Deadly electric bike
Deadly electric bike
China News Weekly reporter/Du Weiwen/Tian Ran
Published in the 1016th issue of China News Weekly on October 18, 2021
In the early morning of September 20, a fire broke out in a unit in Xingfu Yiju Community, Tongzhou District, Beijing, which eventually killed a family of five on the fifth floor.
The culprit of the accident was the electric bicycle battery of a tenant on the third floor.
On the night of the incident, the husband of the renting family brought the lithium battery of the electric car home to recharge, which caused the fatal fire.
From January to July this year, there were 6,462 electric bicycle fire accidents nationwide, with an average of more than 900 accidents per month.
80% of electric bicycle fires occur when charging, and most of the accidents are caused by the explosion of lithium batteries.
At present, there are 320 million electric bicycles in the country, which is equivalent to one for every 4.4 people.
Ma Guilong, a retired teacher at Tsinghua University and known as the "first person in electric bicycles in China," told China News Weekly that compared to electric vehicles that also use lithium batteries, electric bicycles should not have problems with lithium battery safety at all.
"The lithium battery pack of an electric bicycle is composed of only a few dozen batteries in series and parallel, and there are thousands of batteries in an electric car, which is a hundred times different."
The fewer the number of batteries, the lower the capacity of the battery pack, the lower the voltage, and the easier it is to control battery safety.
But in reality, the number of explosion accidents of electric bicycles is higher than that of electric vehicles.
Behind the safety of lithium batteries for electric bicycles, what is exposed is the lack of mandatory standards for batteries and chargers, and the status quo that the industry is at a low level of development.
The safety of lithium batteries for electric vehicles is not a technical or scientific problem. It is essentially a management problem.
The tricky of the battery
The tricky of the battery
There are two types of batteries for electric bicycles-lead-acid batteries and lithium batteries.
Simply put, lithium batteries are lighter than lead-acid batteries, have higher specific energy (a measure of battery energy density), and have a longer lifespan.
Lithium batteries can be divided into ternary lithium batteries, lithium manganese oxide batteries, and lithium iron phosphate batteries according to different battery cathode materials.
The so-called superconducting graphene battery that is popular on the market now uses graphene as the negative electrode to improve the battery's electrical conductivity.
However, due to the poor performance and characteristics of the added graphene, and the content is too small, the battery performance cannot be fundamentally changed.
Compared with lead-acid batteries, the biggest advantage of lithium batteries is light weight.
At the same 48 volts and 12 ampere hours, a set of lead-acid batteries can weigh about 16 kilograms, while lithium batteries are about 4 kilograms.
But lithium batteries are more expensive.
A common 48-volt, 24 amp-hour lead-acid battery costs about 350 yuan, a ternary lithium battery is about 600 yuan, and a lithium iron phosphate battery costs about 900 yuan.
In China, the main manufacturers of lead-acid batteries for electric vehicles are Chaowei and Tianneng in Changxing County, Zhejiang. The two companies have a total market share of 70 to 80%. The leading companies in the lithium battery market are Suzhou Xingheng and Tianneng. can.
In 2018, a new national standard for electric bicycles was released, stipulating that the weight of the entire vehicle including the battery must not exceed 55 kg.
This regulation makes lithium batteries begin to seize the lead-acid battery market.
In recent years, 35% of the 30 million new electric vehicles produced in the country each year use lithium batteries.
The share of lithium battery electric vehicles has increased from 3% before the promulgation of the new national standard to about 15%.
At present, on the eve of the deadline for the transition period of over-standard vehicles in many parts of the country, the demand for trade-in or purchase of new vehicles is soaring.
But for a long time, the charging safety problem of lithium batteries for electric vehicles has not been solved.
In the impression of Lu Jinlong, vice chairman of the China Bicycle Association and honorary chairman of the Jiangsu Bicycle and Electric Bicycle Association, before the promulgation of the new national standard in 2018, the probability of electric vehicles exploding is much lower than now.
Xue Yu, director of the business management department of Wuxi Institute of Inspection, Testing and Certification/National Light Electric Vehicle and Battery Product Quality Supervision and Inspection Center, and researcher-level senior engineer, explained that lithium batteries are more prone to safety accidents than lead-acid batteries.
This is mainly determined by the material and structure of the battery.
Lithium batteries have higher energy density than lead-acid batteries, and the chemical properties of lithium are more active than lead. The electrolyte in lithium batteries is also more combustible than the dilute sulfuric acid electrolyte of lead-acid batteries.
