Why Do EV Batteries Swell? Causes and What to Do

In daily life, electric vehicles are a handy assistant for short-distance travel thanks to their convenience. However, many EV users may encounter a worrying problem: battery swelling. Why does an EV battery swell? Is it safe to continue using a swollen battery? These questions are a major concern for countless EV users.

Battery swelling has a specific professional term: thermal runaway. Thermal runaway is essentially a positive feedback cycle of energy, and its internal mechanism is quite complex. Rising temperature increases the total heat of the system, and the increased heat in turn pushes the temperature higher. Repeating this cycle creates an intensifying vicious circle that drives the system temperature higher and higher, eventually causing the battery to swell.

So which factors trigger thermal runaway in an EV battery and cause it to swell? There are several contributing factors.

First, frequent overcharging is an extremely common and critical factor. In modern life, many people lack proper charging habits. Some leave the EV connected to power for convenience, even after the battery is fully charged, without unplugging the charger in time. This overcharging is like force-feeding someone who is already full. In an overcharged state, the internal chemical reactions of the battery lose balance. Excess energy cannot be released and consumed properly, so it accumulates inside the battery, generating a large amount of heat. As heat builds up, the internal pressure gradually increases, eventually causing the battery casing to expand and deform. It is like a balloon being inflated continuously: when the internal pressure exceeds the limit the balloon can withstand, it swells or even bursts.

Second, unauthorized modification of the casing can also cause battery swelling. The EV casing is not just a simple decorative part; it plays a vital role in battery heat dissipation and protection. When designing the casing, manufacturers fully consider the battery's cooling needs, using reasonable structural design and heat dissipation channels to ensure the battery can effectively release heat during normal operation. However, some users modify the casing at will without professional guidance or authorization, in pursuit of personalization or special needs. Such modification may destroy the original heat dissipation structure, preventing the battery from releasing heat in time. Heat keeps accumulating around the battery, like putting a thick cotton coat on it and making it uncomfortably hot. As the temperature rises, the internal chemical reactions become unusually active, eventually triggering thermal runaway and causing the battery to swell.

Third, high ambient temperature is also a factor that cannot be ignored. EV batteries need a suitable temperature range to work properly. Generally, when the ambient temperature exceeds 60 degrees Celsius, the battery's performance and stability are significantly affected. In high-temperature environments, the internal chemical reactions speed up and ion movement becomes more active. This greatly increases the heat generated during charging and discharging, while the battery's own heat dissipation capacity can hardly keep up. Just as people feel exhausted and uncomfortable working in hot summer weather, batteries also struggle under such high temperatures. Excessive heat accumulates inside the battery, gradually damaging its internal structure and performance, and eventually causing swelling.

Fourth, tearing of the separator that isolates the negative and positive electrodes of a lithium-ion battery is another important cause of swelling. The separator plays a vital role in a lithium-ion battery: it acts like a solid barrier that isolates the negative and positive electrodes, preventing short circuits between them. Once the separator tears, the insulation between the electrodes is destroyed and a short circuit forms instantly. A short circuit causes the battery's internal energy to be released suddenly, generating enormous heat. This sharp increase in heat quickly triggers thermal collapse, which in turn causes the battery to swell. Think of the separator as a critical part of a precision instrument: once this part is damaged, the normal operation of the whole instrument is seriously affected.

In summary, frequent overcharging, unauthorized casing modification, ambient temperatures above 60 degrees Celsius, and separator tearing are the main causes of EV battery swelling. In daily use, never take battery swelling lightly. A swollen battery not only suffers serious performance loss, with range possibly dropping sharply, but also poses major safety hazards such as explosion or fire. Therefore, when you find the battery swollen, the best approach is to take the EV to a professional repair shop for a comprehensive and careful inspection and appropriate maintenance. Only in this way can we ensure safe use of the EV, extend battery life, and keep the EV providing convenient travel.