18650 lithium rechargeable battery

　　Full technical analysis of the reasons why 18650 lithium rechargeable battery explosion news continues

　　The application of lithium batteries in daily life has suddenly expanded, mainly due to the widespread use of smartphones, wearable devices, electric bicycles and new energy vehicles. Over the years, there have been constant news about how to use batteries, lithium battery explosions and other news, but among them Often contains a lot of misleading information. In this article, we start from simple applications to complex applications of lithium batteries.

　　To measure battery performance, there are several important indicators:

　　1. Charge and discharge rate: the higher, the better

　　"C" is a special symbol that describes the size of battery charging and discharging current. 1C discharge represents the amount of current that takes the battery from fully charged to empty within 1 hour. The iPhone 6 battery capacity is 1810mAH, so the 1C discharge current of this battery is 1.81 amps; the capacity of each battery used in BYD e6 electric car is 200AH, so the 1C discharge current of this battery is 200 amps. If a battery is discharged at a high rate, it usually releases less energy than at a low rate.

　　Full technical analysis: Why lithium battery explosion news continues

　　The amount of electricity released at different discharge rates

　　From the test results in the picture above, we can see that the energy released by this power battery using 10C discharge is 85% of that under 1C discharge, and the energy released using 20C discharge is only 70% of that under 1C discharge.

　　2. Number of charge and discharge cycles: the more, the better

　　500 times is a common value for lithium batteries. The number of charge and discharge times of lithium batteries made of different materials ranges from 300-3000 times. The specific meaning of this value may be slightly different in each factory, but it can be roughly understood as: according to the charge and discharge rate specified by the manufacturer (for example, 1C discharge, 0.3C charge; charge from 0% to 100% each time, and follow this cycle) , after 500 cycles, the battery capacity remains 80% of the original. The relationship between the number of charge and discharge times and usage habits is huge. Let’s give a few examples.

　　1. The influence of charge and discharge intensity on the number of cycles

　　Factory label: Each time, charge from 0% to 100%, discharge at 1C, charge at 0.3C, the capacity will decay to 80% after 500 times. This is the most stringent test cycle. It does not need to be so strict. See below.

　　If each power cycle is between 25% and 75%, discharge at 1C and charge at 0.3C, the capacity will decay to 80% after 2000 times.

　　If each power cycle is between 50% and 100%, discharge at 1C and charge at 0.3C, the capacity will decay to 80% after 1800 times.

　　2. The impact of shallow charging and shallow discharge on lifespan

　　Factory label: Each time, charge from 0% to 100%, discharge at 1C, charge at 0.3C, and the capacity will decay to 80% after 500 times. This is the most stringent test cycle. It does not need to be so strict. See below.

　　Each power cycle is between 25% and 75%. Discharge at 1C and charge at 0.3C. The capacity will decay to 80% after 2000 times.

　　Each power cycle is between 50% and 100%. Discharge at 1C and charge at 0.3C. The capacity will decay to 80% after 1800 times.

　　It can be seen from the above two examples that the smaller the charging and discharging rate is, the more beneficial it is to improve the lifespan; shallow charging and shallow discharging are also beneficial to improving the lifespan.

　　3. Internal resistance: the smaller, the better

　　This parameter changes at any time with the load, temperature and other factors. As the battery life decreases, the internal resistance gradually increases. The smaller the internal resistance of the battery, the better it can be charged and discharged at a high rate. The internal resistance of ordinary 18650 batteries is around 50mΩ, and the internal resistance of power-type 18650 batteries is around 15mΩ. If you want to know the internal resistance, you need to use special equipment to measure it, and ordinary multimeters will not work.

　　Full technical analysis: Why lithium battery explosion news continues

　　Tools for measuring battery internal resistance

　　4. Battery consistency

　　The higher the consistency in capacity, internal resistance, and charge and discharge curves of batteries produced using the same materials and processes, the better. Whether batteries can be assembled into battery packs on a large scale is very critical. The larger the size of the battery pack, the higher the consistency requirements.

　　On the following pages we will rank the application in order of difficulty:

　　1. Lithium batteries in mobile phones, tablets, and wearable devices

　　2. Lithium batteries in laptops and mobile power supplies

　　3. Lithium batteries in electric bicycles

　　4. Lithium batteries in electric vehicles

　　Introduce some knowledge respectively, so that the paging is sorted from small to large according to the battery size.

　　I. Lithium batteries in mobile phones, tablets, and wearable devices

　　Why is this field the easiest? Because there is only one lithium battery in these devices, and they are basically ternary lithium batteries. Ternary means three elements: nickel, cobalt, manganese, and the cathode material Li (NiCoMn) O2 of this lithium battery. They are not exactly the same. The ratio of the three elements will be adjusted according to the usage requirements.

　　Full technical analysis: Why lithium battery explosion news continues

　　1. Charge and discharge rate

　　Batteries used in digital devices usually have very low requirements on this. It’s hard for you to use a fully charged mobile phone until it automatically shuts down within an hour, right? No one would design a digital device with a battery life of only 1 hour. They can last for at least 3 hours, so the battery’s discharge rate of about 0.3C can meet the demand, and the charging requirements are often very low. Generally, everyone can accept a digital device that is fully charged in 3-4 hours, so the charging requirements for the battery are It is also about 0.3C.