In the event of an accident, lead-acid batteries will swell or crack at most and will not react more violently.
Lithium batteries will explode when thermal runaway occurs due to low internal resistance and thin separators.
In the same way, the energy density of a ternary lithium battery is higher than the other two, and the three elements that constitute the positive electrode of the battery, nickel, cobalt, and manganese, are also more active than iron, so the safety is not as good as that of a lithium iron phosphate battery.
However, due to the highest energy density, ternary lithium batteries have a market share of more than 50% in the lithium battery market for electric vehicles.
At the moment, some manufacturers are introducing sodium batteries. The chemical properties of sodium are not as active as lithium and are relatively safer, but the energy density is not as good as that of lithium batteries.
Ma Guilong said that the ultimate solution for lithium battery safety is to produce solid-state lithium batteries, that is, the electrolyte is solid.
"Current lithium batteries are prone to swell and explode because the electrolyte is liquid."
Solid-state lithium batteries are also a problem that the world is conquering. How to combine solid-state electrolytes with positive and negative electrodes is the current difficulty.
However, it will take time for this technical route to succeed and go to the market.
In the past two years, due to frequent accidents, electric bicycle manufacturers have begun to use lithium iron phosphate batteries with relatively better safety factors and higher prices.
But in the field of new energy electric vehicles, ternary lithium batteries are still the mainstream.
In the electric bicycle industry, most vehicle companies do not produce batteries themselves, but directly customize, purchase batteries or purchase individual batteries from battery manufacturers for assembly.
Head companies usually cooperate with Xingheng, Tianneng, Chaowei and other manufacturers to provide batteries.
Ma Guilong said that some large manufacturers of automobile lithium batteries pack and assemble batteries with the same voltage and the same internal resistance and sell them to automobile OEMs, but pack batteries with uneven internal resistance and voltage that are not suitable for the group. Sold to some small factories that do not have the qualification to produce lithium batteries.
These small factories directly connect the batteries in series and parallel, package them into battery packs, and then sell them to some small-brand electric vehicle manufacturers and secondary markets such as maintenance points and sales stores.
The difference between the batteries in such a battery pack is very large, which will lead to frequent accidents.
Ma Guilong said that the core of the battery explosion accident is overcharging.
The way to form a large-capacity battery is to connect more than ten individual batteries in series, and then connect these battery packs in parallel.
If a battery with poor consistency is configured, a certain battery will be fully charged quickly, and the other batteries will not be "full".
The charger does not "know" this and will continue to charge all these batteries.
As a result, a fully charged battery will generate heat due to overcharging, and the battery contains electrolyte, which vaporizes when heated, increases the pressure, and explodes.
Ye Zhentao is the technical leader of the National Center for Quality Supervision and Inspection of Light Electric Vehicles and Battery Products (CEVT) and one of the drafters of the new national standard for electric vehicles.
He pointed out that in the past two years, the first batch of new energy vehicle batteries put into use has entered the stage of large-scale replacement and decommissioning.
In 2018, retired lithium batteries for new energy vehicles can reach 1.1 billion watt-hours.
In this process, some batteries will be retired and eliminated, and they will flow into the field of electric bicycles through some small manufacturers.
"Some batteries that look good on the surface may have expired, and there are risky fault points inside. If they are randomly assembled without detection, they will be very dangerous."
In the sales model of electric bicycles, the whole vehicle and the battery can be sold separately.
When the electric bicycles are shipped from the factory, the major vehicle manufacturers will provide batteries. Some small companies only sell the racks, and the battery installation is carried out by the dealership.
This sales model will enable some low-quality batteries to be installed on electric bicycles after being sold online or offline.
Such a battery circulation chain is particularly evident in the car purchase experience of takeaways.
Liu Gang is a takeaway from Ele.me in Beijing. He joined the business in 2017. So far, he has replaced at least two electric vehicles and three electric vehicle batteries.
Liu Gang said that when the delivery staff buy an electric bicycle, they often have already bought the battery.
He used to ride a second-hand Yadi electric car that was bought for 500 yuan, using a 72V, 65AH Hengwei brand high-capacity battery, which was purchased from the Internet for nearly 3,000 yuan, with a cruising range of more than 100 kilometers.
The pursuit of high speed and long mileage is the work requirement of delivery staff. 72V has exceeded the 48V rated voltage of the battery specified by the new national standard.
