1 Introduction

The two-wheeled electric vehicle, as a crucial short-distance transportation tool, has penetrated various fields such as instant delivery, shared mobility, and consumer commuting, with a national fleet exceeding 300 million units. The controller serves as the "control brain" of the two-wheeled electric vehicle, primarily responsible for the motor's start, run, forward/reverse, speed, stop, and other operations. Its performance directly determines the vehicle's reliability and service life. Among the key components of the controller, the electrolytic capacitor plays an indispensable role due to its unique energy storage and filtering characteristics. Particularly under the impetus of the "New National Standard" policy, which promotes industry standardization and intelligent development, the performance requirements for electrolytic capacitors are increasingly heightened. This report comprehensively evaluates the application status, technical challenges, innovative solutions, and future development trends of electrolytic capacitors in two-wheeled electric vehicles.

2 Core Functions of Electrolytic Capacitors in E-bike Controllers

Electrolytic capacitors in e-bike controllers primarily serve three core functions: filtering, energy storage, and protection, directly impacting the controller's stability and overall vehicle performance.

• Filtering and Decoupling: E-bike controllers use PWM chopping technology to control motor speed, with switching frequencies as high as tens of kHz. The motor, being an inductive load, cannot change current instantaneously, leading to instantaneous high voltages and high-order harmonics. Electrolytic capacitors can promptly smooth and suppress these voltage fluctuations, preventing output voltage variations from causing system interference. While controllers already have electrolytic capacitors paralleled on the bus internally (e.g., two 330μF capacitors in series for a 24V system), external expansion can further enhance high-frequency filtering.

• Instantaneous High-Current Compensation: During startup, climbing, and other conditions, the current can reach 2-5 times the rated value (instantaneously over 20-30A). Lead-acid batteries have limited discharge capability (0.5C for power batteries), making it difficult to meet instantaneous high-current demands. Due to their very low internal resistance and fast charge/discharge speed, electrolytic capacitors can instantly release stored energy, supplementing the battery's insufficient discharge, ensuring sufficient motor output torque, and avoiding controller shutdown due to voltage sag.

• Component Protection and Battery Life Extension: By reducing the instantaneous load on the battery, electrolytic capacitors help inhibit battery polarization, delaying sulfation and capacity decay. When the motor drive frequency is high, the capacitor exhibits "AC short-circuit" characteristics, reducing the impact of battery internal resistance on the driver and protecting power devices (like MOSFETs) from voltage spike impacts.

Table: Functional Comparison of Electrolytic Capacitors in a 48V E-bike System

3 Performance Requirements and Evaluation Standards

The complex operating conditions of two-wheeled vehicles (vibration, temperature changes, etc.) impose stringent requirements on the performance of electrolytic capacitors. Evaluation must cover three main dimensions: electrical performance, environmental adaptability, and lifespan.

3.1 Electrical Performance Standards

• Capacitance and Tolerance Range: The deviation between the actual capacitance and the nominal value should be controlled within ±20% to ensure stable filtering performance.

• Equivalent Series Resistance (ESR): The ESR value directly relates to the capacitor's high-frequency characteristics. Lower ESR means better performance at high temperatures and high frequencies. Quality capacitors must ensure ESR remains within standard limits.

• Leakage Current and Withstand Voltage: Excessive leakage current can lead to static power consumption, especially when the vehicle is idle, exacerbating battery drain. Electrolytic capacitors must remain stable at 1.2-1.5 times the rated voltage without breakdown.

3.2 Environmental Adaptability and Life Testing

E-bike controllers face harsh environments (temperatures from -20°C to 85°C), requiring electrolytic capacitors to have a wide operating temperature range. Testing includes:

• High Temperature and High Humidity Test: For example, under conditions of 85°C/85% relative humidity for 1000 hours, the capacitance change rate should be ≤20%.

• Temperature Cycling Test: Checks the mechanical stability of the capacitor under rapid temperature changes to prevent cracking or leakage.

• Vibration Test: Simulates road vibrations, requiring the capacitor structure to remain intact, with no fatigue fracture of leads.

Lifespan testing often uses accelerated aging methods, such as applying ripple current at the maximum operating temperature, monitoring the decay curves of parameters like capacitance (C), leakage current (IL), and loss tangent (tanδ) to estimate the actual service life. Specialized versions of aluminum electrolytic capacitors can operate at temperatures up to 150°C or even 175°C, and their theoretical lifespan can be about 15 years at 65°C.

4 Technical Challenges and Solutions

Although adding capacitors can theoretically improve performance, practical application faces several challenges that need to be addressed from design and process perspectives.

• Leakage Current and Static Power Consumption: The inherent leakage current of electrolytic capacitors (especially in low-quality products) continuously consumes battery power when the vehicle is idle, potentially shortening range. Solutions include selecting low-leakage-current capacitors (e.g., polymer solid aluminum electrolytic capacitors) or adding a manual switch to the capacitor circuit, disconnecting it when parked.

