The harm of lithium-ion battery inconsistency and how to deal with it
Power lithium-ion batteries have steadily occupied the position of the leader in the power supply of electric vehicles. Long service life, high energy density, and great potential for improvement. Safety can be changed and energy density can continue to rise. In the foreseeable time (the legend is about 2020), it can catch up with the endurance and cost performance of fuel vehicles and enter the first mature stage of electric vehicles. However, lithium ion batteries also have the troubles of lithium ion batteries.
1. Why are most lithium-ion batteries small
The lithium-ion batteries, cylindrical batteries, soft-pack batteries, and square batteries we have seen are generally pretty and beautiful, and there is no such thing as a traditional lead-acid battery. Why?
With high energy density, lithium-ion batteries are often afraid to design large capacity. The energy density of lead-acid batteries is around 40Wh/kg, while lithium-ion batteries have exceeded 150Wh/kg. With the increase in energy concentration, the requirements for safety are rising.
First of all, it is very dangerous for lithium-ion batteries that can only use excessively high amounts of lithium-ion batteries in an accident, causing thermal runaway, and a sharp reaction inside the battery. In a short time, too much energy is nowhere to be released, which is very dangerous. Especially when the development of safety technology and management and control capabilities is not sufficient, the capacity of each battery should be restrained.
Secondly, once an accident occurs, the energy wrapped in the lithium-ion battery shell will be inaccessible and powerless by firefighters and extinguishing agents. They can only isolate the scene in the event of an accident and allow the accident battery to react by itself until the energy is exhausted.
Of course, for safety reasons, current lithium-ion batteries have been designed with multiple safety measures. Take cylindrical batteries as an example.
The safety valve, when the internal reaction of the battery exceeds the normal range, the temperature rises, and the side reaction gas is generated, the pressure reaches the design value, the safety valve automatically opens to release the pressure. The moment the safety valve opens, the battery fails completely.
Thermistor, and some cells are equipped with thermistor. Once overcurrent occurs, after the resistance reaches a certain temperature, the resistance value increases sharply, and the current in the loop drops, preventing further temperature rise.
Fuse, the battery cell is equipped with a fuse with over-current fusing function, once the risk of over-current occurs, the circuit is disconnected to prevent the occurrence of vicious accidents.
2. Li-ion battery consistency problem
Lithium-ion batteries can't be made into a large one, so many small batteries have to be organized. Everyone can work hard and cooperate with each other, and they can also fly with electric cars. At this time, we have to face a problem, consistency.
Why be consistent
Our daily relevant experience is that if the positive and negative poles of two dry batteries are connected, the flashlight can emit light. Who cares about the same and inconsistent things. The large-scale application of lithium-ion batteries is not so simple.
The inconsistency of lithium-ion battery parameters mainly refers to the inconsistency of capacity, internal resistance, and open circuit voltage. If inconsistent batteries are used together in series, the following problems will occur.
1) Capacity loss. Single cells form a battery pack. The capacity conforms to the "barrel principle". The capacity of the worst cell determines the capacity of the entire battery pack.
In order to prevent the battery from overcharging and overdischarging, the logic of the battery management system is set up as follows: when discharging, when the lowest cell voltage reaches the discharge cut-off voltage, the entire battery pack stops discharging; when charging, when the highest cell voltage reaches the charge cut-off voltage To stop charging.
Take two batteries in series as an example. One battery has a capacity of C, and the other has a capacity of only 0.9C. In a series connection, two batteries pass the same current.
When charging, the battery with small capacity must be fully charged first, and the charging cut-off condition is reached, and the system will not continue to charge. When discharging, a battery with a small capacity must emit all available energy first, and the system immediately stops discharging.
In this way, batteries with small capacity are always fully discharged, while batteries with large capacity always use part of the capacity. Part of the capacity of the entire battery pack is always idle
2) Life loss, similarly, the life of a battery pack is determined by the cell with the shortest life. It is very likely that the cell with the shortest life span is the cell with a small capacity. Small-capacity batteries are fully charged and discharged every time, and the output is too strong, which is likely to reach the focus of life first. The life of the battery cell has ended, and a group of battery cells welded together will die.
3) The internal resistance increases, the same current flows through different internal resistances, and the cells with large internal resistance generate more heat. The battery temperature is too high, causing the deterioration rate to accelerate, and the internal resistance will further increase. Internal resistance and temperature rise form a pair of negative feedback, which accelerates the deterioration of high internal resistance cells.
The above three parameters are not completely independent. The internal resistance of the battery with a deep aging degree is larger, and the capacity attenuation is also more. Separate explanations, just to express clearly their respective directions of influence.
3. How to deal with inconsistencies
The inconsistency of battery core performance is formed during the production process and deepened during use. The battery cells in the same battery pack are always weak for the weak, and become weaker at an accelerated rate. The degree of dispersion of the parameters between the single cells increases as the degree of aging deepens.
At present, engineers should consider three aspects to deal with the inconsistency of single cells. Single battery sorting, thermal management after grouping, the battery management system provides equalization function when a small amount of inconsistency occurs.
Different batches of batteries should not be used together in theory. Even batteries of the same batch must be screened, and batteries with relatively concentrated parameters are placed in a battery pack and in the same battery pack.
The purpose of sorting is to select cells with similar parameters. The sorting method has been studied for many years, and it is mainly divided into two categories: static sorting and dynamic sorting.
Static sorting is to screen the open circuit voltage, internal resistance, capacity and other characteristic parameters of the cells, select target parameters, introduce statistical algorithms, set screening criteria, and finally divide the same batch of cells into several groups.
Dynamic screening is based on the characteristics of the battery cell during the charging and discharging process. Some choose the constant current and constant voltage charging process, some choose the pulse shock charge and discharge process, and some compare their own charging and discharging curves. relationship.
Combining dynamic and static sorting, static screening is used for preliminary grouping, and dynamic screening is performed on this basis, so that more groups can be divided and screening accuracy is higher, but the cost will rise accordingly.
Here is a small reflection of the importance of the production scale of a power lithium-ion battery. Large-scale shipments allow manufacturers to perform finer sorting and obtain battery packs with closer performance. If the output is too small and there are too many groups, one batch cannot be equipped with a battery pack, and the best method will not be used.
2) Thermal management
For batteries with inconsistent internal resistance, the problem of different heat appears. The addition of the thermal management system can adjust the temperature difference of the entire battery pack to keep it in a small range. Cells that generate more heat will still have a high temperature rise, but will not be separated from other cells, and there will be no significant difference in degradation level.
The inconsistency of the battery cells, the terminal voltage of some battery cells is always ahead of other battery cells, reaching the control threshold first, resulting in the reduction of the entire system capacity. In order to solve this problem, the battery management system BMS has designed a balancing function.
A certain cell first reaches the charging cut-off voltage, while the voltage of the other cells is obviously lagging behind. The BMS activates the charge equalization function, or connects a resistor to discharge part of the high-voltage cell's power, or transfer the energy away and put it to low The voltage cell goes up. In this way, the charge cut-off condition is lifted, the charging process restarts, and the battery pack is charged with more power.
Until now, the inconsistency of batteries is still an important area of research in the industry. No matter how high the energy density of the battery cell is, the battery pack capacity will be greatly reduced if it encounters inconsistency.
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