　　Full technical analysis: Why lithium battery explosion news continues

　　Regardless of whether it is domestically produced or imported lithium batteries such as Panasonic and Samsung, 1C discharge is the minimum specification, and the battery discharge requirements of digital equipment are far lower than the general standards of the battery industry. The charging rate is roughly equivalent to the basic indicators that the battery industry can currently achieve. If there is no extremely special design, there is generally no need to worry.

　　Full technical analysis: Why lithium battery explosion news continues

　　OPPO’s battery flash charging feature puts forward higher requirements for charging

　　Recently, there have been special cases in the mobile phone industry. OPPO, as a representative, has proposed the feature of fast charging, which actually places special demands on battery manufacturers. Taking OPPO's latest N3 as an example, VOOC flash charging promises to charge 75% of the battery in 30 minutes. This works out to a charging current of 1.5C, which is 10 times the charging rate of ordinary mobile phone batteries. It makes sense that the N3 is priced at 3,999 yuan. At least this battery will be one or two times more expensive than ordinary batteries.

　　2. Number of charge and discharge cycles

　　Among digital devices, mobile phones are the most intensively used products. If we charge it once a day, the number of cycles is 500 times, which means a lifespan of 500 days. Is this correct? In fact, it is out of use conditions, so it is wrong. 500 cycles refers to 1C discharge and 0.3C charging for 500 consecutive cycles, and the battery capacity remains 80% of the original. But when we usually use mobile phones, the discharge rate is much less than 1C, often between 0.01C-0.5C. Therefore, the number of cycles can usually be 700 times and 80% of the battery is left. This has been 2 years, and the mobile phone is almost outdated. Apple is very smart in its design in this regard. It deliberately does not use high-power batteries. Not only can it gain the advantage of being thin and light, but also because you will inevitably upgrade to the next generation of iPhone in two years, so why spend more on batteries? ? As for some people who obviously feel that the battery life is reduced after one year, there is indeed a reason, which will be analyzed later.

　　Full technical analysis: Why lithium battery explosion news continues

　　It’s not good to have 100% battery at all times

　　Other digital devices, such as tablets, have longer battery charging and discharging cycles, but sometimes it is found that the battery life drops significantly in less than 2 years. There is an explanation for this reason, and you will know after reading the entire article.

　　3. Battery internal resistance

　　No special explanation.

　　4. Battery consistency

　　No special explanation.

　　II. Lithium batteries in laptops and power banks

　　Common laptop battery specifications are 4-cell, 6-cell, and what does 8-cell mean? This refers to the number of 18650 battery cells. Taking 8 cores as an example, there are two combinations, one is 2 series and 4 parallel, and the other is 4 series and 2 parallel. The specific one to use depends on the voltage conversion circuit of the notebook manufacturer. Design, and battery compartment shape.

　　Full technical analysis: Why lithium battery explosion news continues

　　4-cell laptop battery

　　This statement can also be used to describe mobile power supplies. The 10400mAh capacity product of a serious brand of mobile power supply on the market is composed of four 2600mAh 18650 batteries connected in parallel. In order to seize the Chinese market, Samsung SDK adopts low prices for large orders, and even sells them at a loss of US$1 per unit. Therefore, many big-name mobile power supply manufacturers like to use this kind of cheap and high-quality batteries, and because the capacity of a single unit is 2600, so the capacity of mobile power supplies is often multiples of 2600mAh: 5200mAh, 7800mAh, 10400mAh.

　　1. Charge and discharge rate

　　New laptop batteries rarely discharge within an hour, so a discharge rate of 1C is enough for them, and the charging requirements are not high. For example, everyone can accept a laptop battery being fully charged within 3-4 hours. For mobile power supplies, the requirements for battery charge and discharge rates are even lower, which is almost the lowest among all digital devices.

　　Full technical analysis: Why lithium battery explosion news continues

　　Internal structure of Xiaomi 10400 mobile power supply

　　Taking the current 10400mAh (3.7V) mobile power supply on the market, the maximum output current is 2A (5V), which means that it takes 3.7 hours to discharge photoelectricity with the maximum current. As long as the discharge rate meets 0.3C, it is enough. In terms of charging, the usual specification is 10400mAh with a 1.0A input port. The charging rate is only 0.13C. This is why there are so many inferior products in mobile power supplies: even regular manufacturers can purchase products with the lowest performance in the battery market (note that low performance and poor quality are not exactly the same, but they are still highly related) for use in mobile power supplies. , and can also meet the needs of usage specifications.

　　2. Number of charge and discharge cycles

　　The frequency of use of laptops and mobile power supplies is significantly lower than that of mobile phones. Mobile phones can be charged once every three days at most. However, mobile power supplies and laptops may not be able to complete one charge and discharge in a week on average. Calculated in this way, as long as the life span of 50 charges and discharges is guaranteed, Can last a year. For mature lithium batteries, the worst-case cycle life can be 300 times, which is enough for people who use laptops and mobile power supplies to last 6 years of normal use.