• Heat Dissipation and Stability Issues: The controller space is compact, and capacitors can heat up during prolonged operation. High temperatures accelerate the drying of the electrolyte, leading to decreased capacitance and increased ESR. New installation structures (e.g., aluminum substrate embedded slot design) embed the rear half of the capacitor into the heat sink, filling gaps with thermal conductive paste to improve heat dissipation efficiency. Patent processes also optimize the core package drying conditions (85°C/60min) to ensure even electrolyte penetration.

• Size and Cost Constraints: To significantly enhance performance, Farad-level supercapacitors (10F and above) are needed, but they are expensive and not cost-effective. The current mainstream solution still relies on expanding capacity with traditional aluminum electrolytic capacitors, such as paralleling multiple 3300μF capacitors, to balance cost and effect.

Table: Common Technical Challenges and Solutions for Electrolytic Capacitors

5 Innovative Technologies and Development Trends

To address the challenges mentioned above, electrolytic capacitor technology continues to innovate in materials, structure, and system integration.

• New Preparation Processes: Specifically designed aluminum electrolytic capacitors for e-bike controllers improve the anode foil riveting process (rivet flower thickness ≤42μm, impedance <0.5mΩ) and use high flashover voltage electrolyte (≥370V), enabling them to withstand 2 times higher current (instantaneous current ≥10A). Such products show no significant failure after 1000 cycles, significantly improving controller reliability during climbing and heavy loads.

• Integration and Intelligent Development: Driven by the "New National Standard," the demand for e-bike intelligence is surging (e.g., OTA upgrades, remote control). Capacitors need to collaborate with the BMS (Battery Management System) and VCU (Vehicle Control Unit) for more refined energy management. For example, working with MCU chips' UART/CAN interfaces, the capacitor's operating status can be monitored in real time, with active alarms for abnormalities. Upstream suppliers like GigaDevice have introduced GD32 series MCUs that support protocol interconnection between capacitors, intelligent instruments, and battery packs, laying the foundation for dynamic energy allocation.

• Structural Innovation: Traditional through-hole electrolytic capacitors are difficult to install on aluminum substrates and can short-circuit with the metal aluminum. New mounting structures use supportive conductive sheets and insulating posts to secure the pins, enhancing mechanical strength and avoiding short-circuit risks. This design is compatible with different capacitor models, reduces overall volume, and improves heat dissipation efficiency.

6 Reliability Assessment Methods

To ensure the long-term reliability of electrolytic capacitors in e-bike applications, multi-dimensional assessment methods are required:

• Electrical Performance Testing: Use capacitance measuring instruments or digital bridges to measure capacitance, ESR, and leakage current, assessing compliance with design standards.

• Environmental Adaptability Testing: Includes temperature cycling tests, high temperature/high humidity tests, etc., simulating actual usage conditions to check capacitor performance and stability under different environmental conditions.

• Lifespan Testing and Failure Analysis: Simulate the aging effects of electrolytic capacitors in actual use through accelerated aging tests, analyzing common failure modes such as electrolyte drying and seal failure to provide basis for design improvements.

• Markov Model Assessment: Adopt a reliability assessment method based on Markov models, dividing the electrolytic capacitor state into normal, aging damage, and complete failure, predicting reliability trends through state transition probabilities.

Combining these assessment methods allows for a comprehensive evaluation of the reliability of electrolytic capacitors under practical conditions, providing data support for product optimization.

7 Conclusion and Outlook

Electrolytic capacitors play a significant but limited role in two-wheeled electric vehicle controllers. Their core value lies in optimizing instantaneous current response, improving startup/climbing performance, and assisting in filtering and voltage stabilization. However, capacitors themselves do not generate energy and have inherent drawbacks like leakage current and heat dissipation. Therefore, the cost-effectiveness of adding external capacitors highly depends on capacitor quality, vehicle operating conditions, and control strategies.

Future development trends will focus on high reliability, low ESR, and small size. On one hand, new materials (e.g., polymer electrolytes) and processes (e.g., ultra-fine riveting) will continue to push the limits of current tolerance. On the other hand, intelligent integration will drive capacitors to evolve from "passive components" to "active nodes in energy management," collaborating with BMS and MCUs for more precise dynamic regulation. For vehicle manufacturers, selecting quality capacitors that meet automotive standards (vibration resistance, wide temperature range) and optimizing heat dissipation layout are crucial to truly leverage the performance advantages of electrolytic capacitors and enhance the competitiveness of two-wheeled electric vehicles in the "New National Standard" era.

In summary, the evaluation of electrolytic capacitor applications in two-wheeled vehicles requires comprehensive consideration of technical performance, economy, and reliability factors. Through technological innovation and system optimization, overall vehicle performance and user experience can be continuously improved.