The back cover of the phone suddenly tilted up while using it.

After riding the electric bike for several years, the battery casing began to deform.

Even the battery pack of new energy vehicles may experience local swelling.

Many people's first reaction when they see this scene is:

Did the battery explode from charging

In fact, what really makes the battery "fat up" is not just a simple charge, but the fact that a large amount of gas may have already been generated inside the battery.

The question is: Where do these gases come from? Why do some batteries last for several years without any issues, while others quickly start to swell?

More importantly, can the swollen battery still be used?

1. Why does gas produce inside the battery

Under normal circumstances, during the charging and discharging process of lithium-ion batteries, lithium ions move back and forth between the positive and negative electrodes without producing a large amount of gas inside the battery.

But if there is an abnormal side reaction inside the battery, the situation is different.

For example, if the electrolyte decomposes under high temperature, overcharging, or unstable material surfaces, it may produce gases.

Especially during the initial use of the battery, a layer of SEI film will form on the negative electrode surface.

If this layer of film is unstable, side reactions may continue to occur, consuming electrolyte and continuously producing gas.

As the gas increases, the internal pressure of the battery keeps rising, which may eventually manifest as bulging.

2. High temperature is the catalyst for battery bulging

Many people are unaware that temperature has a significant impact on the internal reactions of batteries.

After the temperature rises, processes such as electrolyte decomposition and material side reactions will accelerate.

It can be understood as:

The higher the temperature, the easier it is for many "undesirable reactions" to occur inside the battery.

Therefore, batteries that are exposed to high temperatures for a long time are more prone to problems such as capacity decay, increased internal resistance, and gas production.

This is also why new energy vehicle batteries require complex thermal management systems.

3. Overcharging can also cause the battery to become increasingly 'fat'

If the battery is in an overcharged state for a long time, both positive and negative electrode materials may exhibit abnormal reactions.

Especially when the battery is close to full charge and continues to be forcefully charged, internal side reactions may significantly increase.

In severe cases, not only may gas production occur, but more dangerous problems such as lithium evolution and internal short circuits may also occur.

So:

Sometimes bulging is just an "external manifestation" of battery problems, and the real danger may occur internally.

4. After bulging, can it still be used

Not recommended.

If it is found that the lithium battery in a mobile phone, electric vehicle, or other device has visibly bulged, it indicates that there is an abnormality inside the battery cell.

At this point, do not continue to squeeze, needle or disassemble the battery, and do not attempt to eliminate the bulge through "discharge".

For batteries with obvious deformation, heating, or odor, they should be stopped from use and professionally disposed of according to local regulations.

In conclusion

The battery is bulging, it just looks like the outer shell has become 'fat'.

But inside this shell, a series of complex electrochemical reactions may be occurring.

High temperature, overcharging, material side reactions, unstable SEI film, and manufacturing defects may all be the reasons behind bulging.