　　And we often update it in less than such a long time, so especially in the mobile power industry, the light load and low frequency of charge and discharge times in actual use make it difficult for us to detect the quality of the product. Some unscrupulous manufacturers will use the worst batteries, and even use disassembled batteries in mobile power supplies. The disassembled battery cells may have been used in other devices in the past and have been cycled 400 times. According to the lifespan, they will be eliminated after 100 cycles. And 100 cycles is enough for the mobile power supply to last for two years, so the waste is purchased and installed. No one will notice it inside the mobile power supply.

　　Compared with mobile power supplies, the original battery cells in notebooks come from major international manufacturers. But we often find that the battery life of someone’s notebook has dropped significantly within 2 years after buying it. What is the reason for this?

　　Still remember the previous page mentioned that the use cycle of tablet computers is infrequent, but some also have 1-2 years, and the battery life drops significantly, right? In fact, they are all caused by the same reason: improper battery storage.

　　If you often charge the battery fully when there is nothing to do, or if the charging cable is always plugged into the device, then the battery capacity will be reduced quickly if you maintain the maximum power at all times. This reason involves the structure of lithium batteries.

　　Full technical analysis: Why lithium battery explosion news continues

　　Internal diagram of lithium battery

　　The positive electrode of a lithium battery is composed of a metal oxide containing lithium ions. The negative electrode is generally a crystal lattice composed of graphite. When charging, the lithium ions move to one end of the graphite and eventually penetrate into the stable crystal lattice composed of graphite, ready to go. . The more lattice that can hold lithium ions, the more lithium ions can be moved, and the greater the battery capacity. Long-term fully charged storage mainly affects the number of movable lithium ions, because the battery reaches a voltage of 4.2V after being fully charged, and the maintained high voltage causes some reactions to occur in the electrolyte and the positive and negative electrodes of the battery, and these reactions occur at 3.0V- Although it also occurs in the 3.7V state, it is very weak. This reaction generates a passivation film on the electrode. The higher the voltage, the thicker the film. The thicker the film, the fewer lithium ions that can move to the negative electrode and drill into the graphite lattice. So the macroscopic manifestation is battery capacity fading.

　　Full technical analysis: Why lithium battery explosion news continues

　　A "little house" composed of negative electrode graphite for lithium ions to drill in and out

　　What counts as “long term”? In my opinion, 7 days is enough to cause bad effects. How bad? Here is an example. A person who plays model airplanes has two brand-new ternary material lithium batteries A and B with the same specifications. They bought them in June and stored them at the same time. A has 30% remaining power and B is 100% full. After 3 months, Test the battery capacity. A capacity is 98% of the original capacity and B capacity is 60% of the original capacity. Yes, the impact is so bad, so it’s not surprising that your laptop battery often maintains high voltage for a long time, and the battery life drops significantly after a year. Mobile phones are the least likely devices to be stored fully charged for long periods of time, because even if they are fully charged, they are constantly being used and the voltage will drop in less than a few hours. But mobile phones occasionally encounter this problem. For example, when the phone is turned off after being fully charged, there is no power consumption at this time, and the battery maintains high voltage. Maybe a week later when you open the box, you will find that the battery has bulged.

　　The correct approach is to charge 30%-40% of the battery and then store it for a long time.

　　3. Battery internal resistance

　　No special explanation.

　　4. Battery consistency

　　The barrel effect says that how much water a barrel can hold depends on the height of the shortest piece of wood in the siege wood. There is no better way to describe the consistency than putting it on a lithium battery pack. Poor battery consistency performance has a greater impact on batteries with a large number of parallel groups.

　　Full technical analysis: Why lithium battery explosion news continues

　　Notebook 6-cell battery diagram

　　Assume that the capacity of the third 18650 in the 6-cell battery in the picture decreases quickly for some reason, and soon the capacity is only 75%. The 6 of them are connected in parallel, then the third battery will The discharge end voltage is reached at the earliest, so regardless of whether the other five batteries have power, the battery pack will stop discharging. The same goes for charging. As a result, there was no problem with the other five batteries, and they were all charged and discharged simultaneously with the problematic 18650. From the outside, this set of batteries showed severe capacity attenuation. In fact, only one of them is defective. This is where the importance of battery consistency comes into play.

　　III. Lithium batteries in electric bicycles

　　Brands such as Luyuan and Xinri began to enter every household in 2010, but at that time most electric vehicles used lead-acid batteries. Since 2012, electric bicycles using lithium batteries have appeared. Common specifications are voltage 36V-48V and capacity 8Ah-12Ah. If an electric bicycle with pedals uses a 48V12Ah lithium battery, its pure electric range can reach 50 kilometers.

　　1. Charge and discharge rate

　　There are no special requirements for charging when driving a bicycle. The common charging rate is between 0.1C and 0.3C. The battery factory easily meets the standard, but the discharge rate is slightly higher, at least 1C., which puts little pressure on battery factories. But there is currently an electric motorcycle that adopts the shape of a scooter and can even exceed 100km/h. The battery requirements for this kind of motorcycle have been increased to 2C-3C, so if you assemble this high-performance electric motorcycle yourself, Be careful when purchasing batteries.

