Lithium battery has high consistency requirements
Issuing time:2019-03-20 19:32Source:baidu
There is an inconsistency in the lithium battery, which will cause the lithium battery to be prone to capacity loss, loss of life, and increase in internal resistance.
Inconsistent performance of the battery core is formed during the production process and deepens during use. For example, the battery cells in the same battery pack are weaker and weaker, and the acceleration is weak.The degree of dispersion of the parameters between the individual cells increases as the degree of aging increases.
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? Energy density has increased, and lithium batteries are often not designed. High capacity. Lead-acid batteries have an energy density of around 40 Wh/kg, while lithium batteries have exceeded 150 Wh/kg.
Of course, the concentration of energy is increased, and the safety requirements are also rising.
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 be touched, powerless, can only be isolated in the event of an accident, the accident battery itself reacts, energy burned.
For safety reasons, current lithium batteries have been designed with multiple safety measures.
Waterproof performance, Izola Energy's lithium battery research has been upgraded to battery waterproof module, professional structural technology research and development, lithium battery has been able to penetrate 100 meters underwater.
2) Internal structure safety of lithium battery
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 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, causing the loop current to drop and prevent the temperature from rising further.
The fuse is equipped with a fuse with an overcurrent fuse function. Once there is a risk of overcurrent, the circuit is disconnected to avoid a malignant accident.
2. Lithium battery pack consistency problem
Lithium batteries can't be made into a big one, so we have to organize many small batteries, and everyone is going to make a concerted effort. At this time, you need to face a problem and consistency.
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.
The capacity loss, the battery cells constitute a battery pack, 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 overdischarging of the battery, the logic of the battery management system is set such that when discharging, when the lowest cell voltage reaches the discharge cutoff voltage, the entire battery pack stops discharging; when charging, when the highest cell voltage reaches the charging cutoff voltage , stop charging.
Take two batteries in series for example. One battery has a capacity of 1C 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.
Life loss, similar battery life, is 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.
The internal resistance increases, the different internal resistances, the same current flows, and the cells with large internal resistance generate relatively more heat. If the battery temperature is too high, the rate of deterioration will increase and the internal resistance will increase further. The internal resistance and temperature rise form a pair of negative feedback, which accelerates the degradation of the high internal resistance battery.
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.