There are two types of rechargeable batteries, lead-acid and lithium. Lithium batteries are lighter, have more storage capacity, and a longer life. Lead-acid batteries were the norm until the late 2010s, but in 2018, a new national standard for electric bikes was released, stipulating that the total weight of an e-bike including batteries should not exceed 55 kilograms. Since then, 35 percent of the 30 million new e-bikes produced nationwide use lithium batteries.
Lu Jinlong, vice president of the China Cycling Association, said that before 2018, the number of e-bike explosions was much lower.
Xue Yu, a manager at Light Electric Vehicle and Battery Product Quality Supervision and Inspection Center (CEVT), a government agency affiliated to the State Administration for Market Regulation in Wuxi, Jiangsu Province, said lithium batteries are more prone to accidents than lead-acid batteries, mainly due to the material and structure of the battery. Xue said lithium batteries have higher energy density than lead acid batteries, and lithium elements have more active chemistry than lead. Also, the electrolytes in a lithium battery – the liquid or paste which allows ions to move – burn easier than the dilute sulfuric acid electrolytes in lead acid batteries. Lead acid batteries can bulge or crack in an accident without causing further damage. Lithium batteries can combust because of thermal runaway, which happens when the heat produced inside a battery builds up and exceeds the amount of heat that is dissipated externally. If this occurs in a single battery, it affects adjacent batteries, which causes the runaway effect.
Ma said the ultimate solution would be to develop a solid-state lithium battery, with the electrolytes in solid form too.
“Now it’s easy for lithium batteries to expand and explode because the electrolytes are liquid,” Ma said. As a solid-state battery does not use a liquid electrolyte solution, it cannot combust and has no need of safety components, which makes it smaller. These batteries hold more charge and charge quicker as well. Development of a solid-state lithium battery, the holy grail of e-vehicles, is in the preliminary research stages, although a Harvard team led by Associate Professor of materials science Li Xin revealed in May 2021 that they have designed a solid-state battery which can last for the lifetime of a car, about 10- 15 years, and charge in 20 minutes, Nature reported.
In the e-bike industry, most enterprises do not produce their own batteries, but buy custom batteries from manufacturers to assemble. Leading e-bike producers often work with lithium battery manufacturers including Phylion, Tianneng Group and Chilwee.
According to Ma, some big manufacturers of automobile lithium batteries pack them with uniform voltage and internal resistance and sell them to auto factories, while selling lower-quality batteries with different internal resistance and uneven voltage to small, unlicensed factories. These factories connect batteries in parallel, pack them and then sell them to secondary markets such as repair shops and e-bike stores. If a battery pack is comprised of batteries of different voltages and internal resistance, it can cause accidents. Overcharging is responsible for most battery explosions, Ma said.
“The way to form large-capacity batteries is to connect more than a dozen individual batteries in a series, and then combine them into a pack,” Ma said. “So if the configuration of the batteries is inconsistent, some batteries might be fully charged while others are not, and the battery pack continues to charge.” As a result, some batteries overcharge and heat up. The liquid electrolytes vaporize, increasing pressure which leads to an explosion.
Ye Zhentao is technical head of CEVT and one of the drafters of the new national standard for electric vehicles. Because e-bikes and batteries are often sold separately in China, inferior batteries are often installed, Ye said. He added there is no special recycling system for waste lithium batteries or any special recycling outlets so far.
Lu Jinlong said dozens of companies produce substandard lithium batteries in Dongguan, Guangdong Province with no safety guarantees. The price of these sub-standard batteries is normally 20-30 percent cheaper than leading brands. He suggests there should be compulsory national certification.
Ma added that e-bike manufacturers should step up to ensure the consistency of battery panels. Violation of the regulations should lead to legal penalties.