　　Full technical analysis: Why lithium battery explosion news continues

　　Lithium battery is at the arrow

　　There is usually no need to worry about the batteries supplied by the manufacturer. There are generally three types of batteries supplied by yourself: lithium iron phosphate batteries, ternary lithium batteries, and power ternary batteries. Their highest discharge rates are: 2C, 1.5C, and 10C. You can also increase the current corresponding to 1C discharge by increasing the capacity of the battery pack.

　　2. Number of charge and discharge cycles

　　The service life of lithium iron phosphate battery is about 2000 times, that of ternary lithium battery is about 800 times, and that of power ternary lithium battery is about 800 times. However, since there are often no single batteries in electric bicycles, the barrel effect causes the first capacity failure after the group is assembled, which will occur earlier, about 1/4 to 1/2 of the life cycle of the single battery.

　　Of course, this is also related to usage habits. All the rules just mentioned apply here, because we involve another cathode material: lithium iron phosphate, so we have to talk about it in isolation.

　　Full technical analysis: Why lithium battery explosion news continues

　　Modified lithium iron phosphate battery pack used in electric motorcycles

　　The energy density of lithium iron phosphate is 40% lower than that of ternary lithium batteries. Lithium iron phosphate batteries with the same energy are large in size and heavy in weight. The advantage is that they have many cycles and are easy to maintain. For example, under the conditions just mentioned, if a ternary lithium battery is stored at full capacity for 3 months, the battery capacity will decline to 60% of its original value. However, lithium iron phosphate can maintain 90% of its capacity in the face of such harsh storage conditions. Although it is also lost, Far less serious than ternary lithium batteries.

　　This reason is related to the fact that most of the energy (more than 85%) of lithium iron phosphate is concentrated on the 3.2V voltage. Although the cut-off voltage of this kind of battery charging is 3.6V, the energy stored in the range from 3.6V to 3.2V is not yet To 1% of the total energy, even after it is fully charged, the voltage will fall back to 3.2V if left for a few minutes. Therefore, lithium iron phosphate automatically maintains low voltage stress. It is not easy to form a passivation film.

　　3. Battery internal resistance

　　No special explanation.

　　4. Battery consistency

　　The issue of consistency is more important. Currently, there are roughly two types of single batteries used in electric bicycles:

　　1. Small single battery, that is, 18650 battery, with a capacity of 2.2Ah-2.6Ah, and the number of battery cells in each group is 200-500.

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　　Use small cells to form a battery pack, a 120V100Ah battery pack is required

　　2. Large single battery, the capacity is generally between 20Ah-40Ah, and the number of battery cells in each group is 15-30.

　　Let's take the 72V40Ah battery pack as an example. If a small single battery is used, it will require 20 series and 19 parallel, a total of 380 18650 batteries. Every 20 batteries are connected end to end, and the voltage difference of the 19 battery packs is required to be within 0.02V at any time when charging and discharging. It sounds very demanding, but it is not as difficult as imagined because the process of 18650 batteries has been Very mature and very consistent with batteries from the same batch.

　　Full technical analysis: Why lithium battery explosion news continues

　　Large single lithium iron phosphate battery, various specifications

　　If you use 20Ah large single cells, you need 20 in series and 2 in parallel, for a total of 40 large single cells. As long as these 40 work in sync, there will be no problem. Generally speaking, the consistency of the large single cells will improve after they are grouped. There is less chance of problems occurring. There are also disadvantages. If it is a large single battery of lithium iron phosphate, the consistency will be much worse than that of ternary lithium battery. Therefore, the battery pack needs to add an automatic balancing circuit to determine which battery needs to be charged at the end of each charge. Charge it alone for a while to solve the weakness of poor consistency of lithium iron phosphate.

　　The issue of consistency has risen to a very important position here, but not to the extreme.

　　IV. Lithium batteries in electric vehicles

　　Lithium batteries in electric vehicles are the most complex applications, because batteries face demands such as high-power charging and discharging, large-capacity storage, high-frequency use, and long-term driving. The batteries used are high-standard products from regular manufacturers and are eliminated in electric vehicles. The used batteries that come out are often installed on electric bicycles, and they still have a use (of course, this is a profiteer behavior). The usual approach is to eliminate them and use them in energy storage power stations to store wind and solar energy.

　　1. Charge and discharge rate

　　There are big differences depending on the car. The well-known BYD Qin uses 152 lithium iron phosphate batteries with a single capacity of 26Ah. The discharge voltage curve is integrated to get a total energy of 10.5KWh. The maximum power of Qin’s motor is 110KW. Qin’s battery is A discharge rate of 10C is required. The battery of BYD's e6, another pure electric vehicle, has the largest battery capacity for passenger cars at present: 63KWh, so to achieve greater power does not require a high battery discharge rate. The discharge rate of the e6 battery is 1.5C. The buses with the largest battery capacity of pure electric vehicles, such as BYD's 13.8-meter-long electric bus K9, have a battery capacity of 324KWh. Due to the huge battery capacity, the battery discharge rate only needs 0.5C for a power of 180KW.

