You wake up in the morning to a jolt from your electric toothbrush, and your electric bike whisks you through crowded streets on your way to work. In the office, laptops and mobile phones support the day’s work.
Lithium batteries have long been integrated into every corner of life.
However, have you ever thought about why these “energy blocks” that provide convenience for life become dangerous “flame items” when the news suddenly shows the flaming flames of electric cars or the breathtaking pictures of power banks smoking in their bags?
The answer is often hidden in the fatal moment that can not be seen, the short circuit.
According to statistics, 80% of serious lithium battery accidents are due to short circuit.
What is more alarming is that short circuit is not a single phenomenon: it is like a pair of “dangerous twins”, divided into internal short circuit and external short circuit.
The two have different causes and different manifestations, but they are equally fatal.
We will analyze the definition, causes, manifestations and prevention of them.
- What are internal and external short circuits?
Internal short circuit:
Short circuit in lithium battery refers to the abnormal state of forming a low-impedance conductive path between the positive and negative electrodes inside the battery.
External short circuit:
When the external positive and negative poles of the battery encounter conductor intervention, the short circuit phenomenon occurs immediately.
- What causes it?
Internal short circuit:
Under normal working conditions, the separator inside the lithium-ion battery, as a key insulating component, can effectively isolate the positive and negative materials. However, when the battery is overcharged, lithium metal will deposit on the surface of the negative electrode and form lithium dendrites, which will penetrate the separator with the continuous growth of lithium dendrites, or in the case of thermal abuse, the separator material will melt and shrink due to high temperature, resulting in insulation failure. Or in the process of over-discharge, the collapse of the cathode material structure produces sharp debris to pierce the diaphragm, which will lead to direct conduction between the anode and cathode, and then lead to internal short circuit fault.
External short circuit:
External short circuit may be caused by accidental contact of metal tools with positive and negative electrodes, infiltration of liquids containing conductive ions (such as rainwater, electrolyte) into electrode gaps, or residual metal debris during battery assembly and overlapping of two electrodes. This kind of short circuit process is similar to directly binding the positive and negative poles of the battery with iron wire, forming a low-resistance conductive path instantaneously, making the current bypass the normal load, generating a large amount of heat in a very short time, which may cause the battery to bulge, smoke or even catch fire in serious cases.
- What are the manifestations of releasing short circuit?
Internal short circuit:
In the initial stage of short circuit in lithium battery, because the short circuit current is small and the heat is in the slow accumulation stage, the appearance and performance parameters of the battery may not change significantly.
With the continuous decrease of internal impedance and the gradual increase of short-circuit current, the internal temperature of the battery shows a nonlinear upward trend, which is typically manifested as abnormal temperature rise when the equipment is idle. When the temperature reaches the critical value to trigger the runaway reaction, the battery will release a large amount of heat rapidly, triggering a violent chain reaction, eventually leading to safety accidents such as combustion or explosion.
External short circuit:
When a short circuit occurs, a low impedance path will be formed instantly inside or outside the battery, which will generate a large current several times or even tens of times higher than normal working current.
Under the action of Ohm’s law, the large current passes through the internal or external resistance of the battery, and the electric energy is rapidly converted into heat energy, resulting in a sharp rise in the temperature of the battery, the surface temperature of the shell can soar to hundreds of degrees Celsius in a few seconds, and the naked eye can see white smoke pouring out from the gap. If the short circuit is not blocked in time, the internal pressure of the battery will increase sharply due to side reactions such as electrolyte vaporization and diaphragm melting. When the pressure exceeds the limit of the battery shell, it will cause severe.
- How to prevent it?
Internal short circuit:
Control the metal chips and burrs, and optimize the material system design. And avoid overcharge and over discharge at that use terminal.
External short circuit:
Attention shall be paid to shell protection and scene control for external short circuit.
In a word, the short circuit in the lithium battery is the damage and failure inside the battery core, which causes the positive and negative electrodes to form a direct electrical connection path inside the battery. External short circuit is mostly caused by external accidents such as humid battery environment, metal foreign body contact or human misoperation. An internal short circuit is an “internal fault” and an external short circuit is an “external accident”.
