Is the lithium battery of an electric car unsafe? These four methods keep you away from danger!
With the implementation of the new national standard for electric bicycles, the weight of the whole vehicle has been strictly limited, and lithium-ion batteries with higher energy density have been widely used in electric bicycles. However, the quality of lithium-ion batteries in the electric two-wheeler market is uneven, and most store shopping guides and consumers lack understanding of lithium-ion batteries, which may lead to safety accidents. This is also the reason why a certain two-wheeler head brand recently issued a document prohibiting its offline stores from participating in the battery swap project and stocking large amounts of lithium batteries.
So, what are the safety hazards of lithium batteries? We must first understand the working principle of lithium batteries.
1. The working principle of lithium ion battery
Lithium-ion batteries use carbon materials as the negative electrode and lithium-containing compounds as the positive electrode (depending on the positive electrode compound, common lithium-ion batteries include lithium cobaltate, lithium manganate, lithium iron phosphate, ternary lithium, etc.). There is a diaphragm in the middle to avoid short-circuiting the positive and negative poles. During charging and discharging, Li+ travels back and forth between the positive and negative electrodes: when charging, lithium ions are extracted from the positive electrode and inserted into the negative electrode through the electrolyte; the opposite is true when discharging. During the intercalation and deintercalation of lithium ions, current is also generated along with the intercalation and deintercalation of equivalent electrons.
2. Why does the lithium battery of electric vehicles explode?
Knowing the working principle of lithium batteries, you can also know the reasons why lithium-ion batteries explode, which are mainly divided into the following two points.
1. Overcharge causes too much lithium to be released, and the capacity of the negative electrode is 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, these lithium atoms will grow dendrites from the surface of the negative electrode toward the direction of the lithium ions. These lithium metal crystals will pass through the diaphragm paper and short-circuit the positive and negative electrodes. Sometimes the battery explodes before the short circuit occurs. This is because during the overcharging process, the electrolyte and other materials will crack to produce gas, causing the battery shell or pressure valve to swell and rupture, allowing oxygen to enter and react with the lithium atoms accumulated on the surface of the negative electrode. And then exploded.
2. Current limitation is also necessary when charging and discharging. When the current is too large, lithium ions will not have time to enter the interlayer structure and will also collect on the surface of the negative electrode material. After these lithium ions obtain electrons, they will produce lithium atom crystals on the surface of the material, which is the same as overcharging, which is dangerous.
Therefore, generally in the lithium battery pack, in addition to the lithium battery core, there will be a protective board, which is what we often call BMS (BATTERY MANAGEMENT SYSTEM), the Chinese name is battery management system. It can monitor the voltage, current and temperature of the lithium battery and intervene when needed according to the status of the lithium battery.
Whether it is an electric car or an electric bicycle, lithium battery accidents are no longer new. In the old national standard era, the electric vehicle industry has a small amount of lithium battery applications, and many safety accidents have occurred. The main reasons are the unstable lithium battery cells, the immature PACK process, and the poor use of lithium batteries due to lack of knowledge. Habits and so on. With the rapid increase in the use of lithium batteries in the era of the new national standard, if some measures are not taken, the probability of accidents can be imagined.
Three, lithium battery safety measures
1. Safety measure 1: Software BMS protection board, remote safety monitoring
Nowadays, the domestic battery cell manufacturing level cannot reach the consistency accuracy of Japan and South Korea. The battery cell itself has certain safety hazards. If only a pure hardware BMS board is used for protection, there is a probability of failure of such electronic components. Once the BMS fails, Then there will be a safety accident.
The solution is to add a remote data transmission module to realize the data interconnection of key electrical components of the vehicle, including the charging and discharging data of the lithium battery. Through the data monitoring platform of the headquarters, we can predict battery failure modes, inform users in advance and provide quick services, and eliminate potential safety hazards in the bud.