　　Full technical analysis: Why lithium battery explosion news continues

　　BYD K9 electric bus sold to Israel

　　There are also some non-plug-in hybrid cars with a battery capacity of only 1.5KWh, but the maximum power of the motor reaches 30KW, so the battery discharge rate used in these cars may be 20C, which is compared to the application scenario of mobile power supply of 0.2C It's more than 100 times bigger.

　　Full technical analysis: Why lithium battery explosion news continues

　　The Camry hybrid version requires a power battery, which has a higher discharge rate.

　　Most pure electric vehicles are equipped with two charging methods: fast charging and slow charging. The fastest charging of fast charging is 2 hours, so the charging rate is 0.5C. There are even more exaggerations. For example, Tesla’s boss Musk hopes that the charging time of the next generation Tesla can be reduced to 10 minutes. In this case, the charging rate will be at least 6C.

　　2. Number of charge and discharge cycles

　　When designing new energy vehicles, the battery life should be longer than the vehicle life. Therefore, the battery life ensures that the vehicle can travel normally for at least 300,000 kilometers. This is no problem for most pure electric and hybrid vehicles that have been released. Take the BYD e6 as an example. It has a pure electric range of 300 kilometers and a battery capacity of 63KWh. Based on a cycle life of 2,000 times, it is 600,000 kilometers (actually slightly less than 600,000 kilometers. Think about it?). This is far more than the average car owner expects. Total mileage required.

　　Full technical analysis: Why lithium battery explosion news continues

　　BYD e6 taxi driver

　　Full technical analysis: Why lithium battery explosion news continues

　　e6 with a total mileage of more than 500,000 kilometers

　　In actual tests, BYD has operated 850 e6 taxis in Shenzhen since 2010. So far, dozens of vehicles have a total mileage of more than 500,000 kilometers. I went to Shenzhen in July to investigate the situation of e6 pure electric taxis. See Are they a set of batteries that have been used since leaving the factory? Fortunately, I met a driver with a total mileage of 500,000 kilometers. He guaranteed by his reputation that not only has the battery not been replaced, but the battery life has not significantly declined. Coincidentally, Autohome I once tested the battery life of this driver's car. In urban + highway + full air conditioning conditions, it ran out of power for a total of 260 kilometers.

　　Full technical analysis: Why lithium battery explosion news continues

　　BYD pure electric car e6

　　Among the new energy vehicles that are currently emerging, especially pure electric vehicles, except for BYD which is 5-7 years ahead of the entire industry, the products released by other self-branded car companies were all developed after receiving the notice from the central government to develop new energy 4 years ago. , they are all modified from the fuel vehicle frame by throwing away the engine and fuel tank and inserting the electric motor and battery. The characteristic is that the driving range under working conditions is 150-180 kilometers (it will definitely reach 150 kilometers, because the national and local subsidies will reach a higher level when reaching 150 kilometers!), and the actual driving range of these cars is often 120-150 kilometers in summer and 100 kilometers in winter. -120 kilometers. Are the number of battery cycles sufficient for these impure pure electric vehicles?

　　Full technical analysis: Why lithium battery explosion news continues

　　Lithium iron phosphate battery

　　This depends on the specific situation. If a lithium iron phosphate battery is used, then 120 kilometers multiplied by 2,000 times, it will also have a battery life of 240,000 kilometers. In addition, because you do not always use the accelerator to the end, and every time it is discharged from 100% Recharge to 0%, so it is normal for the number of cycles to exceed 2,000. This also ensures that the batteries of these vehicles can at least last longer than the entire vehicle.

　　But the other type of pure electric vehicles that use ternary lithium batteries is a bit uncertain. Although China's current new energy vehicles use ternary lithium batteries as batteries, no one buys them, and statistics after long-term use are impossible to talk about. . But there is precedent abroad, and it is a very famous example. The Nissan LEAF is the world's largest-selling pure electric vehicle. It has sold 130,000 units since its launch in 2011. Large sales does not mean quality. Well, mainly because it is cheap. It has been the lowest price among pure electric vehicles since its launch in Europe and the United States, with a US dollar price of RMB 180,000.

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　　NEC modified lithium manganate ternary material 35Ah battery, for Chenfeng Automobile

　　After 3 years of use, the problem of significant decline in battery mileage of this car broke out. This car uses NEC's modified lithium manganese oxide battery (which also belongs to the category of ternary lithium battery, but the positive electrode material ratio and doping are slightly adjusted), with a soft pack of 35Ah. Although Nissan does not necessarily admit this problem, the disassembled lithium batteries sold in Taobao stores can illustrate the problem. In the list of battery merchants of the Electric Vehicle Alliance, regardless of whether the merchants are in Shenzhen, Guangzhou, Wuhan, Changsha, Beijing, or Dalian, they all have the same The kind of disassembled battery cell: NEC modified lithium manganate 35Ah soft pack large three yuan. Some of the monomers are in very new condition, and there are no traces of shearing or welding on the tabs. They are actually from Nissan Leaf electric cars. There are a lot of problems with the cars themselves, and the factory's after-sales control is not strict, so the dismantled batteries are flowing out in large quantities, and they are harvested by merchants and sold to people who make DIY electric motorcycles.

