Lithium-ion batteries, particularly lithium iron phosphate (LiFePO₄) variants, have emerged as critical components in modern hybrid electric vehicle (HEV) systems due to their high energy density, thermal stability, and extended cycle life. This study focuses on the design, testing, and lifecycle analysis of a 48V mild hybrid lithium-ion battery pack under accelerated aging conditions. The goal is to evaluate its performance and durability in real-world applications, specifically aligned with the World Light Vehicle Test Procedure (WLTP).
1. Introduction
The automotive industry faces increasing pressure to reduce fuel consumption and emissions. Traditional internal combustion engines (ICEs) struggle to meet stringent regulations, prompting the adoption of 48V mild hybrid systems. These systems integrate a belt-driven starter generator (BSG) and a lithium-ion battery pack to enable energy recuperation, engine start-stop functionality, and torque assist during acceleration. According to prior research, such systems can reduce fuel consumption by 10–15%.
The 48V lithium-ion battery pack is central to this technology. Its lifespan and performance directly impact the viability of hybrid systems. This study investigates a 48V/37Ah LiFePO₄ battery pack composed of high-rate pouch cells. We analyze its capacity, direct current resistance (DCR), and lifecycle under WLTP-mimicked conditions.
2. Experimental Design
2.1 Battery Pack Configuration
The battery pack was assembled using 15 series-connected LiFePO₄ pouch cells (3.2V nominal voltage, 37Ah capacity). Key specifications are summarized in Table 1.
Table 1: Battery Pack Specifications
| Parameter | Value |
|---|---|
| Nominal Voltage | 48 V |
| Nominal Capacity | 37 Ah |
| Cell Configuration | 15S1P |
| Dimensions (L × W × H) | 429.0 mm × 174.0 mm × 189.0 mm |
2.2 Test Methodology
2.2.1 Capacity Testing
Capacity tests were conducted at 25°C using a CE-7004-120V400A-R28GC battery tester. The protocol included:
- Charging: 1.0C constant current (CC) to 52.0V, followed by 0.5C CC to 54.0V.
- Discharging: 1.0C CC to 28.0V.
Three cycles were performed, with results averaged.
2.2.2 DCR Measurement
DCR was calculated using:R=ΔUIR=IΔU
where ΔUΔU is the voltage drop during a 10-second 200A pulse, and I=200 AI=200A. Tests were repeated at 25°C for SOC levels of 25%, 50%, and 85%.
2.2.3 Accelerated Aging via WLTP Cycles
To simulate real-world conditions, the battery underwent WLTP cycles at 50°C. Key parameters:
- SOC Range: 20%–85% (preventing over-discharge).
- Cycle Count: 14,400 cycles (equivalent to 334,800 km).
- Evaluation Intervals: Capacity and DCR measured every 1,200 cycles.
3. Results and Analysis
3.1 Capacity Retention
Initial capacity measurements (Table 2) confirmed the pack’s compliance with design specifications.
Table 2: Capacity Test Results
| Cycle | Charge Capacity (Ah) | Discharge Capacity (Ah) |
|---|---|---|
| 1 | 40.34 | 37.55 |
| 2 | 40.37 | 37.57 |
| 3 | 40.35 | 37.40 |
After 14,400 cycles, capacity decay followed an exponential model:Cremaining=C0⋅e−k⋅NCremaining=C0⋅e−k⋅N
where C0=37 AhC0=37Ah, k=8.2×10−5 cycle−1k=8.2×10−5cycle−1, and NN = cycle count. Final capacity retention was 89.6% (Figure 1).
3.2 DCR Evolution
DCR increased with SOC and cycling. At 50% SOC, initial DCR was 11.54 mΩ, rising to 12.15 mΩ post-testing (5.2% growth). Table 3 details DCR variations.
Table 3: DCR at Different SOC Levels
| SOC (%) | Discharge DCR (mΩ) | Charge DCR (mΩ) |
|---|---|---|
| 25 | 11.50 | 10.53 |
| 50 | 11.54 | 10.57 |
| 85 | 12.20 | 12.12 |
The relationship between DCR and SOC is linear:RDCR=R0+α⋅SOCRDCR=R0+α⋅SOC
where R0=10.8 mΩR0=10.8mΩ and .
3.3 Cycle Life Projection
The WLTP-equivalent mileage (334,800 km) exceeds industry standards (300,000 km/10 years). Capacity retention above 80% validates the lithium-ion battery pack’s suitability for automotive applications. Accelerated aging at 50°C further ensures robustness under extreme conditions.
4. Discussion
The 48V lithium-ion battery demonstrated exceptional durability, aligning with the lifecycle requirements of mild hybrid systems. Key factors contributing to its performance include:
- Cell Chemistry: LiFePO₄’s inherent thermal stability mitigates degradation.
- Thermal Management: Elevated test temperatures (50°C) accelerated aging without catastrophic failure.
- Energy Recovery: 38.9% energy recuperation efficiency during WLTP cycles enhanced sustainability.
5. Conclusion
This study confirms that 48V lithium iron phosphate battery packs meet the rigorous demands of mild hybrid vehicles. With 89.6% capacity retention and minimal DCR growth after 14,400 WLTP cycles, the design ensures long-term reliability. Future work will explore advanced thermal management strategies and hybrid configurations to further optimize lithium-ion battery performance.