We really have seen too many electric bicycle refit lithium battery explosion combustion events. The cause of lithium battery explosion and combustion can be seen from the following four aspects.
1. Types of lithium batteries and battery quality grade
The first is lithium iron phosphate battery, good chemical stability, under extreme conditions only smoke not fire, the disadvantage is that the volume is large.
The second is Li-ion NCM battery, wide discharge platform, high efficiency, and the volume can be extremely small (such as LG). But more power means more danger, so some manufacturers have designed more secure protective cases (such as Panasonic 18650, CATL), although not as small as LG, but it’s much smaller than lithium iron phosphate. So at present, including Tesla, BYD, Xiaopeng and other major new energy vehicle manufacturers use this NCM cell.
The quality of the cell is relative to the “Brand New”, which is the so-called A grade cell. Have A will inevitably have a B, the so-called B grade cell is the defective cell eliminated on the assembly line or the dismantled cell. As well as the cells removed from shared electric vehicles like Halo and Meituan by criminals. These cells naturally become B grade cells, and B grade cells have no quality assurance. That’s why the lithium-ion battery industry for electric vehicles is in hot water.
2. Assembly of lithium battery pack
When assembling lithium-ion battery packs, large manufacturers will use a compartment to accurately match the volume of each cell, while small workshops only need a multimeter to do the job. In this way, the internal resistance of the battery is not consistent, which seriously affects the battery life. It is easy to cause a few strings of overcharge in the same group of batteries and a few strings of undercharging, which increases the risk factor.
3.Welding of lithium batteries
There are only a few regular lithium battery welding, resistance welding, ultrasonic welding, laser welding. The rest of what aluminum polar flux, high-power soldering iron, and so on are not regular welding. There are several reasons for this. Underneath the battery pole is a layer of insulating plastic film. Once exposed to high temperatures, it is easily damaged. After using for a period of time will find leakage, bulging, and other conditions. There was a slight short circuit inside the cell because the insulation layer was damaged. So it’s not safe for long-term use.
The BMS is the most important threshold to protect lithium-ion batteries from being overcharged or over-discharged. How much the electric vehicle controller can release, multiplied by the proportion of 2.5 is the most suitable BMS are carrying (such as 100Acontroller with 300A BMS), because of lithium-ion batteries more support 1C – 3C discharge. If the voltage is too high, the BMS will stop discharging. At the same time, the BMS will balance the battery series and protect the life of the battery pack.
But the small workshop only uses a soldering iron on the assembly, the soldering tin pile is not good or the hand operation error will greatly affect the safety of the lithium battery. Especially in the rationality of BMS wiring and welding completeness, if not done well, it will cause a greater disaster.
4. Bad usage habits
On the lithium battery fire explosion, as long as the object with energy, as long as the conditions are enough to cause the explosion. At present, there are many cases of lithium battery explosion, most of which are caused by bad usage habits. As long as the lithium battery is overcharged, it will certainly produce unsafe factors for the battery. Light is the reduction of capacity and increases resistance to protect safety, heavy explosion combustion. The cut-off voltage of the NCM cell is 4.2V, above which the chemical structure of the battery will become unstable, releasing excess energy through heating, expansion, and other ways. If the high-pressure filling continues, the best result is that the safety valve is opened, the internal circuit is broken, and the input is completely cut off to ensure safety.
Another common bad usage habit that causes explosions is over-discharge. Discharge over the designed safe current will not only affect the customized battery life but more likely to cause safety accidents. Therefore, it is suggested that lithium battery peers install current limiting devices inside the battery to avoid safety accidents caused by over-rate discharge or short circuits. The super-rate discharge causes the dissolving of the inner membrane of the battery, which will increase the internal resistance and cause serious heating or even fire.
Common 18650 cell, even Panasonic or Tesla brand, after the actual measurement of 3 times the battery power for a long time discharge, the battery temperature rise of more than 50 degrees, so you can imagine very dangerous being in a box.
Finally, a few points that must be noted.
- If you can, it is best not to buy a privately made lithium battery pack.
- Buy the battery from a professional battery manufacturer which has real technology and product insurance.
- Do not charge the lithium battery in the corridor, home!
- Be sure to use qualified lithium battery chargers. Do not overcharge or over-discharge.
SmartPropel Original Writer: Michelle Pan