　　The root cause of such problems in the Leaf is that the number of battery cycles it uses is much less than that of lithium iron phosphate, which should be about 800 times. If you are interested, you can search "Leaf battery ages too fast" on Baidu, or search on Google Search for "nissanleafbatterycapacityloss" and you will see this problem. Nissan also established an investigation team for this purpose and subsequently provided battery replacement services. The price of replacing the entire battery pack is US$5,500.

　　Unfortunately, this car only changed its name when it came to China and became Dongfeng's pure electric car "Vucina-Morning Wind". With the new name, each car received a national and local subsidy of 95,000 yuan. The current domestic price of this car is 267,800-281,800.

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　　Nissan LEAF or Dongfeng Morrowind

　　Including Morrowind, all pure electric vehicles with a test range of less than 200 kilometers under working conditions are not worth buying. Their mission at the beginning of their birth was to communicate with their superiors to obtain the qualifications for subsequent expansion of fuel vehicle production. In 2010, car companies such as FAW and BAIC did not expect that new energy would become an irreversible strategic development direction of automobiles in 2014. It would not be possible to submit pure electric vehicles that had undergone several modifications to apply for the national catalog of new energy vehicles and everything would be fine. Even if they have plans to re-develop a model specifically designed for electric vehicles, it will only be launched four years later. Do they really have the research and development capabilities? I personally prefer that in a few years these car companies will still rely on the introduction of joint venture brand models to sell pure electric vehicles developed by others. The operation method can be referred to the Leaf to Morrowind.

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　　BYD Qin

　　Among all car companies, BYD is the only exception. BYD has been working hard to build electric vehicles since before it acquired Qinchuan Automobile in 2003. Not only does it conduct the research and development and production of motors, electronic controls, and batteries by itself, but it also connects the source of positive and negative electrode materials: mines. , are all fighting for control. The 49% shareholder of the world's second largest spodumene mine is the cousin of BYD Chairman Wang Chuanfu. In addition, there are shares in many mines. It is no exaggeration to say that whether it is a pure electric vehicle or a plug-in hybrid vehicle, the level of BYD's products is at least 5-7 years ahead of other domestic car companies. Even if it is taken worldwide, if we only discuss new energy vehicles, then the motors and electronic controls BYD's three most important components, battery and battery, are also among the best.

　　For BYD, new energy vehicles are products related to the survival of the company. For other car companies, new energy vehicles are related to the amount of subsidies. .

　　3. Battery internal resistance

　　No special explanation.

　　4. Battery consistency

　　Since many car companies use lithium iron phosphate batteries, the single capacity of this battery can reach 200Ah, so despite the huge capacity of car batteries, the number of cells is not too many. But there are exceptions, and that is Tesla. The Model S took a different approach and used Panasonic 18650NCA batteries, with a total of 7623 cells and a single cell capacity of 3.1Ah. In 2011, Tesla bought a total of 200 million of these batteries, which is only enough to produce high-end Model S. Edition of 26,000 vehicles.

　　Full technical analysis: Why lithium battery explosion news continues

　　If the remaining 7622 normal batteries cannot use the full 3.1Ah capacity due to a certain battery, then the power system of this car has failed. Here we see the most extreme application of lithium batteries in terms of consistency. . In order to ensure consistency, in addition to imposing very strict requirements on Panasonic, Tesla also built an expensive battery management system for the battery. This system can not only monitor the voltage and current of each battery, but also achieve reasonable Charging and discharging can also ensure that their temperatures are consistent. Whether the battery charge and discharge curves are consistent has a lot to do with the battery temperature. The Tesla chassis is covered with batteries, and the battery temperatures at different locations vary greatly. Therefore, in order to ensure that the charge and discharge curves are consistent, we must first ensure that the temperatures of the 7623 batteries are consistent. This is achieved by a two-way flow liquid cooling system, the liquid is 50% water and 50% ethylene glycol.

　　According to analysis by industry professionals, even if Tesla's battery management system is entirely produced in China, the cost is still 6,000-8,000 US dollars, which is compared to the 100-200 yuan for a charge and discharge protection board with automatic balancing function on an electric bicycle battery pack. It's much more complicated, after all, the number of cells to be controlled has increased from four hundred to seven thousand.

　　We spend so much ink on lithium batteries in electric vehicles because it’s where the situation is most complicated. Let’s talk about common sayings about lithium batteries.

　　VI. Various rumors about lithium batteries (1)

　　Rumor 1: A new battery needs to be charged and discharged three times before it activates?

　　This is a statement copied from nickel-cadmium batteries many years ago. The activation of lithium batteries is carried out in the factory. The raw materials are mixed, coated, sliced, rolled, and laminated., assemble the battery cells, bake, inject electrolyte, form, high current test, and then the activation is completed. Ordinary people don’t have this kind of equipment at home. This kind of advice even appeared in the manual of a certain brand of mobile phone. Maybe the person who wrote the manual copied the previous version and found no errors.

　　When you buy lithium batteries, you will find that most of them still have about 30% of the remaining power, which is the ideal remaining power when in factory inventory.

