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How Tesla Calculates Usable vs. Rated SOC and Range Estimates (Summary version)

  • Writer: rory lee
    rory lee
  • Jul 28
  • 2 min read

How Tesla Calculates Usable vs. Rated SOC and Range Estimates (Summary)

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1. Usable State of Charge (SOC)

Definition: Usable SOC shows the battery percentage indicating the actual amount of energy available for driving, excluding energy reserves (buffers) Tesla maintains for battery health and safety.

Factors Considered:

  • Battery Degradation: Over time, battery capacity naturally decreases, directly reducing available usable energy. A battery degraded by 5%, for instance, will show fewer miles at full charge despite still showing “100%” SOC.

  • Temperature Effects: In cold weather, battery energy becomes temporarily restricted, displayed by Tesla's blue snowflake icon. Usable SOC only shows currently accessible energy, not the energy locked due to cold. This locked energy becomes available again when the battery warms up.


Calculation Method: Tesla’s Battery Management System (BMS) uses coulomb counting and voltage measurements. Usable SOC is calculated by subtracting Tesla’s reserved energy buffer from the current estimated battery capacity.


2. Rated State of Charge (Rated Range)

Definition: Rated SOC (displayed as rated range in miles/km) is an estimate of the driving distance possible under standardized (EPA) test conditions, assuming a fixed efficiency rate.

Characteristics:

  • Fixed Efficiency: Rated range assumes a fixed Wh/mile consumption constant based on EPA testing results, unaffected by driving style or real-time environmental factors.

  • Battery Degradation Effect: While rated SOC uses a fixed efficiency, battery degradation indirectly reduces the displayed rated range by decreasing the total usable energy over time.

  • Temperature Effects: When cold temperatures restrict battery energy, rated range temporarily decreases to reflect currently accessible energy. It returns to normal once the battery warms.


3. Tesla Mileage Estimations

Tesla provides two main types of range estimates:

(a) EPA-Rated Range

  • Calculated using standardized EPA tests (a combination of city and highway driving cycles).

  • Assumes moderate speed, temperatures, and gentle driving conditions.

  • A static and standardized benchmark for vehicle comparison, not adjusting dynamically for real-world driving scenarios.

(b) Projected Range (Real-Time Estimation)

  • Dynamically calculated based on recent driving efficiency (Wh/mile).

  • Adjusted by real-time factors: speed, terrain, acceleration, climate control, and external temperatures.

  • Provides realistic, personalized predictions reflecting immediate driving conditions.


4. Dr.EV Battery Level and Mileage

  • Battery Level (SOC): Active Degradation Adjustment: Continuously updates SOC based on real-time battery health (State of Health, SOH), clearly reflecting current battery capacity.

  • Mileage (Range Estimations): Real-World Efficiency: Combines recent driving efficiency metrics with real-time battery health data to calculate mileage.

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