Reason Analysis of Why Battery Fire&Exploded?

- Mar 10, 2019-

Guide: The types of battery cell explosion can be summarized as external short circuit, internal short circuit and overcharge. The outer part of the battery refers to the outer part of the cell, which includes the short circuit caused by poor insulation design of the battery pack.

 

Cause analysis of battery drum shell and explosion:

  

I. Characteristics of lithium-ion batteries

  

Lithium is the smallest and most active metal on the chemical periodic table. Because of its small size and high capacity density, it is widely welcomed by consumers and engineers. However, the chemical characteristics are too active, which brings high risk. When lithium metal is exposed to air, it will react with oxygen intensely and explode. To improve safety and voltage, scientists have invented materials such as graphite and lithium cobalt oxide to store lithium atoms. The molecular structure of these materials forms nanoscale small storage lattices that can be used to store lithium atoms. In this way, even if the battery shell ruptures and oxygen enters, the oxygen molecules will be too large to enter these tiny storage cells, so that lithium atoms will not contact with oxygen and avoid explosion. This principle of lithium-ion batteries enables people to achieve high capacity density and safety at the same time.

 

When lithium-ion batteries are charged, the lithium atoms in the cathode lose electrons and oxidize to lithium ions. Lithium ion swims to the negative electrode through the electrolyte, enters the storage cell of the negative electrode, and obtains an electron, which is reduced to lithium atom. When discharging, the whole program is reversed. In order to prevent short circuit caused by direct contact between positive and negative electrodes, a kind of diaphragm paper with many holes will be added to the battery to prevent short circuit. Good diaphragm paper can also automatically close the fine holes when the battery temperature is too high, so that lithium ions can not pass through, in order to self-discard martial arts, to prevent danger.

 

protective measures

 

When the overcharge voltage of lithium batteries is higher than 4.2V, side effects will begin to occur. The higher the overcharge voltage, the higher the risk. When the voltage of the lithium cell is higher than 4.2V, the number of lithium atoms remaining in the cathode material is less than half. At this time, the storage cell often collapses, causing a permanent decline in battery capacity. If the charge is continued, subsequent lithium metals will accumulate on the surface of the negative material because the storage cell of the negative electrode is filled with lithium atoms. These lithium atoms grow dendritic crystals from the negative surface toward the direction of lithium ions. These lithium crystals will pass through the diaphragm paper and short the positive and negative electrodes. Sometimes the battery explodes before the short circuit occurs. This is because in the process of overcharging, materials such as electrolyte will crack to produce gas, causing the battery shell or pressure valve to swell and burst, allowing oxygen to react with lithium atoms accumulated on the surface of the negative electrode, and then explode. Therefore, when recharging lithium batteries, it is necessary to set a voltage ceiling, so as to take into account the life, capacity and safety of the batteries at the same time. The optimal upper limit of charging voltage is 4.2V.

 

There is also a lower voltage limit when lithium cells are discharged. When the cell voltage is less than 2.4V, some materials will begin to be destroyed. Because the battery will self-discharge, the longer the discharge time, the lower the voltage. Therefore, it is better not to put 2.4V to stop discharging. During the period from 3.0V discharge to 2.4V discharge, the energy released by lithium batteries only accounts for about 3% of the battery capacity. Therefore, 3.0V is an ideal discharge cut-off voltage.


In addition to voltage limitation, current limitation is also necessary when charging and discharging. When the current is too high, lithium ions can not enter the storage cell, and will accumulate on the surface of the material. When these lithium ions acquire electrons, lithium atoms will crystallize on the surface of the material, which is as dangerous as overcharging. If the battery case breaks, it will explode.

 

Therefore, the protection of lithium-ion batteries should include at least three items: the upper limit of charging voltage, the lower limit of discharging voltage and the upper limit of current. In general, besides the lithium battery cell, there will be a protective plate in the lithium battery pack, which mainly provides these three kinds of protection. However, these three protections are obviously insufficient, and the explosion of lithium batteries is still a frequent occurrence in the world. In order to ensure the safety of battery system, the cause of battery explosion must be analyzed more carefully.

 

II. Causes of battery explosion:


1: Internal polarization is bigger!

 

 

2: The pole absorbs water and reacts with the electrolyte.

 

3: The quality and performance of electrolyte itself.

 

4: The injection volume can not meet the technological requirements.

 

5: In the assembly process, the sealing performance of laser welding is poor, gas leakage and leakage detection.

 

6: Dust, polar dust is easy to cause micro-short circuit, the specific reasons are unknown.

  

7: The positive and negative plates are thicker than the process range and difficult to shell.

  

8: Injection sealing problem, poor sealing performance of steel balls lead to air drum.

 

9: The shell wall is thicker when the material is supplied, and the deformation of the shell affects the thickness.

 

 

III. Analysis of Explosion Types

 

The types of battery cell explosion can be summarized as external short circuit, internal short circuit and overcharge. The outer part of the battery refers to the outer part of the cell, which includes the short circuit caused by poor insulation design of the battery pack.

 

When the short circuit occurs outside the cell and the electronic components fail to cut off the circuit, high heat will occur inside the cell, which will cause some electrolytes to vaporize and expand the battery shell. When the temperature inside the battery reaches 135 degrees Celsius, the good quality diaphragm paper will close the fine hole, terminate or nearly terminate the electrochemical reaction, and the current will drop sharply and the temperature will drop slowly, thus avoiding the explosion. However, the poor closure rate of fine holes, or the diaphragm paper that the fine holes will not close at all, will make the battery temperature continue to rise, more electrolytes will vaporize, and eventually the battery shell will burst, or even the battery temperature will be raised to make the material burn and explode.