　　Full technical analysis: Why lithium battery explosion news continues

　　Lithium battery activation process, do you have such equipment?

　　Rumor 2: Lithium batteries need to be recharged with light?

　　This is also a statement from nickel-cadmium 10 years ago. That kind of battery has a memory effect, so it must be discharged before charging. Previously we had an example of the impact of charging and discharging percentage on lifespan:

　　Factory label: Charge from 0% to 100% each time, discharge at 1C, charge at 0.3C, the capacity will decay to 80% after 500 times

　　Each power cycle is between 25% and 75%. Discharge at 1C and charge at 0.3C. The capacity will decay to 80% after 2000 times.

　　Each power cycle is between 50% and 100%. Discharge at 1C and charge at 0.3C. The capacity will decay to 80% after 1800 times.

　　Although the latter two methods only use half of the power each time, the number of cycles is 4 times and 3.6 times that of the previous one, so the discharge capacity during the entire life is still 1 times more than the deep charge and deep discharge method. This is the so-called "shallow charging and shallow discharge are beneficial to extending life." 0%-100% is deep charge and deep discharge, which is the most life-consuming usage. Therefore, the mobile phone battery can be charged as you use it. Don’t pursue full charge all the time, and don’t use it to automatically shut down frequently. This is the most beneficial to battery life.

　　Full technical analysis: Why lithium battery explosion news continues

　　This does not mean that there is no electricity left in the battery.

　　But it should also be noted that when we use mobile phones and tablets to automatically shut down, it is not when the battery power is 0%. Generally speaking, mobile phone manufacturers design to automatically shut down when the battery voltage is lower than 3.0V. The actual battery is discharged to 2.7V. The battery will be damaged only when the voltage is below 3.0V. There is still at least 5% remaining power in the battery at 3.0V, so even if you use automatic shutdown, don’t worry too much about damaging the battery. However, if you use this light every time, it is considered a deep charge and a deep discharge. .

　　Rumor 3: If you charge while making a call, your phone will explode?

　　The basis for this rumor may be that doing so will cause the battery to be hotter than when it's just charging. It makes sense to say that the temperature will be higher, because the energy conversion efficiency during charging is usually only 85%, and the remaining 15% of the energy is waste heat; the baseband chip is also in working condition and generating heat when making phone calls, especially when making phone calls. At that time, one side of the phone was still attached to the face, which has poor heat dissipation. These three factors increased the temperature of the battery.

　　But if it’s true as the rumors say, you should hear about a certain colleague being bombed in the face every week, right? So the truth is that it will only cause the internal temperature of the mobile phone battery to rise, and there will be no serious consequences.

　　Full technical analysis: Why lithium battery explosion news continues

　　Rumor 4: The charging time should not exceed 12 hours. After the light turns green, charging for an extra hour can charge 10% more power?

　　In order to prevent battery explosions, the industry is designed based on a one-billion-level failure rate. However, no matter how the process and process are controlled, a single step cannot achieve such a high level of safety, so two levels are used. There is a first level on the charger and a first level on the battery protection board. It is achievable to ensure a failure rate of one in 100,000 for each level, so if two problems occur at the same time and cause the battery to explode, it will be one in 10 billion.

　　The charger side is designed in this way. It is first charged in a constant current mode, and then charged in a constant voltage mode of 4.2V when the battery is about 75% charged. At this time, the current will slowly decrease due to the increase in battery voltage. The charger will Continue to judge the current size until the current is less than a certain value, such as 20mA, and then the green light turns on.

　　What does the charger do next? It depends on what control chip you use to make the charger. The control method of the chip determines what the charger will do afterwards.

　　Full technical analysis: Why lithium battery explosion news continues

　　Charge and discharge management chip in charger

　　1. TI's BQ2057 series charging chips and Linear's LT1800 series allow the cradle to completely shut off the charging circuit, so leaving it on the cradle for another 10 hours will have no effect.

　　2. The cradle charger continues to perform constant voltage charging and strictly controls the voltage not to exceed 4.2V. There is no doubt that charging for a few more hours can indeed increase the power. However, the power increase from the green light to 4.2V is less than 0.1%, which is of no use.

　　3. Maxim's 1679 chip can also be seen inside many mobile phones. It uses pulse charging. When it displays a green light, it means that the battery is 100% full. Of course, it will not be overcharged if left for another hour. . In other words, not only do many chargers stop charging when the green light comes on, but the mobile phone itself refuses to receive any more current.

　　4. The cradle charger continued to charge, but its current control was very poor. It accidentally caused the battery to exceed 4.2V and continued to increase. Then overcharging occurs. When the voltage reaches 5V, it is not far from explosion. However, in addition to the charger, there is also protection on one side of the battery, so there are many junk chargers sold, but battery explosions are not common.

　　Full technical analysis: Why lithium battery explosion news continues

　　Typical junk charger, don’t buy it

　　The last one is a typical junk charger. If you plug in a mobile phone to charge with this kind of thing, the charging management chip inside the mobile phone can also cut off the current when it is fully charged. But if you take out the battery and connect it to this type of charger, it may bulge or even explode.

　　Rumor 5: The power grid voltage fluctuates greatly at night, and lithium batteries are more delicate, so you shouldn’t charge them at night?

