In recent years, the electric vehicle market in China has experienced rapid growth, with consumers increasingly favoring these vehicles due to their low noise levels and environmental benefits. However, as an engineer working on electric vehicle powertrain systems, I have encountered a persistent issue: the high noise generated by high-voltage relays in the battery distribution unit (BDU) during power-on and power-off cycles. This noise, often perceived as unsettling by users, can detract from the overall driving experience. In this article, I will share our team’s comprehensive investigation into the noise propagation mechanisms and optimization strategies for high-voltage relays in electric vehicle battery packs. Through experimental validation, we have identified effective methods to mitigate this noise, contributing to the refinement of China EV technologies.
The high-voltage direct current (DC) relays in electric vehicle battery packs serve as critical components for controlling electrical circuits and ensuring safety. During operation, the internal moving contacts and iron cores collide with stationary parts, generating transient noise. This noise primarily propagates through structural paths within the battery pack, as the sealed design of electric vehicle battery systems limits airborne transmission. To understand this better, we analyzed the installation methods of relays, which can be mounted either inside the BDU or directly on the battery pack’s metal substrate. Our findings indicate that structural transmission via bolts and enclosures dominates the noise propagation, leading to audible vibrations that affect user perception.
To address this, we explored multiple optimization approaches. First, we considered modifying the noise source by using horizontal relays instead of vertical ones. Horizontal relays, with their actuation mechanism oriented horizontally, reduce the impact force during collisions, thereby lowering noise. Second, we investigated the use of rubber vibration damping pads to decouple the BDU from the battery pack structure, minimizing bolt-induced transmission. Third, we evaluated acoustic insulation materials, such as sound-absorbing cotton, though this posed challenges related to safety and cooling. Finally, we optimized the structural modal properties of the battery pack外壳 to avoid resonance with relay-induced excitations. These strategies were tested in a semi-anechoic chamber using a Lond sound collector to measure A-weighted sound pressure levels (SPL) and loudness in sone units.
The sound pressure level in decibels (dB) is calculated using the formula: $$ L_p = 20 \log_{10} \left( \frac{p}{p_0} \right) $$ where \( p \) is the measured sound pressure and \( p_0 \) is the reference pressure of 20 μPa. Loudness, in sone, provides a perceptual measure of sound intensity and is derived from psychoacoustic models. For our experiments, we focused on reducing both SPL and loudness to enhance the comfort of electric vehicle users.
In the following sections, I will detail our experimental methodology and present results in tabular form to summarize the effectiveness of each optimization technique. Our work underscores the importance of structural design in mitigating noise for China EV applications, and we hope these insights will guide future developments in electric vehicle battery systems.
Noise Propagation Analysis in Electric Vehicle Battery Packs
As part of our research on electric vehicle components, we delved into the mechanisms of noise generation and propagation in high-voltage relays. The noise originates from the collision of moving parts within the relay, such as the moving contact hitting the stationary contact and the moving iron core impacting the yoke plate. In a typical electric vehicle battery pack, the relay is housed in a sealed environment with high ingress protection (e.g., IP67), which confines the noise and emphasizes structural transmission paths.
We identified two primary propagation pathways: airborne and structural. Airborne transmission involves sound waves traveling through the air inside the battery pack, partially reflecting off or penetrating the BDU plastic外壳. However, due to the密闭 nature of electric vehicle battery packs, this path is secondary. Structural transmission, on the other hand, occurs when the impact forces from the relay actuation vibrate the mounting bolts, BDU enclosure, and battery pack壳体. This vibration radiates noise externally, similar to how striking an object produces sound. To quantify this, we conducted tests where we removed the bolts securing the BDU, resulting in a noise reduction of up to 11 dB(A) and a loudness decrease of approximately 9 sone. This confirmed that bolt connections are a major contributor to noise in China EV battery systems.
The installation orientation of relays also plays a role. Vertical relays, with their actuation direction aligned vertically, exert gravitational forces that amplify collisions, whereas horizontal relays minimize this effect. Our initial assessments showed that horizontal relays could reduce noise by 2–7 dB(A) on average, making them a preferable choice for electric vehicle designs where space permits.
Optimization Strategies for Relay Noise Reduction
In our pursuit of quieter electric vehicle battery packs, we implemented and tested several optimization strategies. Below, I describe each approach in detail, supported by theoretical reasoning and experimental data.
1. Using Horizontal Relays
We replaced conventional vertical relays with horizontal variants in select electric vehicle models. The horizontal design aligns the actuation mechanism horizontally, reducing the gravitational contribution to impact forces. This results in lower collision energy and, consequently, reduced noise. Our tests indicated that horizontal relays could achieve a noise reduction of 2–6 dB(A) compared to vertical relays, with loudness decreasing by 1–3 sone. This makes horizontal relays a straightforward solution for improving the acoustic performance of China EV battery packs.
2. Implementing Rubber Vibration Damping Pads
To address structural transmission, we introduced rubber vibration damping pads at the BDU mounting points. Rubber, with its high damping properties, absorbs vibrational energy and decouples the BDU from the battery pack structure. Initially, we used pads with a hardness of 70 HB and steel bushings, but this still involved bolt connections that limited effectiveness. We then optimized the design by incorporating double-studded rubber pads that eliminate direct bolt contact, as shown in the following formula for vibration isolation efficiency: $$ \eta = \frac{1}{\sqrt{1 + \left( \frac{2 \zeta \omega}{\omega_n} \right)^2}} $$ where \( \eta \) is the isolation efficiency, \( \zeta \) is the damping ratio, \( \omega \) is the excitation frequency, and \( \omega_n \) is the natural frequency of the system. This optimization led to significant noise reductions, as detailed in our experimental results.