 

The internal short circuit is mainly caused by the burr of copper foil and aluminium foil breaking through the diaphragm, or the dendritic crystal of lithium atom breaking through the diaphragm. These tiny needle-like metals can cause a slight short circuit. Because the needle is very thin and has a certain resistance value, the current may not be very large. The burr of copper and aluminium foil is caused by the production process. It can be observed that the leakage of battery is too fast. Most of the burrs can be detected by the cell factory or assembly plant. Moreover, because of the small burr, sometimes it will be burned off, making the battery back to normal. Therefore, the probability of explosion caused by burr short circuit is not high.

 

In this way, it can be said that there are often bad batteries with low voltage shortly after charging, but few explosions occur, which is supported by statistics. Therefore, the explosion caused by internal short circuit is mainly caused by overcharge. Because there are needle-like lithium crystals everywhere on the overcharged rear pole sheet, puncture points are everywhere, and micro-short circuits occur everywhere. Therefore, the temperature of the battery will gradually increase, and finally the high temperature will make the electrolyte gas. In this case, whether the material is burned and exploded due to excessive temperature or the shell is broken first, which causes the air to enter and the lithium metal to oxidize intensely, it is the end of the explosion.

 

But the explosion caused by internal short circuit caused by overcharge does not necessarily occur at the time of charging. It is possible that consumers will stop charging and take their mobile phones out before the temperature of the battery is high enough to make the material burn and produce enough gas to break the battery shell. At this time, the heat generated by numerous micro-short circuits slowly increases the temperature of the battery, and after a period of time, the explosion occurs. The common description of consumers is that when they pick up the phone, they find it very hot, and when they throw it away, it explodes.

 

Based on the above types of explosion, we can focus on the prevention of overcharge, external short circuit, and enhance the safety of the cell. Overcharge prevention and external short circuit prevention belong to electronic protection, which is closely related to battery system design and battery assembly. Chemical and mechanical protection is the key to improve the safety of the battery, which is closely related to the battery cell manufacturer.

 

IV. Design Specifications

  

Since there are hundreds of millions of mobile phones in the world, the failure rate of security protection must be less than one in 100 million to achieve safety. As a result, the failure rate of circuit boards is generally much higher than one in 100 million. Therefore, when designing battery system, there must be more than two safety lines. A common mistake is to use the adaptor directly to charge the battery pack. In this way, the protection of overcharged batteries is entirely handed over to the protection board on the battery pack. Although the failure rate of protective panels is not high, even if the failure rate is as low as one in a million, the probability of explosion accidents will occur everyday in the world.

 

If the battery system can provide two safety protections for overcharge, overdischarge and overcurrent respectively, the failure rate of each protection can be reduced to one in 100,000 if the failure rate of each protection is one in 10,000. Common battery charging system block diagram is as follows, including charger and battery pack two major parts. The charger includes adapter and charge controller. The adapter converts AC to DC, while the charging controller limits the maximum current and voltage of DC. The battery pack consists of a protective panel and a battery cell, as well as a PTC to limit the maximum current.

 

Text box: adapter AC/DC text box: Charge controller current limiting and voltage limiting text box: Charger text box: Protective text box: Protective text box such as overcharge and overdischarge overcurrent; Battery text box: Current limiting text box: Battery cell takes mobile phone battery system as an example, overcharge protection system uses the output voltage of charger to set at about 4.2V to achieve the first layer of protection. Protection, so that even if the protective plate on the battery pack fails, the battery will not be overcharged and dangerous. The second protection is the overcharge protection function of the protective board, which is generally set to 4.3V. In this way, the protection board usually does not need to be responsible for cutting off the charging current, only when the charger voltage is abnormally high, it needs action. Over-current protection is the responsibility of the protection board and current limiter, which are also two ways of protection to prevent over-current and external short circuit. Because overdischarge can only occur in the process of electronic products being used. Therefore, the circuit board of the electronic product is generally designed to provide the first protection, while the protection board on the battery pack provides the second protection. When an electronic product detects that the supply voltage is less than 3.0V, it should shut down automatically. If this function is not designed in the design of the product, the protective plate will close the discharge circuit when the voltage is low to 2.4V.

  

In a word, when designing battery system, it is necessary to provide two electronic protections for overcharge, overdischarge and overcurrent respectively. The protective plate is the second protection. Remove the protective panel and charge it. If the battery explodes, it means bad design.

 

 

Although the above-mentioned methods provide two protections, consumers often buy non-original chargers to charge after the charger breaks down, and charger manufacturers, based on cost considerations, often remove the charge controller to reduce costs. As a result, bad money drives out good money, and there are many bad chargers on the market. This makes overcharge protection lose the first and most important line of defense. Overcharge is the most important factor that causes battery explosion, so poor charger can be called the culprit of battery explosion.

 

Of course, not all battery systems use the scheme shown above. In some cases, there will be a charge controller design in the battery pack. For example, many notebook computers have charging controllers for their external batteries. This is because notebook computers generally use charge controllers in their computers and only give consumers an adapter. Therefore, the external battery pack of notebook computer must have a charging controller to ensure the safety of the external battery pack when using the adapter to charge. In addition, products charged by car cigarette lighters sometimes use charging controllers in battery packs.

 

The Last Defender HD

 

If electronic protection fails, the last line of defense will be provided by the cell. The safety level of the cell can be roughly classified according to whether the cell can be short-circuited or overcharged. Because before the battery explodes, if lithium atoms accumulate on the surface of the material, the explosion power will be greater. Moreover, the protection against overcharge is often left with only one line of defense because consumers use inferior chargers. Therefore, the ability to resist overcharge is more important than the ability to resist external short circuit.

 

Compared with steel cell, aluminum cell has higher safety advantages than steel cell.