How dangerous is it for electric scooters to privately change lead-acid batteries to lithium batteries?
The Hangzhou electric scooter suddenly exploded while driving, causing severe burns to the two fathers and daughters. Experts and the fire brigade concluded after analysis that the lithium battery exploded, and the safer and more stable lead-acid battery that the vehicle was equipped with when it left the factory, how could it become a lithium battery? It turns out that among the users of electric scooters, there are a large number of cases of privately modifying the vehicle to change the battery in order to improve the battery life and reduce the weight...
In 2021, at 11 o'clock in the morning on July 18, a battery car suddenly ignited while driving on Yuhuangshan Road in Hangzhou. Three members of a family were injured, of which the father and daughter were in serious condition. The family members of the injured claimed that the little girl had been issued a critical illness notice three times by the hospital and may need to be intubated for life.
According to the fire video released by the Hangzhou traffic police, the whole car suddenly turned into a fireball to envelop people, and the scene was terrifying. The person in charge of the power supply department of a well-known electric bicycle brand research and development center in Zhejiang Province, after watching the video of the spontaneous combustion of electric bicycles, analyzed that, "This situation is very likely to be caused by lithium batteries."
Only lithium batteries can burn so suddenly and so fiercely. Experts say that the probability of this situation in lead-acid batteries is extremely low, or even almost impossible, because lead-acid batteries are basically unlikely to cause spontaneous combustion.
Two conditions are required for spontaneous combustion or explosion of lead-acid batteries. The first is that hydrogen and oxygen will be produced in the case of overcharging. The second is that an electric spark ignites during the discharge process. "In other words, lead-acid batteries basically do not ignite or explode when they are charged. Only when they are discharged, that is, when they are driving, will there be electric sparks or short-circuits, so that they will spontaneously ignite or explode when they react with hydrogen and oxygen.
However, there is an overflow valve in the lead-acid battery. When the battery is overcharged and the pressure generated is high, the valve will automatically open to discharge the gas (hydrogen and oxygen), so the probability of spontaneous combustion or explosion during driving is small. At most, the circuit board is burned out. Moreover, the lead-acid battery itself is a negative feedback system, and the battery pack has a fuse. If an external short circuit occurs, the fuse will be blown immediately. If there is an internal short circuit, the lead plate itself will not burn, it will only heat and vaporize and dry the electrolyte. If the electrolyte is gone, the battery will be naturally disconnected.
The lithium battery is a positive feedback system. "The electrolyte in the organic material (lithium hexafluorophosphate) is flammable. The positive electrode is lithium ion and the negative electrode is graphite. Normally, the lithium ion is embedded in the positive electrode. When it is fully charged, If a short circuit occurs, it is easy to cause lithium to precipitate. In addition, its chemical properties are relatively active and cannot be in contact with oxygen in the air. Once in contact, it will easily cause thermal runaway, the chemical reaction is relatively violent (such as explosion), and the electric vehicle is naturally easy to burn.
Professionals said that there are several common protection measures in lithium batteries:
The most important thing is the lithium battery protection board, which mainly prevents the current from overcharging, overdischarging, overcurrent or short circuit, and preventing the abnormal battery from causing high temperature or even burning. If there is no protection board, if one of the batteries in a group has a problem, it will cause an abnormality or even a short circuit. Of course, the function and quality of the protection board are also very different, ordinary tens of yuan, many good protection boards cost hundreds of yuan or even large-capacity battery protection boards.
2Battery holder. Our common lithium battery looks like a big piece, but in fact, it is composed of countless small batteries. It is very important to fix the battery combination. A bracket is needed. For the sake of safety, many big brands have a bracket structure that allows every small battery. Separate the distance of a few millimeters and fix it (called pack rack), which is good for heat dissipation and makes the battery more stable. However, such a bracket will affect the density of the battery in the same volume. As a result, some batteries pursue small size and large capacity, so they will cancel more professional brackets and stick the batteries together, so that although the capacity is large, the temperature is difficult to control, and the stability is also poor.
Waterproof performance. This is easier to understand, that is, the tightness of the casing, and lithium batteries are most afraid of water ingress.
The power of the charger. Generally speaking, big brands will balance charging time and efficiency, and give a safe solution. If you use a modified charger and increase the current, it will cause the rechargeable battery to overheat, especially in summer, which is also a hidden danger
Generally, electric scooter batteries are installed under the pedals or under the seat. There are many teaching videos on the Internet about replacing lead-acid batteries with lithium batteries.
Composition and function of lithium battery protection board
The protection board usually includes control IC, MOS tube, resistor, capacitor and auxiliary devices FUSE, PTC, NTC, ID, memory and so on. Among them, the control IC controls the MOS switch to turn on under all normal conditions to make the cell and the external circuit conduct, and when the cell voltage or loop current exceeds the specified value, it immediately controls the MOS switch to turn off to protect the cell’s Safety. The main functions of other components are as follows:
01 Resistance: plays the role of current limiting and sampling;
02 Capacitor: For direct current, the resistance value is "∞", for alternating current, the resistance value is close to zero, and the voltage at both ends of the capacitor cannot be changed suddenly, which can play a role in instant voltage stabilization and filtering;
03 FUSE: Fuse the fuse, play the role of over-current protection;
04 PTC: PTC is the abbreviation of Positive Temperature Coefficient, which means positive temperature coefficient resistance (the higher the temperature, the greater the resistance), which can prevent the occurrence of high-temperature battery discharge and unsafe high current, that is, overcurrent protection.
