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SAE J1666-2002: Standardized Test Procedures for Electric Vehicle Acceleration, Gradeability, and Deceleration
The SAE J1666-2002 document, though cancelled, remains a foundational recommended practice for evaluating the acceleration, gradeability, and deceleration performance of battery-powered electric vehicles. Developed by SAE International, it provides standardized test methods that enable cross-comparison of vehicle performance in specifications, research, and engineering discussions. The procedures address the total vehicle system rather than individual components, ensuring that real-world driving characteristics are assessed under controlled conditions.
🛠️ Key Test Conditions and Instrumentation
Consistent, repeatable results depend on strictly controlled vehicle and environmental conditions. The standard mandates specific preparations for both the vehicle and its battery system before any performance tests begin. Below is a summary of the core requirements:
| Condition | Requirement |
|---|---|
| Vehicle Stabilization | Minimum 1600 km (1000 miles) accumulation or battery conditioning per the manufacturer. |
| Battery Aging | Aged with the vehicle or via an equivalent bench procedure (e.g., USABC EV Battery Test Procedures). |
| Test Weight | Curb weight + 300 lb (136 kg). |
| Tire Pressure (dynamometer) | Same as used to establish dynamometer road load power setting; not to exceed levels in SAE J1263 or SAE J2264. |
| Ambient Temperature Soak | Vehicle and battery stored for minimum 8 hours at 16–32 °C (60–90 °F) before fully charged tests. |
| Road Grade | Level to within ±1% for road tests. |
| Wind Speed | At test site ≤ 16 km/h (10 mph). |
| Dynamometer Load | Programmable to simulate road load vs. speed; settings based on SAE J1263 or frontal area method per 40 CFR 86.129-80. |
| Instrumentation Accuracy | Overall error ≤ ±2% of maximum measured value; includes DC watt-hour meter, speed vs. time recorder, distance vs. time recorder, tire pressure gauge, ambient temperature indicator, and wind measurement for road tests. |
Note: The standard requires that tests initiated with a fully charged battery include an 8-hour soak after charging. For partial discharge tests, the battery is discharged at a C/3 rate, and testing must begin within 10 minutes of reaching the desired state of charge.
🔍 Performance Test Methods
The recommended practice defines five core tests, each targeting a specific performance attribute. Procedures are provided for both road and dynamometer execution, with road tests preferred for final specifications.
- Acceleration Characteristics on a Level Road (Section 6) – Measures vehicle acceleration capability on a level surface, typically from a standstill to various speeds.
- Gradeability Limit (Section 7) – Determines the maximum percent grade the vehicle can just move forward from a stop, reflecting low-speed tractive force.
- Gradeability at Speed (Section 8) – Evaluates the maximum grade the vehicle can sustain at a specified speed for a given duration, illustrating sustained climbing ability.
- Deceleration (Section 9) – Assesses the vehicle’s braking deceleration, often from a specified initial speed on a dry, level surface.
- Coastdown Testing (Section 10) – Measures vehicle deceleration due to road load (rolling resistance, aerodynamic drag, driveline losses) for dynamometer load simulation and performance characterization.
Engineering Considerations and Best Practices
Obtaining reliable and comparable results demands attention to several practical details identified in the standard. Below are key insights and frequently asked questions that engineers should review before implementing these test procedures.
Design Insights
- Battery Conditioning is Critical – The battery must be aged with the vehicle (minimum 1600 km) or an equivalent bench cycle to ensure stable capacity and performance. Extended storage requires additional cycling per manufacturer recommendations.
- Road Test Direction – When using a road route, tests must be run in opposite directions to cancel the effects of grade and wind. Closed tracks do not require reversal.
- Soak Time and Ambient Temperature – A minimum 8-hour soak at 16–32 °C is mandatory before fully charged tests. Road testing ambient range is extended to 5–32 °C, but the soak window is tighter for consistency.
- Dynamometer Road Load Simulation – The load must be programmable and set via coastdown techniques (SAE J1263 or SAE J2264) or the frontal area alternative. Tire pressure must match the setting used during the load establishment to avoid errors.
- Test Initiation Timing – For partially discharged tests (e.g., gradeability), start within 10 minutes of reaching the target state of discharge to minimize battery recovery effects.
Common Mistake: Failing to run road tests in opposite directions can introduce significant grade and wind bias. Additionally, using the same tire pressure for dynamometer load setting and the actual test is essential for accurate road load replication.
Frequently Asked Questions
How are acceleration characteristics measured on a level road?
The test typically uses a fifth wheel or speed-distance recorder to capture time to accelerate from a standstill to specified speeds, with runs performed in both directions on a road within ±1% grade. The average of runs is used to minimize wind influence.
How is the gradeability limit determined?
The vehicle is driven on a measured grade (or simulated on a dynamometer) until the maximum grade at which the vehicle can just move forward is identified. This reflects the peak tractive force at near-zero speed.
What are the required environmental conditions for testing?
Road tests require a hard, dry surface with grade ≤ ±1%, ambient temperature 5–32 °C, and wind speed ≤ 16 km/h. For dynamometer tests, the ambient soak temperature is 16–32 °C, and the dynamometer must be located in a controlled environment with a fixed-speed cooling fan.
How does battery state of charge affect test results?
Battery SOC directly influences power delivery and voltage. The standard specifies tests at 0%, 40%, and 80% discharge when investigating SOC effects, and partial-discharge tests must be started within 10 minutes of reaching the target SOC to ensure consistency.
