
Additional checks include inspecting for corrosion on battery terminals, ensuring the sense wire is connected correctly, and verifying that the alternator is delivering current when there is demand.
Subaru-specific notes
Subaru models use a range of alternators, commonly in the 60–90 amp range for standard equipment; higher-output units can exceed 90 A on some trims. The regulated voltage remains around 13.8–14.4 V, with the amp rating determining how much current the alternator can supply at once.
Common alternator sizes by model range
Exact amperage and regulator configurations vary by model and year. When in doubt, check the service manual for your Subaru’s year and engine code to confirm the original alternator rating and regulator behavior.
In summary, while the Subaru alternator isn't a fixed-voltage device, it is designed to maintain a charging voltage in the mid-teens (roughly 13.8–14.4 V) when the engine runs, with the available current depending on the specific model and condition of the charging system.
Summary
The output voltage of a Subaru alternator is regulated to about 13.8–14.4 volts during typical operation, with the current output determined by model-specific amperage ratings and engine/electrical load. For reliable charging, keep the belt in good condition, the battery healthy, and the regulator functioning.