Around 2019, he bought two sets of 72V, 20AH Chaowei lithium batteries near the community at a cost of more than 3,000 yuan.
One of the sections is still in normal use, and the other section can’t be used after nearly a year.
There are more than ten electric bicycle shops on both sides of Zhao Dengyu Road in Xicheng District, Beijing.
In front of some of these stores, there are unbranded or over-standard electric vehicles that do not contain batteries.
The main buyer of this type of electric car is the takeaway.
Ye Zhentao said that China has not established a special recycling system for waste lithium batteries, and has set up special recycling outlets.
In the time period for the formulation and promulgation of the new national standard, it was found that electric vehicles used obsolete batteries less. Therefore, the new national standard did not stipulate this. On the contrary, this also shows that these batteries have not been tested and flowed directly. market.
Lu Jinlong said that there are dozens of companies in Dongguan, Guangdong that produce unnamed lithium batteries. The batteries produced by them do not have safety guarantees. "Something will definitely happen." The price is 20% to 30% cheaper than that of big-brand batteries.
He suggested that from a national level, manufacturers of lithium batteries for electric vehicles should be subject to mandatory certification.
Ma Guilong said that in order to solve the safety problem of electric bicycles, rules must be established. Lithium batteries entering the field of electric bicycles must have an entry barrier.
The remaining batteries after screening by electric vehicle manufacturers shall not be allowed to flow into the manufacturers of electric bicycle batteries at will.
Manufacturers engaged in electric bicycle battery must have the ability and qualifications to detect and rationally assemble batteries to ensure the consistency of battery assembly.
Violators shall bear severe legal responsibilities.
He also said that now some well-known electric bicycle companies have put out the slogan of "car-grade lithium battery". Its true meaning is to achieve the same battery configuration as electric cars. This should be a basic requirement for electric bicycle batteries. .
Behind the frequent occurrence of electric bicycle battery safety accidents, it reflects the lack of related management.
There are strict mandatory access standards for automotive lithium batteries.
In contrast, Ye Zhentao said that "Lithium-ion Battery for Electric Bicycles" GB/T 36972-2018 is only a recommended standard.
"Some companies selectively implement some of the standard items in the standards, and only specify items that can pass the test. Or formulate corporate standards to avoid the stricter items in the implementation of the standards." There is a "overcharge" requirement in the standard, that is, to remove the battery After protecting the device, overcharge the fully charged battery for 90 minutes.
According to the standard, the battery cannot explode or catch fire after being overcharged.
After charging, install the protective device and the battery can still be used normally.
"But as far as I know, after testing by an organization, some products directly caught fire or exploded during testing."
Battery management system and charger
Battery management system and charger
The battery management system (BMS) is an important line of defense to protect battery safety.
This is an electronic circuit containing a chip, also known as a battery protection circuit or protection board.
The lead wire of the BMS is connected to the lead wire of the battery, which plays a role in battery management.
The electric vehicle BMS is usually installed at the end of a set of batteries, and then the outer shell is sealed.
Ma Guilong explained that BMS has three functions: First, it monitors the voltage, current and battery pack temperature when each battery is charged and discharged.
When a battery is fully charged, the BMS will adjust the current to pass through the bypass. When discharging, if the voltage of a battery cell is too low, it will adjust the discharge voltage of each battery cell to equalize.
Furthermore, when a problem with the battery pack is detected, the charging current will be cut off.
BMS is only found in lithium batteries. Lead-acid batteries are not equipped with BMS because they are not easy to explode.
Xue Yu said that although there are no mandatory regulations, for safety reasons, lithium batteries must be equipped with BMS.
In reality, the price of an electric bicycle BMS varies greatly, ranging from a few dollars cheaper to more than ten dollars to twenty or thirty dollars.
Correspondingly, the functions and reliability of the electric vehicle BMS are also different.
Poor-quality BMS may be useless and will not do anything.
In contrast, the BMS of an electric vehicle can manage thousands of batteries more accurately, and its cost is correspondingly greatly increased, with a price of 20,000 to 30,000 yuan.
Xue Yu said that when the lithium batteries of electric vehicles leave the factory, they all have the original matching BMS. If the battery is changed to high voltage and large capacity, the replacement BMS may not be compatible with the battery and will not be able to protect it.
For electric bicycles, a BMS worth more than a dozen dollars has been able to play an effective protective role. There is no technical difficulty, but it is just a matter of cost.
The charger is also an important part.