2. Safety measures 2: Improve the structure to enhance the safety of lithium batteries
Compared with electric vehicles, the use environment of lithium batteries in electric bicycles is even harsher. The vehicle has poor shock absorption performance; the riding conditions are complex; the vehicle is often placed outdoors, and the battery is directly exposed to the outside, experiencing wind, sun, and rain.
The solution is to independently design a fully sealed, breathable, dry, water-proof and moisture-proof structure, and the interior is filled with high thermal conductivity silica gel. While improving the seismic level of the internal battery and BMS board, it also further strengthens the waterproof and moisture-proof performance. At the same time, it achieves the purpose of equalizing the temperature between the cells inside the battery, ensuring the consistency of the cells, and thereby improving the service life of the entire battery pack.
3. Safety measure 3: Two-way handshake identification circuit to ensure that the battery is not used indiscriminately
Electric bicycles on the market are basically straight-through output solutions, which provides convenience for users to abuse batteries. If the lithium battery is used in lead-acid vehicles, that is, lead-to-lithium sales; or used as a test treasure for lead-acid vehicle trial riding; there will also be maintenance personnel using lead-acid controllers or chargers to replace faulty lithium battery control And charger. These are bad habits left behind in the lead-acid battery era. Once the lithium battery is misused on a lead-acid vehicle, the lead-acid controller has EABS reverse charging and is inconsistent with the undervoltage point of the lithium battery, which will cause safety hazards to the lithium battery.
The solution is to add a two-way handshake recognition circuit between the lithium battery BMS board and the controller and before the BMS board and the charger. As long as they do not match each other, the lithium battery will close the charge and discharge loop, and illegal controllers are not allowed to discharge. , Illegal charger charging, reduce lithium battery safety accidents caused by external factors.
4. Lithium battery safety measures 4: PGR motor avoids reverse charging of lithium battery
EABS reverse charging is a very good point of interest for lead-acid batteries, but for lithium batteries, it has become a potential safety hazard.
What are the hazards of reverse charging of lithium batteries? We have already introduced the charging principle of lithium batteries, that is, the process in which lithium ions are extracted from the positive electrode and migrate to the negative electrode porous graphite through the diaphragm and electrolyte. If the lithium ion migration speed is not fast enough and the migration amount is too large, it will stay on the surface of the negative electrode to form metallic lithium dendrites, which will pierce the diaphragm to a certain extent and cause a short-circuit explosion. The factors that affect the number and speed of lithium ion migration during the charging process are mainly the ambient temperature and the charging current.
Although the EABS function has been cancelled for all lithium electric vehicles, reverse charging current will still occur during downhill sliding: the existing industry lithium electric bicycles generally use low-speed hub direct-drive motors, which are electric motors during riding, but downhill. When the slope slides at high speed, it becomes a generator. When the motor speed exceeds the rated speed, the back-EMF voltage will be higher than the battery voltage, and the energy will be injected back into the battery. The faster the vehicle slides, the higher the induced back-EMF, that is, the greater the counter-charging current.
Especially in the case of the new national standard electric bicycle with a maximum speed limit of 25 kilometers per hour, the glide speed on a downhill section will easily exceed the maximum speed of 25 kilometers per hour, and the reverse charging current is extremely large, especially in the cold winter, the electrolyte conducts electricity. The rate of lithium ion decreases, the lithium ion migration rate decreases, and the probability of metallic lithium dendrite formation on the surface of the negative electrode is greater. The puncture of the diaphragm is more likely to occur. In the light, the battery capacity decreases sharply, the life is accelerated, and in the worst case, large-area short-circuit heating occurs. , And eventually exploded and burned out of control.
Therefore, the lithium battery models are equipped with a self-designed PGR motor with a clutch device. During coasting, the internal motor is disconnected from the outer wheel and stops rotating, that is, no induced electromotive force will be generated, and of course no reverse charging will occur.
Through in-depth analysis of the failure mechanism of lithium batteries, and through the above multiple technical improvements, the safety and reliability of lithium batteries can be ensured, and it is possible to popularize lithium-ion batteries on electric bicycles, and at the same time allow users to use them more comfortably, reliably and with less trouble.