Recently, in discussing some technical details, it is worth discussing some of the differences in the design details of the battery system in the context of the PED Test. In this very important method, there are some differences with our understanding.
1.1 Test Object The test object must be the battery system or battery subsystem, and the test object must be consistent with that described in GB/T 31467.3-2015.
1.2 Test Procedure Perform the following steps at room temperature (25 ° C ±2 ° C) :
1) Discharge at the current not less than I3(A) specified by the enterprise to the discharge termination condition specified by the enterprise, and stand for not less than 30min; 2) Charge according to the charging method specified by the enterprise to the charging cut-off conditions specified by the enterprise (charging time is no more than 8h), and stand for no less than 30min;
3) Repeat Step 1), measure the discharge energy E(Wh);
4) Repeat steps 2) ~ 3) twice to obtain the average value of the discharge energy E of the three times.
5) Measure the mass M of the test object with A weighing instrument (measured in kg, including at least the components specified in Appendix A.1 of GB/T 31467.3-2015 when weighing);
6) Calculate the discharge energy density PED(Wh/kg) of the test object, the calculation formula is as follows: PED = Eaverage/M 2
This is the approach that we all use, but there's something there's something worth talking about. As shown below:
Although carried out at room temperature, but this test for convenience, so that the standing time is a little short, and there is no regulation of the standing inside the cell temperature
After the battery is charged and discharged, if the thermal management of the battery management system is not turned on, the temperature of the whole battery will gradually accumulate. In the following two experiments, the temperature will start to rise from 25 degrees, according to the heating characteristics of the battery
The actual capacity of the battery is one of the most important battery characteristic parameters, and the ambient temperature and the average discharge current of the battery are two major factors that affect the actual capacity of the battery. Different chemical systems and electrolytes have different sensitivity to temperature. In the lithium battery, the working temperature of lithium battery to maintain good performance is 0℃ ~ 45℃, below 0℃ belongs to the low temperature stage, above 45℃ belongs to the high temperature stage. When the battery is at low temperature, the available capacity of the battery will decrease. At high temperatures, the available capacity of the battery is a little more than at room temperature, about 3 to 6 percent more. The main reason for this phenomenon is that some of the performance of the electrolyte will change with the temperature: when the surface temperature of the cell rises, the viscosity of the electrolyte decreases, the activity improves, the diffusion ability of the ions in the electrolyte increases, the utilization rate of active substances improves, and the actual battery capacity increases. The real available capacity of a battery is roughly proportional to temperature. In the study of particle motion in lithium ion battery, the deembedding of Li+ and the transport capacity of electrons in the reaction process at different temperatures were compared, and it was found that the diffusion rate of Li+ was more sensitive to temperature. At the appropriate temperature, the diffusion rate of Li+ increases significantly with the increase of temperature, while the electronic conductivity does not substantially increase, indicating that the increase of temperature may increase the discharge capacity of lithium ion batteries by accelerating the diffusion rate of ions. When the temperature of the cell exceeds 35℃, the capacity of the battery increases with the increase of temperature, and the temperature has relatively little effect on the capacity of the phase above 45℃ compared with the capacity of the phase above 25℃. When the temperature is high, although the capacity of the battery can be improved to a certain extent, it will produce irreversible substances in the process of chemical reaction, and the generation rate is strengthened. And with the increase of the number of charges and discharges, the rate of decay of the available capacity of the battery increases.
For this problem, at the core level, if we further pass different temperature tests, we can do some different tests at this point, and the following capacity deviation can be obtained.