　　If this is the case, millions of batteries are broken every night around the world. When you wake up early, you will see that the battery is dead? The actual situation is that the voltage is changed from 220V mains power to 5V, and then from 5V voltage to the voltage for charging the battery. After several steps of voltage transformation, the output voltage of each step is qualified. Even if the 220V terminal fluctuates, it will not affect the battery charging voltage. Another scenario for this rumor is: Typhoon Catfish will land in Fujian, so Beijing residents are asked to stay away from parks, moats and other water bodies to avoid endangering their lives. It seems to be quite considerate, but in fact no matter how strong the typhoon is over there, the moat will not become violent due to it.

　　Rumor: Lithium polymer batteries are safer than 18650 batteries?

　　There is some truth to this statement. There are two types of explosions: one is when the battery internal short-circuit causes the temperature to increase sharply and burn, resulting in an explosion; the second is when gas is generated inside the battery but does not burn, and the gas bursts the outer shell. Polymer batteries largely avoid the second type of explosion. The polymer batteries are not enclosed in a fixed volume. Once used improperly, the electrolyte produces gas and the battery core bulges, sometimes even bursting the outer skin. , the gas leaked out, and although the faulty battery would also fail, an explosion was avoided.

　　Full technical analysis: Why lithium battery explosion news continues

　　The same type of polymer battery, the left side is normal and the right side is bulging, but it did not explode because of this.

　　Full technical analysis: Why lithium battery explosion news continues

　　Polymer batteries have room for expansion

　　The 18650 battery core also has a safety valve. Once the gas pressure increases to a certain level, the valve will cut off the circuit, so the battery will have no voltage and cannot be used anymore. However, many domestic 18650 valves are not reliable, so compared to the characteristics of polymers that can bulge Generally speaking, 18650 batteries are indeed more likely to explode.

　　Full technical analysis: Why lithium battery explosion news continues

　　The steel shell has no space to release gas and explode

　　Moreover, lithium batteries using ternary materials will easily burn as long as the positive electrode material is exposed to the air and comes into contact with oxygen and water. I used to think that brands such as Panasonic and Sanyo would not, and only domestic junk batteries would. It will burn and explode. Later, I watched a popular science program about lithium battery explosions and found that they would all burn as long as the 18650 battery core was pierced. The only difference between imported high-end products and domestic ones is the safety valves, which makes the imported products safer.

　　Rumor: Why is the battery indicator often inaccurate?

　　There are currently two ways to display the battery power of mobile devices. One is the Coulomb meter that Apple has used since the iPhone 3GS to measure the amount of charge flowing through it, so it is very accurate. The other is the voltage fitting method to determine the electric charge, which has a large error.

　　Full technical analysis: Why lithium battery explosion news continues

　　Discharge curve of ternary material lithium battery

　　This is the discharge curve of a ternary material lithium battery with a capacity of 2600mAh, 1A discharge, and 25°C. The vertical axis is voltage and the horizontal axis is time. When purchasing the same type of battery in mobile phone production, the discharge curves of batteries with good consistency are almost the same. Recording the voltage points corresponding to the percentage of power can form a [voltage-power] comparison table. As long as the voltage is measured, the corresponding information can be known remaining battery. This is the principle of voltage fitting. But we will find that the voltage of section A changes significantly with the amount of electricity. The electricity of section B changes a lot, but the voltage does not change much. Therefore, the error in the B-segment power display is more likely to be large. This is one of the reasons for the error.

　　—————————————————-

　　Another reason has a greater impact on error: temperature.

　　Full technical analysis: Why lithium battery explosion news continues

　　Effect of Temperature Changes on Electricity Measurement Error

　　The same battery has different discharge curves at different temperatures. The picture above is the discharge curve of the battery at 15℃ and 25℃. One temperature corresponds to a line, and the two green dots are points with the same voltage. Press [Voltage-Electricity] 】The comparison table to display the power will cause an error of 10%. So you say it doesn’t matter. You can make a few more corresponding tables after testing at 5°C, 15°C, 25°C, 35°C... at different temperatures to display the power. If you measure the temperature first and then decide which [voltage-electricity] comparison table to use, wouldn’t the problem be solved? Yes, this can indeed further reduce the error, but I have to say that the change in the discharge curve also changes considerably with the load, and also changes slightly with the position of the battery in the life cycle. In order to display the power, do you need to make a comparison table of [n types of temperatures × m types of loads × q types of life points - power levels]? That's pretty much it. It's not an experiment. Therefore, low-cost mobile devices are unwilling to use coulomb counters, so they simply do not use digital displays. The cost is high and inaccurate. They simply install four LED lights, each of which represents 25% of the power. No matter whether it is accurate or not.

　　Summary of the full text:

　　This article does not explain everything. For example, the capacity of lithium iron phosphate batteries is halved at low temperatures (-10°C) and cannot even work at -30°C; why lithium batteries cannot be charged below 0°C; overcharging and overcharging How did the battery get damaged after being discharged? Is the bulging cell phone battery a problem with the battery quality, etc. But this article is already very long, I will write it again when I have time...

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