Around 2016, a 22-year-old girl who came to Beijing to work in Beijing took the lead-acid battery of a Chaowei electric car home to recharge it.
In the middle of the night, the charger caused a short circuit due to overcharging the battery. After it caught fire, the girl was burned to death.
The charger manufacturer slipped away, Chaowei was sued in court, and Ma Guilong appeared as an expert witness.
In the past few years, Ma Guilong has participated in the preparation of the national standard of "Technical Requirements for Chargers for Electric Bicycles". During the discussion, he strongly urged manufacturers of lead-acid battery chargers for electric vehicles to add chargers to prevent thermal runaway.
This will increase the cost of the manufacturer by a few yuan, and was once opposed by individual manufacturers, but in the end, thanks to everyone's efforts, it was passed and the relevant regulations were written into the file.
However, "Technical Requirements for Chargers for Electric Bicycles" GB/T 36944 is a recommended standard.
Ye Zhentao said that after the release of this standard, only a few companies have implemented it. In 2020, Jiangsu Market Supervision and Administration Bureau randomly checked 50 batches of chargers, and only 2 implemented this recommended national standard; some companies have explicitly implemented corporate standards or similar standards. , Some of the safety items are lower than the GB/T 36944 standard.
Due to the insufficient strength of the recommended national standards, the National Standards Committee of the State Administration for Market Supervision has set up a mandatory national standard for chargers, which is drafted by Xue Yu.
On the afternoon of September 28, this mandatory national standard was reviewed and is expected to be released at the end of this year or early next year.
Ma Guilong said that for lithium battery chargers, the most important thing is to establish an open agreement between the BMS and the charger to realize the communication between the two.
BMS is equivalent to the "intermediary" between the lithium battery and the charger.
Just like a doctor prescribing medicine to a patient, BMS monitors the battery and tells the charger how to charge the battery under which conditions.
Once the battery temperature is too high, the charger should stop charging immediately.
A temperature sensor must be installed in the lithium battery pack for BMS monitoring.
This is currently the only way to solve the burning of lithium batteries and should be included in the mandatory national standard.
In the market, the so-called battery of a certain brand must be matched with a charger of a certain manufacturer, which is actually just a bundling of commercial interests.
Domestic manufacturers of lead-acid battery chargers for electric two-wheeled vehicles (including light motorcycles, electric motorcycles, and ultra-standard electric vehicles) show obvious concentration of production enterprises and regions.
In the primary market, the annual sales volume of vehicle chargers is 25 million, of which Nanjing Xipuer, Nanjing Teneng, and Zhejiang Juyuan account for 70% of the total.
Taking 48V, 20AH specification chargers as an example, the cost of chargers in the primary market is about 25 yuan.
The secondary market is mainly occupied by more than 70 companies in Qinlan Town, Tianchang City, Chuzhou, Anhui. The market share accounts for 70% of the secondary market. The quality of chargers is lower than that of the primary market. The annual sales volume is about 100 million. Only the charger costs 18 yuan.
Lithium battery chargers are mainly from Wuxi Sanshi, Shenzhen Medirui and other manufacturers.
Compared with lead-acid battery chargers, the lithium battery charger market is still in its infancy. There are no large companies, and many small factories still have a lot of irregular production.
"China News Weekly" searched on the e-commerce platform. The price of a lithium battery charger with full self-stop function is around 60 yuan, and a brand of lithium battery charger with overheating, overcharge, and overcurrent protection is priced at 159 yuan.
Ye Zhentao said that from the analysis of supervision and spot check data, the failure rate of network platforms is high, and the failure rate of chargers is as high as 80%.
Ma Guilong said that to solve the problem of lithium battery charging safety, it is necessary to consider the trinity of lithium batteries, BMS, and chargers. It is necessary to prevent inferior lithium batteries and chargers from entering the market, otherwise, it will only be scratched.
If the safety is only improved from the perspective of battery packs and chargers, the manufacturer’s increased cost will only be 20 to 30 yuan, "not more than 50 yuan, and consumers will not spend more than 100 yuan at most."
"Grassroots industry" that is not taken seriously
"Grassroots industry" that is not taken seriously
Under the implementation of the new national standard, Beijing has established a three-year transition period for vehicles that exceed the standard.
Starting from November 1, over-standard vehicles equipped with temporary yellow cards will be eliminated, and all vehicles with "compliant" white plates will pass on the road.
