What Exactly Happens During Overcharging and Puncturing?

Lithium - polymer (LiPo) batteries have become ubiquitous in our lives, powering everything from our smartphones to drones and electric 

vehicles. However, their convenience comes with a hidden risk: thermal runaway. This catastrophic failure mode can lead to fires, explosions, 

and serious injuries. To better understand this phenomenon, let's take a deep dive into what happens inside a LiPo battery during two common 

triggers of thermal runaway: overcharging and puncturing.

The Science Behind LiPo Batteries

Before delving into thermal runaway, it's essential to grasp the basic structure and operation of a LiPo battery. A typical LiPo battery consists of 

multiple layers of positive and negative electrodes, separated by a porous membrane soaked in an electrolyte. The positive electrode is usually 

made of lithium cobalt oxide (LiCoO₂) or other lithium - based compounds, while the negative electrode is often graphite. The electrolyte, a 

lithium salt dissolved in an organic solvent, allows the flow of lithium ions between the electrodes during charging and discharging.

During normal operation, lithium ions move from the positive electrode to the negative electrode when charging and reverse direction when 

discharging. This movement of ions is what stores and releases electrical energy. The battery's casing keeps all these components contained 

and protected.

Overcharging: A Recipe for Disaster

Overcharging is one of the most common causes of LiPo battery thermal runaway. When a LiPo battery is overcharged, the voltage across its 

terminals exceeds the safe limit, typically around 4.2 volts per cell. This excess voltage triggers a series of chemical reactions that can quickly 

spiral out of control.

At first, the overcharging causes lithium ions to accumulate on the surface of the negative electrode faster than they can be inserted into the 

graphite structure. This leads to the formation of lithium metal dendrites, which are needle - like structures. These dendrites can grow over time 

and eventually pierce the separator membrane between the positive and negative electrodes. Once the separator is pierced, a short circuit 

occurs, allowing a large current to flow between the electrodes.

The short circuit generates a significant amount of heat. As the temperature rises, the organic electrolyte begins to decompose, releasing 

flammable gases such as carbon monoxide, hydrogen, and methane. The increased pressure from these gases can cause the battery casing 

to rupture.

Meanwhile, the high temperatures also trigger exothermic reactions in the electrode materials. For example, lithium cobalt oxide can decompose 

at high temperatures, releasing oxygen and more heat. This oxygen further fuels the combustion of the flammable gases, leading to a violent fire 

or explosion.

Puncturing: Breaking the Barriers

Puncturing a LiPo battery, whether by a sharp object or due to physical damage, can also initiate thermal runaway. When the battery is punctured, 

the separator membrane is immediately broken, creating a direct short circuit between the positive and negative electrodes.

Similar to overcharging, the short circuit results in a rapid release of energy in the form of heat. The heat causes the electrolyte to vaporize and 

decompose, producing flammable gases. The puncture itself may also create a path for these gases to escape, but if the release is restricted, 

pressure can build up inside the battery.

In addition to the short circuit, the puncturing object can cause mechanical damage to the electrodes. This damage can disrupt the normal 

structure of the electrodes, leading to more localized heating and further chemical reactions. The combination of heat, flammable gases, and 

oxygen from the surrounding air (if the casing is breached) can quickly ignite a fire.

It's important to note that even a small puncture can be dangerous. The initial short circuit may be small, but the heat generated can spread to 

other parts of the battery, causing a chain reaction that leads to full - scale thermal runaway.

Preventing Thermal Runaway

Understanding what happens during LiPo battery thermal runaway is crucial for preventing such incidents. Here are some key preventive 

measures:

Use a proper charger designed for LiPo batteries and never leave a charging battery unattended. Chargers with built - in safety features, such 

as overcharge protection, can significantly reduce the risk of overcharging.

Inspect LiPo batteries regularly for signs of damage, such as swelling, cracks, or punctures. If a battery is damaged, discontinue use immediately 

and dispose of it properly according to local regulations.

Store LiPo batteries in a cool, dry place away from flammable materials. Specialized battery storage bags or containers can provide an extra 

layer of protection in case of a thermal event.

Avoid exposing LiPo batteries to extreme temperatures, both high and low. High temperatures can accelerate chemical reactions, while low 

temperatures can reduce battery performance and increase the risk of damage during use.

In conclusion, LiPo battery thermal runaway during overcharging and puncturing is a complex process involving a series of chemical and physical 

reactions. By understanding these mechanisms, we can take proactive steps to minimize the risks associated with these powerful energy sources. 

Whether you're a hobbyist using LiPo batteries in a drone or simply a consumer with a smartphone, being informed and cautious is the key to safe 

battery use.