Reading guide:The consistency of the self-discharge is one of the important part of the effect factor,the SOC of battery with inconsistency self-discharge will have a biggish difference after a period of storage and its capacity and security will be greatly affected. It help us to improve the overall level of our battery pack,get longer life and lower the fraction defective of the products through our studying.
When we put a battery which containing a certain amount of electricity at a certain temperature and keep that way of a while,it will lose some of its capacity. To make a long story short, self-discharge is a phenomenon that the battery lose by the subsidiary reaction.
The Importance of Self-discharge
Currently, lithium battery are used more and more widely in laptop,digital camera and other digital devices, besides, it also have board prospects in vehicle,mobile base station, energy b storage power station and some other areas.Under these circumstances,battery is not only show up alone like in just a cell phone but also will show up in series or in parallel.
The capacity and life span of the battery pack is not just only related to every single battery, but also more related to the consistency between every single battery.Poor consistency can greatly drag the manifestation of the battery pack.
The consistency of the self-discharge is one of the important part of the effect factor,the SOC of battery with inconsistency self-discharge will have a biggish difference after a period of storage and its capacity and security will be greatly affected. It help us to improve the overall level of our battery pack,get longer life and lower the fraction defective of the products through our studying.
The Mechanism of Self-discharge
None of the above reaction occurred when battery open circuit,but the electricity still be reduced, that’s because of the self-discharge.The main causes of self-discharge are:
1.Internal electronic leakage caused by partial electron conduction or other internal short circuit of electrolyte
2.External electronic leakage caused by poor insulation of battery sealing ring or gasket or insufficient resistance between external lead shells (external conductor,humidity)
a.Electrode/electrolyte reaction,like the anode is corroded or cathode is restored due to electrolyte and impurities.
b Local decomposition of electrode active materials
3.The electrode is passivated because of decomposition products (undissolved substance and adsorbed gas)
4.Mechanical wear of electrode or the resistance(between the electrode and collector) increases with current collector.
The Influence of Self-discharge
1.Self-discharge leads to the decrease of storage capacity.
Here are some typical problems caused by excessive self-discharge:
- Vehicle park too long and can’t start.
- The battery voltage and other parameters were as usual but the voltage wqas found lower or even down to zero when the battery is delivered.
- We put the vehicle GPS on the car in summer, after a period of time, it seems that the quantity of electricity or service time is obviously insufficient, and even accompanied by battery drum.
2.Self-discharge of the metal impurity type caused the blocking of diaphragm pore diameter, even piercing the diaphragm and bring about local short circuit,endangering the battery safety.
3.Self-discharge leads to the increase of SOCdifference between batteries and make decrease of the battery pack capacity.
Because of the inconsistency of self-discharge, the SOC of the batteries in the battery pack is different after storage ,and the battery function decreases.Clients can often find the problem of performance degradation after getting the battery pack that has been stored for a period of time. When the SOC difference reaches about 20%,the capacity of the combined battery only left 60%~70%.
4.The biggish difference of SOC is easy to make overcharge and over discharge of battery.
Ⅰ.Distinction between chemical and physical self-discharge
1.Comparison between high temperature self-discharge and normal temperature self-discharge.
Physical micro short circuit has obvious relation with time,and long time storage is more effective for physical self-discharge selection.
It is in the way of high temperature 5D and room temperature 14D:if the self-discharge of battery is mainly physical self-discharge, the normal temperature self-discharge /high temperature self-discharge is approximately 2.8;if it is mainly chemical self-discharge, the normal temperature self-discharge /high temperature self-discharge is less than 2.8.
2.Comparison of self-discharge before and after cycling
The cycle will make micro short-circuit melting inside the battery, which will reduce the physical self-discharge.Therefore, if the battery self-discharge is mainly physical self-discharge, it will be significantly reduced after the cycle;if it is mainly chemical self-discharge, then it will has no obvious change after cycle.
3.Leakage current test under liquid nitrogen.
The leakage current of battery is measured by high voltage tester under liquid nitrogen.If the following condition occur, it indicate that the micro short circuit is serious and the physical self-discharge is large.
·Under a specific voltage, the leakage current is large.
·Under different voltages, the ratio of leakage current and voltage are having big differences.
5.Comparison of self-discharge in different SOC
The contribution of physical self-discharge is different in different SOC situation. It is easier to distinguish the battery with abnormal physical self-discharge under 100 %SOC through the experimental verification.
1.Self-discharge detection method
Voltage drop method
The rate of voltage decrease during storage is used to characterize the self-discharge.This method is easy tp work but the disadvantage of method is that the voltage drop can not reflect the loss of capacity directly.Voltage drop method is the most simple and practical method, which is widely used in current production.
Capacity attenuation method
That is, the percentage of capacity reduction per unit time.
Self-discharge current method
According to the relationship between capacity loss and time, the self-discharge current ISD of the battery during storage is calculated.
Calculation of Li + mole number for side reaction consumption
Based on the influence of the electronic conductivity of the negative SEI film on the Li + consumption rate during storage, the relationship between Li + consumption and storage time was deduced.
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