Before the National Day, Liu Gang found an electric car shop, sold his Yadi electric car for 200 yuan, and bought a Luyuan electric car with a rated voltage of 72V for 2,500 yuan. Under the operation of the store maintenance staff, The electric vehicle whose original factory speed is set to 25km/h has been changed to a speed of 50km/h, which is called lifting the speed limit in the industry.
No matter how fast the motor rotates, the speed displayed on the dashboard of the electric vehicle still does not exceed the maximum limit value of the new national standard of 25km/h.
Ma Guilong said that although the vast majority of electric vehicle fire accidents occur during charging, private modification of high-voltage, large-capacity batteries will also bring hidden dangers to fire safety.
High voltage means high speed. When an electric vehicle is running at a high speed, it means that the discharge current will increase and the battery temperature will increase, which will increase the possibility of battery explosion.
A heavy load will also produce a large current, and another greater possibility of deflagration during the driving of an electric vehicle is that the positive and negative electrodes of the battery are short-circuited.
Lu Jinlong said that in fact, the requirements for lithium batteries for electric bicycles should be higher than those for automotive-grade lithium batteries.
"The use environment of electric bicycles is worse than that of automobiles, with bumps, loads, exposure to the sun, and severe cold. Automobiles rarely face such situations, and the safety of their batteries and various equipment is better." But at the same time, electric bicycles are used by the crowd. The economic capacity is limited and the price is more sensitive. "Buy a car will not care about a thousand dollars. An electric bicycle may be 200 yuan more expensive, but consumers can't accept it." This is also the important reason why many other brand electric car batteries have a way out. reason.
Ma Guilong said that the current division of electric two-wheelers into electric bicycles, electric mopeds, and electric motorcycles according to speed and weight is just a convenient way for traffic control departments to manage.
The new national standard does not fundamentally solve the two major problems of electric bicycle traffic safety and fire safety.
Ma Guilong suggested that all two-wheeled vehicles with handlebar speed should be included in the management of motor vehicles.
Once managed by motor vehicles, the riding speed of electric vehicles is regulated according to the road section, which can better guarantee traffic safety, and at the same time, the management of its production and sales can be in line with that of motor vehicles.
As for the vehicles used by the takeaways, special standards should be formulated, a qualified electric vehicle manufacturer should be selected, and the speed should be regulated accordingly. The battery can meet the professional needs of the takeaways to deliver meals. Purchase vehicles.
In this way, take-out vehicles can be transformed from disorder to standard.
Ye Zhentao said that the food delivery platform should also establish an information platform and conduct regular inspections to ensure vehicle safety.
Almost all foreign electric bicycles are exported from China, and 70% to 80% use lithium batteries.
However, the accident rate of electric bicycles abroad is not as high as in China.
This is because, on the one hand, electric bicycles are mostly electric mopeds abroad, and the speed will not be higher than 30km/h.
On the other hand, due to the high price of exported electric bicycles, which can be sold for 8,000 to 10,000 yuan, manufacturers also have enough profit margins to adopt better quality accessories.
For example, all electric bicycles exported to foreign countries are equipped with BMS, and the consistency of battery configuration is also relatively good.
Ma Guilong said that the development of the entire domestic electric bicycle industry is still at a low level, and the manufacturers of electric bicycle parts are basically self-employed.
There are thousands of automakers across the country, and only dozens of them are doing well.
However, if supervision is strengthened, it means that costs and prices will increase, which may cause damage to the interests of low-income consumers among its consumer groups.
This involves the government's management strategy.
In recent years, the electric bicycle industry has flourished, but government departments pay much more attention to electric vehicles than electric bicycles.
"As for electric bicycles, it is a grassroots industry, and there are also certain differences in policies." Lu Jinlong said.
Ma Guilong believes that electric bicycles have incomparable advantages over electric vehicles in terms of floor space, energy conservation and environmental protection, or practicality.
However, in the eyes of many local governments, making lithium batteries is high-tech, making automobiles is high-tech, and making electric vehicles is even more important.
"As for electric bicycles, everyone is not very interested."
(In the text, Liu Gang is a pseudonym)
China News Weekly, Issue 38, 2021
Keywords: bicycles, bms, vehicles., battery, standard, lead-acid batteries, lithium batteries, superconducting graphene battery, ma guilong, batteries.at, vehicles, industry, electrolyte, battery., xingfu yiju community
Statement: The publication of the "China News Weekly" manuscript is authorized in writing
Statement: The publication of the "China News Weekly" manuscript is authorized in writing