Seconds understand the harm of lithium battery inconsistency and how to deal with it

Power lithium battery, has firmly occupied the status of electric vehicle power supply. Long service life, high energy density and great potential for improvement. Security can be changed and energy density can continue to rise. In the foreseeable time (the legend is about 2020), you can catch up with the fuel car's endurance and cost-effectiveness, and enter the mature stage of electric vehicles. However, lithium batteries also have the trouble of lithium batteries.

1. Why are lithium batteries mostly small?

The lithium batteries, cylindrical batteries, soft pack batteries, and square batteries that we have seen are generally handsome and can't find the big ones like traditional lead-acid batteries. Why?

High energy density, lithium batteries often do not dare to design large capacity. Lead-acid batteries have an energy density of around 40 Wh/kg, while lithium batteries have exceeded 150 Wh/kg. The concentration of energy has increased, and the demand for safety has risen.

First of all, the lithium battery can only be over-extended, encountering an accident, causing thermal runaway, and the battery reacts sharply inside. In a short time, too much energy is nowhere to be released, which is very dangerous. Especially in the case of safety technology, when the development of management and control capabilities is not sufficient, the capacity of each battery should be restrained.

Secondly, the energy enclosed by the lithium battery case, in the event of an accident, firefighters, fire extinguishing agents can not touch, powerless, can only isolate the scene in the event of an accident, the accident battery itself reacts, energy is burned out.

Of course, for safety reasons, current lithium batteries have been designed with multiple safety measures. Take a cylindrical battery as an example.

The safety valve, when the internal reaction of the battery exceeds the normal range, the temperature rises, and the reaction gas is generated, the pressure reaches the design value, the safety valve is automatically opened, and the pressure is released. At the moment the safety valve opens, the battery is completely inoperative.

Thermistors, some of the batteries are equipped with thermistors. Once an overcurrent occurs, the resistance increases sharply after reaching a certain temperature, and the current in the loop drops, preventing the temperature from rising further.

Fuse, the battery is equipped with a fuse with overcurrent fuse function, in the event of overcurrent risk, the circuit is disconnected to avoid the occurrence of a malignant accident.

2. Lithium battery consistency problem

Lithium batteries can't be made into a big one, so we have to organize many small batteries. Everyone is going to make it all together, and we can cooperate with them to fly with electric cars. At this time, you need to face a problem and consistency.

Why should we agree?

Our daily experience is that two dry batteries, the positive and negative poles are connected, the flashlight can be illuminated, and whoever manages it is inconsistent. The large-scale application of lithium batteries is not so simple.

The inconsistency of lithium battery parameters mainly refers to the inconsistency of capacity, internal resistance and open circuit voltage. Inconsistent battery strings are used together and the following problems occur.

1) Capacity loss, the battery cells are composed of battery cells, the capacity is in line with the "cask principle", and the capacity of the worst battery cell determines the capacity of the entire battery pack.

In order to prevent overcharging and over discharging of the battery, the logic of the battery management system is set such that when discharging, when the cell voltage of ** reaches the discharge cutoff voltage, the entire battery pack stops discharging; when charging, when the ** cell voltage touches the charging cutoff When the voltage is applied, the charging is stopped.

Take two batteries in series for example. One battery has a capacity of C, and the other has a capacity of only 0.9C. In series, two batteries pass the same amount of current.

When charging, the battery with a small capacity must be fully charged first, and the charging cut-off condition is reached, and the system does not continue charging. When discharging, the battery with a small capacity must first emit all available energy, and the system immediately stops discharging.

In this way, the cell with a small capacity is always fully charged, and the cell with a large capacity always uses a part of the capacity. A portion of the entire battery pack is always idle.

2) Loss of life, similarly, the life of the battery pack, determined by the battery with the shortest life. The possibility is that the battery with the shortest life is the one with a small capacity. Small-capacity batteries, each time full and full, the output is too strong, it is likely to reach the focus of life first. At the end of the life of the battery, a group of batteries that are welded together will follow the end of life.

3) The internal resistance increases, the different internal resistances flow through the same current, and the cells with large internal resistance have relatively more heat. If the battery temperature is too high, the rate of deterioration will increase and the internal resistance will increase further. Internal resistance and temperature rise form a pair of negative feedback, which accelerates the degradation of the high internal resistance cell.

The above three parameters are not completely independent, and the internal resistance of the battery with a deep aging degree is relatively large, and the capacity attenuation is also more. Explain separately, just want to express their respective directions of influence.

3. How to deal with inconsistency

Inconsistent performance of the battery core is formed during the production process and deepens during use. The batteries in the same battery pack are weak and weak, and the acceleration is weak. The degree of dispersion of the parameters between the individual cells increases as the degree of aging increases.

At present, engineers should deal with the inconsistency of individual cells, mainly considering three aspects. Single cell sorting, thermal management after grouping, and battery management system provides equalization when there is a small amount of inconsistency.

1) Sorting

Different batches of batteries are theoretically not used together. Even the same batch of cells need to be screened, and the cells with relatively concentrated parameters are placed in one battery pack and in the same battery pack.

The purpose of sorting is to pick out the cells with similar parameters. The sorting method has been studied for many years and is mainly divided into two categories: static sorting and dynamic sorting.

Static sorting, screening the characteristic parameters such as open circuit voltage, internal resistance and capacity of the cell, selecting the target parameters, introducing statistical algorithms, setting the screening criteria, and finally dividing the cells of the same batch into several groups.

Dynamic screening is to screen the characteristics of the battery during charging and discharging. Some choose constant current and constant voltage charging process, some choose pulse shock charging and discharging process, and some compare their own charging and discharging curves. relationship.

Dynamic and static combined sorting, static screening for preliminary grouping, based on this dynamic screening, so that more groups are divided, the screening accuracy is higher, but the cost will increase accordingly.

Here, the small figure reflects the importance of the scale of production of a power lithium battery. Large-scale shipments allow manufacturers to perform finer sorting and get battery packs with closer performance. If the output is too small, too many groups, and one batch can not be equipped with a battery pack, no matter how good the method is.

2) Thermal management

For the inconsistent internal resistance of the cell, the problem of heat generation is different. With the addition of a thermal management system, the temperature difference across the battery pack can be adjusted to keep it in a small range. The battery that generates more heat still has a higher temperature rise, but it will not open the gap with other batteries, and there will be no obvious gap in the deterioration level.

3) Balance

Inconsistent cell cells, some cell terminal voltages are always ahead of other cells, reaching the control threshold first, resulting in a smaller overall system capacity. In order to solve this problem, the battery management system BMS has designed an equalization function.

A certain battery core first reaches the charge cut-off voltage, and the rest of the battery voltage is obviously delayed. The BMS starts the charge equalization function, or accesses the resistor, discharges part of the power of the high-voltage battery, or transfers the energy to the low. The voltage cell goes up. In this way, the charge cut-off condition is released, the charging process starts again, and the battery pack is charged with more power.

Until now, cell inconsistency is still an important area of ​​research in the industry. The energy density of the battery is higher, and the battery pack capacity will be greatly reduced if it encounters inconsistency.