Why do lithium-ion batteries explode?
In order to understand the principle of lithium-ion batteries, we can
understand the reasons why lithium-ion batteries explode.
1. Overcharging causes too much lithium to be released and the capacity of
the negative electrode to be insufficient. The lithium generated during charging
cannot be inserted into the interlayer structure of the negative electrode
graphite, and metallic lithium will be formed on the surface of the negative
electrode. Over time, lithium molecules will grow a network-like crystal
structure from the negative surface layer into the lithium-ion battery. This
lithium metal crystal can pass through the diaphragm, causing a positive and
negative short-circuit failure. Sometimes rechargeable batteries explode before
a short-circuit failure occurs because raw materials such as lithium battery
electrolyte destroy the vapor, causing the battery sleeve or pressure valve to
burst, allowing carbon dioxide to react with the lithium atoms deposited in the
negative surface layer, which then explodes.
2. When the battery is charging, current limiting is also necessary. When
the current is too large, the lithium-ion battery cannot catch up with the
interlayer structure and will accumulate on the surface layer of the cathode
material. This kind of lithium-ion battery will produce lithium-ion crystals on
the surface of the raw material, which can cause dangerous factors like
overcharging.
Therefore, generally in lithium battery packs, in addition to lithium
batteries, there is also a lithium battery protection board, commonly known as
bms (battery management system), and the Chinese name is rechargeable battery
intelligent management system. It can detect the lithium power supply in the
event of interference based on the operating voltage, current and temperature
information content of the lithium power supply.
Whether it is a pure electric vehicle or an electric vehicle, lithium-ion
safety incidents are not new. In the old national standards period, the electric
vehicle industry already had a small number of lithium-ion applications, and
many safety accidents had occurred. The key reasons were that the lithium
battery core was unstable, packaging and processing technology was immature, and
little was known about lithium-ion application habits. With the increase in the
use of lithium ions during the national standard period, if some countermeasures
are not taken, safety accidents are very likely to occur.
Lithium-ion battery safety measures
1. Safety measure one: Metal lithium will be formed on the surface of the
negative electrode.
At present, China's lithium-ion manufacturing level is still not as high as
that of South Korea and Japan's consistent precision. The lithium-ion core
itself has certain safety risks. If only a purely hardware-configured bms board
is maintained, such electronic components have a probability of being
ineffective. Once bms is ineffective, it will inevitably lead to safety
production accidents.
The solution is to add a remote control transmission data control module to
complete the data interconnection of all important electrical equipment
components, including lithium-ion battery charging data information. Based on
the company's data information monitoring and management platform, it can
predict battery failure modes, warn users and provide convenient services, and
safety risks in their infancy.
2. Safety measure two: Improve the structure to enhance the safety of
lithium batteries
Compared with pure electric vehicles, the application of lithium ions in
electric vehicles is more extreme. The entire vehicle has poor shockproof
performance; real-time road conditions are complex; the vehicle is often placed
outside the room, and the rechargeable battery will be immediately exposed to
sunlight and rain through gentle blowing.
The self-designed tailings design adopts a sealed air-permeable,
water-proof, and waterproof gas structure, with high heat transfer coefficient
silicone rubber added inside. In addition, it also ensures that all lithium-ion
batteries are in the middle of the average temperature to ensure the consistency
of the lithium-ion batteries, thereby increasing the service life of the entire
battery.
3. Safety precautions three: two-way handshake identification circuit to
ensure that the battery is not misused
The basis of electric vehicles in the sales market is a direct connection
discharge scheme that makes it easier for users to use the battery. If lithium
is used as a lead-acid battery for automobiles, that is to say, lead is sold in
the lithium market; or as a trial operation treasure trove for cycle testing of
lead-acid batteries; there will continue to be maintenance personnel using
lead-acid battery control boards Or charging head to replace the lithium
electric vehicle control board and charging head. This is a bad habit with
lead-acid batteries. Once lithium is used in lead-acid battery vehicles, safety
risks will arise because the lead-acid battery control board has anti-battery
charging capabilities and does not meet the low-voltage protection point of
lithium batteries.
The corresponding measure is that a dual-wave identification power circuit
is proposed between the lithium-ion battery board and the control board, and
before the BMS board and charging head. If the lithium-ion batteries do not
match each other, the charging control circuit will be shut down.
4. Lithium battery safety precautions four: PGR motor to avoid reverse
charging of lithium battery
Electrical resistance charging is a good point of interest for lead-acid
batteries, but for lithium-ion it becomes a safety risk.
What are the dangers of reverse charging of lithium batteries? The basic
principle of lithium-ion battery charging is introduced in detail, that is, the
lithium-ion battery is sliding from the normal stage, the entire process of
transition from the diaphragm and lithium-ion battery electrolyte to the
high-purity graphite with negative porous structure. If the transfer rate of the
lithium-ion battery is not fast enough and the total number of transfers is too
high, it will stay in the negative surface layer and produce lithium dendrites.
The main factors that affect the total transfer rate and transfer rate of
lithium-ion batteries are operating temperature and current size.
Although all lithium-ion vehicles eliminate the single-vehicle effect,
reverse charging current is still present throughout downhill operation. When
the rated power of the motor exceeds the rated speed ratio, the self-induced
electromotive force operating voltage will be higher than the battery voltage,
and the kinetic energy will flow back to the rechargeable battery. The faster
the vehicle descends, the higher the magnetically induced electromotive force,
and the greater the reverse current.
Especially when the national standard electric vehicle maximum speed limit
is 25km/h, the sliding rate on the downhill section will exceed 25km/h, and the
reverse current will be large. Especially in cold winter, the conductivity of
the lithium battery electrolyte decreases, and the lithium The transmission rate
of the ion battery decreases, and the lithium dendrites of the negative layer
metal material are more likely to change, and the diaphragm is more likely to
form. If the battery is slightly penetrated, the volume of the battery will
decrease, and the battery life will decrease faster. In severe cases, a large
area of short circuit will generate heat, and eventually it will explode and
burn out of control.
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