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Determining RESS Peak Power for Hybrid Electric Vehicles: A Guide to SAE J2758-2018
SAE J2758-2018 provides a recommended practice for rating the peak power of a Rechargeable Energy Storage System (RESS) used in combustion engine hybrid electric vehicles (HEVs). This standard defines a vehicle-specific test procedure to establish the ratio of RESS peak power to the sum of internal combustion engine power and RESS peak power. The methodology accounts for the RESS control environment within a particular vehicle, meaning that the same RESS may have different peak power ratings when installed in different HEV platforms.
Key Definitions and Limits
The standard introduces critical voltage and current limits that must be determined before testing. These parameters are essential for configuring the test and calculating the predicted peak power.
| Parameter | Definition | Significance |
|---|---|---|
| Vmax | Smaller of the RESS steady-state maximum voltage and vehicle maximum allowable DC-link bus voltage that can be sustained indefinitely. | Defines the upper voltage operating limit for the RESS in the vehicle. |
| Vmin | Larger of the RESS steady-state minimum voltage and vehicle minimum allowable DC-link bus voltage that can be sustained indefinitely. | Defines the lower voltage operating limit. |
| Vmax-pulse | Smaller of the RESS maximum pulse voltage and vehicle maximum allowable DC-link pulse voltage that can be sustained for at least 10 seconds. | Allows temporary voltage excursions above Vmax during power assist events. |
| Vmin-pulse | Larger of the RESS minimum pulse voltage and vehicle minimum allowable DC-link pulse voltage that can be sustained for at least 10 seconds. | Allows temporary voltage drops below Vmin during regenerative braking. |
| Imax | Smaller of the RESS maximum current (battery manufacturer) and maximum allowable current on the vehicle DC-link. | Sets the current limit for the peak power test. |
| Icc | Minimum of Imax or 20 times the C1/1 rate. | Used for conditioning cycles and capacity determination. |
| Rated Capacity | Total Amp·hours discharged at constant current to decrease from 100% to 0% SOC in one hour (C1/1 rate). | Baseline for SOC setup and capacity confirmation. |
| C1/1 Rate | The constant current that removes the rated capacity in one hour. | Key current for preparation and test sequences. |
| State of Charge (SOC) | Percentage of Amp·hours remaining relative to the static capacity at C1/1 rate. | Test target SOC is vehicle-specific and must be set precisely. |
| Test Temperature (TT) | Room temperature defined as 26 °C + 3 °C. | Controlled environment ensures repeatable results. |
Overview of the Test Procedure
The test procedure is performed at the pack level with the RESS controller disabled (by the least intrusive means possible) to allow full SOC control. The following steps outline the process:
- Information Gathering: Collect vehicle-specific target SOC, voltage/current limits, and DC-link constraints.
- Battery Preparation: Capacity determination at C1/1 rate and stabilization cycles.
- Peak Power Test: Apply constant current discharge pulses at the target SOC and measure voltage response.
- Calculation of Predicted Peak Power: Use the voltage at the pulse limit (Vmin-pulse) and the applied current (Imax) to calculate power.
- Correction/Confirmation: Validate the predicted power by repeating the test or applying correction factors as defined in the standard.
⚠️ Important: The peak power rating obtained through this procedure is vehicle-specific. If the same RESS is used in a different HEV platform, it must be retested because the control strategy and DC-link environment can significantly affect the achievable power.
Engineering Design Insights
The vehicle-specific nature of SAE J2758-2018 highlights the tight integration between the RESS and the hybrid system. Key insights for engineers include:
- Voltage and current limits are not solely battery characteristics; they depend on the vehicle’s DC-link capabilities and the RESS controller logic. Defining Vmax, Vmin, and their pulse counterparts requires coordination between battery and vehicle teams.
- The test temperature (25–29 °C) must be representative of normal operation. Peak power under cold or hot conditions would differ, so the standard focuses on a controlled room-temperature environment for a consistent baseline.
- The C1/1 rate capacity determination is critical for setting accurate SOC. Errors in capacity or SOC setup directly propagate into the peak power calculation.
- The predicted peak power is calculated as the product of Vmin-pulse and Imax. This assumes the RESS can deliver Imax while staying above the minimum pulse voltage, representing a worst-case power capability at the target SOC.
Designers should also refer to the USABC and FreedomCAR battery test manuals, which are referenced in the standard for related procedures.
🛠️ Engineering Tip: When implementing this test, ensure that the RESS controller can be safely disabled and that the pack cycling (C1/1 capacity and stabilization) is fully completed before the peak power pulse. Skipping conditioning cycles often leads to erroneous results.
Frequently Asked Questions
1. Can SAE J2758-2018 be applied to fuel cell vehicles or pure electric vehicles?
No. The standard explicitly states that it is intended only for combustion engine HEVs. Fuel cell vehicles and pure electric vehicles have different system architectures and should use other test procedures (e.g., USABC or FreedomCAR manuals).
2. Why is the peak power rating vehicle-specific?
The test measures power capability in the actual vehicle operating environment, including the DC-link bus, thermal management, and RESS control strategy. The same battery pack may behave differently in another vehicle due to variations in voltage/current limits, control algorithms, and cooling capacity.
3. What are the consequences of testing at the wrong state of charge or temperature?
Testing at an incorrect SOC or temperature (outside the 26 °C ± 3 °C range) will yield a peak power value that is not representative of the intended operating conditions. This can lead to incorrect power ratings, potentially causing system underperformance or overstress.
4. What is the difference between steady-state voltage limits (Vmax/Vmin) and pulse limits (Vmax-pulse/Vmin-pulse)?
Steady-state limits are voltages that the RESS and DC-link can sustain indefinitely without damage or operational issues. Pulse limits allow temporary excursions (e.g., during regenerative braking or power assist) for durations up to 10 seconds. The peak power test uses Vmin-pulse as the lower bound because it reflects the allowable voltage dip during a short power pulse.