05 NTC is the abbreviation of Negative Temperature Coefficient, which means negative temperature coefficient. When the ambient temperature rises, its resistance decreases. The use of electrical equipment or charging equipment responds in time and controls internal interruptions to stop charging and discharging.
06 ID is the abbreviation of Identification, that is, the meaning of identification. It is divided into two types: one is memory, which is often a single-wire interface memory, which stores information such as battery type and production date; the other is identification resistance. Both can play a role in product traceability and application restrictions.
Commonly seen lithium battery protection board circuit diagram
The direction of current when the battery is charging is as shown by the arrow
When charging, the control IC X1 will always monitor the voltage between the 5th pin VDD and the 6th pin VSS. When this voltage is greater than or equal to the overcharge cut-off voltage and meets the delay time of the overcharge voltage, X1 will control the third pin To turn off the MOS transistor Q2, after Q2 is turned off, the charging circuit is cut off (the body diode D2 of Q2 is also reversely cut off), at this time, the battery can only be discharged.
When one of the following two conditions is met, the charging protection can be released:
01 The voltage across the cell drops to the overcharge recovery voltage of the protection IC.
02 Add a load to the output terminal of the battery pack to discharge, and discharge until the voltage is less than the overcharge protection voltage.
Over discharge protection
When the two ends of the battery output current to the load, the current flows according to the red arrow in the figure below.
Discharge the battery, and the control chip IC X1 will detect the voltage on C1 through the 5th pin. When this voltage is lower than the discharge cut-off voltage for a period of time, the control IC will control Q1 to cut off through the DO pin, and the discharge circuit is cut off at this time.
When the following conditions are met, IC X1 contacts the over-discharge protection: remove the load and charge the battery pack. When the voltage between VM-VDD reaches the over-discharge recovery voltage value, the control IC X1 will turn on the MOS tube Q1 again.
Over current/short circuit protection
Over-current protection is to detect the voltage flowing through the control MOS tube through the second pin (VM) of IC X1. If the voltage is too large and lasts for a certain period of time, the control IC will turn off Q1 and disconnect the discharge circuit. Remove the output load, the control IC will automatically turn Q1 back on.
The overcurrent protection voltage VM is usually 0.1 ~ 0.2V, and this value is related to the IC model.
The overcurrent protection value is not only related to VM, but also related to the on-resistance of the two MOS transistors Q1 and Q2. If the conduction resistance of the MOS tube is larger, the protection current value is smaller.
For example: a MOS tube with an internal resistance of 20mΩ, and a control IC with an overcurrent value of 0.15V selected, the overcurrent protection current should be: 0.15V/(0.02*2)=3.75A.
FUSE protection when the control IC fails
A fuse is added to some protection boards. After the control IC fails, it serves as a secondary protection to avoid worse results, and of course it will increase the cost.
Why do people always replace lead-acid batteries with lithium batteries?
According to professionals, in essence, both electric bicycle batteries and electric vehicle batteries belong to lithium-ion batteries and secondary batteries. The working principle is that lithium ions flow back and forth between the positive and negative electrodes of the battery to complete the charging and discharging process.
"The most widely used electric vehicles are lithium iron phosphate batteries and ternary lithium batteries." Professionals say that lithium iron phosphate batteries have low energy density and good safety performance. When the battery temperature is between 500°C and 600°C, its internal chemical components begin to decompose. But the cruising range of the vehicle is average. When the temperature is lower than -5°C, the charging efficiency is low, which is not suitable for the demand of charging in the north in winter. The ternary lithium battery has high energy density, high activity, large capacity, and low temperature resistance. The battery is more stable at low temperatures.
Electric bicycles mostly use lead-acid batteries and lithium batteries. "Compared with lithium batteries, lead-acid batteries have lower cost, higher cost performance, heavier weight, lower energy density, and slow charging." Professionals say that lead-acid batteries are relatively safer. If the lithium battery is used improperly, it will explode and fire more often.
According to the Hangzhou Daily, the mother of the injured girl said that the two cars the couple rode were bought at the same store, and they were replaced with new batteries in this store.
The Hangzhou Fire Rescue Detachment conducted a detailed fire investigation on the electric vehicle that caught fire. The electric car in the accident used a lithium battery. Because of the explosion, the weight of the battery has been much lighter than before.
Photos after the accident of the electric scooter burned down
The vehicle purchase invoice and the non-motor vehicle driving permit are completely kept in the middle of the vehicle. The car purchase invoice states that the electric car purchase time is 2018. The reporter found the same electric car in the store based on the model of the explosive electric car, and from the manual of the electric car, it was found that the original battery should be a lead-acid battery.
It is reported that all brand electric vehicles on the market will have the original battery and provide quality assurance, which have passed the national quality inspection. However, some people are not satisfied with the capacity and price of the original lithium batteries, and some of the original manufacturers are lead-acid batteries, which are directly changed to lithium batteries, which gives third-party batteries a market.
Private battery replacement is a big safety hazard
As long as a brand electric car uses non-original batteries, it belongs to the scope of modification. In addition to changing the lead-acid battery to a lithium battery or increasing the lithium battery capacity, the usual modification behavior is to install a new controller on the vehicle and cancel the buzzer alarm. The purpose of this is to increase the vehicle's top speed.
Refitting a vehicle is not an easy task. Usually, to make a regular electric bicycle, it needs to use more than 100 batteries in series and parallel. Each battery needs to have corresponding performance and meet relevant manufacturing standards, and the connection of the battery also needs to strictly abide by the BMS battery management system. However, the merchants who assemble battery packs privately on the market do not have the corresponding technological transformation capabilities, and most of their privately assembled lithium batteries are of poor quality and insufficient safety. Therefore, private modification often means a great safety hazard.