3. Acoustic Insulation with Sound-Absorbing Materials
We explored wrapping the battery pack with sound-absorbing cotton to block airborne noise. However, this approach raised concerns about thermal management and safety in electric vehicle battery systems. Our tests showed modest reductions of 1–5 dB(A) in SPL and 1–3 sone in loudness, but due to the potential risks, we do not recommend this method for widespread use in China EV applications.
4. Optimizing Battery Pack Structural Modality
We enhanced the structural design of the battery pack外壳 to increase its natural frequency and avoid resonance with relay excitations. For instance, we added aluminum bases near relay mounting points and reinforced the BDU plastic外壳 with thicker walls and ribs. The modal frequency improvement can be expressed as: $$ f_n = \frac{1}{2\pi} \sqrt{\frac{k}{m}} $$ where \( f_n \) is the natural frequency, \( k \) is the stiffness, and \( m \) is the mass. By increasing stiffness through structural optimizations, we shifted the natural frequency away from the relay’s impact frequency, reducing noise radiation. This approach proved highly effective, especially in aluminum-based battery packs, which have better damping characteristics than steel ones.
Experimental Validation and Results
We conducted experiments in a semi-anechoic chamber to validate our optimization strategies for electric vehicle battery packs. Using a Lond sound collector and Head software, we measured the A-weighted SPL and loudness during relay actuation cycles. For each test, we performed five power-on and power-off cycles and averaged the results. Below, I present our findings through tables and descriptive analysis.
First, we tested the baseline noise levels of a hybrid electric vehicle battery pack (Battery Pack 1) with standard vertical relays and no damping pads. We then sequentially applied each optimization and recorded the changes. The measurement positions included six surfaces around the battery pack to capture spatial variations.
| Optimization Strategy | Sound Pressure Level Reduction (dB(A)) | Loudness Reduction (sone) | Remarks |
|---|---|---|---|
| Horizontal Relay | 2–6 | 1–3 | Consistent reduction across tests |
| Standard Rubber Damping Pad | 1–3 | 1–3 | Limited by bolt connections |
| Optimized Rubber Damping Pad | 1–5 | 3–7 | Improved isolation efficiency |
| Sound-Absorbing Cotton Wrap | 1–5 | 1–3 | Not recommended due to safety concerns |
For Battery Pack 1, the optimized rubber damping pads showed the most promise, with noise reductions of up to 5 dB(A) and loudness decreases of 7 sone. This aligns with the vibration isolation theory, where the double-studded design minimizes structural transmission. The formula for sound transmission loss through a barrier can be approximated as: $$ TL = 20 \log_{10}(f \cdot m) – C $$ where \( TL \) is transmission loss in dB, \( f \) is frequency, \( m \) is surface density, and \( C \) is a constant. By increasing effective mass and damping, the pads enhance transmission loss.
Next, we evaluated a pure electric vehicle battery pack (Battery Pack 2) with structural optimizations, including reinforced BDU外壳 and added aluminum bases. The results were dramatic, as summarized in Table 2.
| Condition | Average SPL (dB(A)) | Average Loudness (sone) | Noise Reduction |
|---|---|---|---|
| Before Optimization (A-sample) | 45–50 | 30 | Baseline |
| After Optimization (B-sample) | 35–40 | 15 | 7–10 dB(A) SPL reduction, 50% loudness reduction |
The structural optimizations in Battery Pack 2 resulted in a 7–10 dB(A) decrease in SPL and a 50% reduction in loudness (approximately 15 sone). This underscores the importance of modal analysis in electric vehicle battery design. The relationship between structural stiffness and noise can be modeled using the following equation for resonant response: $$ A = \frac{F_0}{m \sqrt{(\omega_n^2 – \omega^2)^2 + (2 \zeta \omega_n \omega)^2}} $$ where \( A \) is the amplitude of vibration, \( F_0 \) is the excitation force, and other terms are as defined earlier. By increasing \( \omega_n \) through stiffening, we reduced \( A \), thereby lowering noise.
Overall, our experiments demonstrate that combining horizontal relays with optimized damping pads and structural enhancements yields the best results for China EV battery packs. The data also highlight that structural propagation is the dominant noise mechanism, necessitating focus on decoupling and modal optimization.
Conclusion
In this study, we systematically addressed the issue of high-voltage relay noise in electric vehicle battery packs. Through first-hand experimentation and analysis, we confirmed that noise primarily propagates structurally via bolt connections and enclosures. Our optimization strategies—including the adoption of horizontal relays, improved rubber vibration damping pads, and battery pack structural modal enhancements—proved effective in reducing noise levels significantly. For instance, structural optimizations alone achieved up to 10 dB(A) SPL reduction and 50% loudness decrease, greatly improving the user experience for China EV owners.
We recommend that electric vehicle manufacturers prioritize horizontal relays and advanced damping solutions in future designs, while also investing in structural modal analysis during the development phase. Although acoustic insulation showed some benefits, its safety limitations make it less viable. Our work contributes to the ongoing efforts to refine electric vehicle technologies, and we believe these findings will aid in the creation of quieter, more comfortable China EV models. As the electric vehicle market continues to expand, such innovations will play a crucial role in meeting consumer expectations and advancing sustainable transportation.