The Solution for Preheating AC Current and Superimposed Ripple Durability Testing of Lithium-ion Batteries

In recent years, frequent cold waves have been attributed to the weakening of polar vortexes, leading to abnormally low temperatures especially in high-latitude regions. Many electric vehicle (EV) owners have found themselves stranded due to being unable to charge their cars when needed, forcing them to call for rescue. The main culprit here is the significant degradation in the performance of lithium-ion batteries at low temperatures. To quickly restore these batteries to their original performance, they need to be warmed up, relying on battery heating technology. In order to shorten the warm-up time, car manufacturers have begun searching for more efficient heating methods. Among numerous technical options, alternating current (AC) heating technology has garnered the most attention from EV manufacturers. When implementing AC heating strategies, the focus of research lies in shortening heating time while avoiding battery degradation. Test equipment capable of superimposing ripple currents across a wide frequency range can assist engineers in simulating various heating frequencies and amplitudes, thereby significantly reducing testing time.

Due to the decrease in ionic conductivity of the electrolyte at sub-zero temperatures, the internal resistance of lithium-ion battery cells increases, in turn reducing their energy and power output/input capabilities. AC heating technology utilizes alternating current of appropriate frequency to pass through the internal resistance, generating heat. The power is primarily determined by the resistance components and the effective value (RMS) of the AC current. The effectiveness of electrolyte heating is influenced by the amplitude and frequency of the AC signal used. Relevant studies indicate that high-frequency (~1kHz) signals can achieve better heating efficiency and avoid inducing battery degradation.

In addition to originating from AC heating devices, AC ripple currents also exist in the operational environment of batteries, such as the power supply noise (ripple) interference from electric vehicles or energy storage grids. These ripple currents associated with charging or energy recovery may lead to issues of overvoltage and long-term battery degradation. Especially at low temperatures, where the internal resistance of the battery increases, charging ripple currents can result in higher ripple voltages. Some overvoltages may evade detection by the battery management system (BMS)'s detection frequency range (e.g., if the ripple frequency is a multiple of the BMS voltage detection frequency). This could potentially lead to abnormal degradation such as through lithium deposition, causing safety risks and performance degradation in the battery. At such times, it's necessary to appropriately reduce the operating voltage or select batteries with better ripple and overvoltage tolerance. However, reducing the operating voltage implies reducing the available energy of the battery, requiring a more rigorous evaluation. Additionally, since ripple currents generate heat and periodic fluctuations, they may have long-term degradation effects. Hence, another experimental focus is to evaluate the tolerable frequency domain and magnitude of ripple currents.

Chroma's Ripple Current Superposition Test System provides ripple currents ranging from 100Hz to 20kHz, with amplitudes exceeding 150App, and a testing platform for DC charging and discharging with a maximum of 1200A. It can be programmatically integrated with environmental chambers for various temperature tests. The system utilizes independent high-frequency AC test power sources and DC charging and discharging test equipment. The advantage of this architecture is the flexible superposition of ripple currents in multiple charging and discharging modes (CC, CV, CP, CR, CC-CV, CP-CV, and Waveform), suitable for simulating power circuit conditions or heating strategies in authentic tests to evaluate ripple current operating specifications. Furthermore, due to the independence of the AC and DC circuits, the impact of charging and discharging modes on the cutoff judgment is minimal, which is of great benefit when performing cycle life comparisons.

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