Power battery normal temperature overcharge and adiabatic overcharge comparison

Power battery normal temperature overcharge and adiabatic overcharge comparison.

For those concerned about battery safety, 2019 is destined to be an extraordinary year for three reasons. First, in the first half of the year, many well-known brands (Tesla, Weilai, Geely, etc.) have caused fire accidents in the first half of the year. Secondly, the CATL NCM811 battery is officially supplied in volume production. Geely Geometry A, GAC Aion S, Xiaopeng G3 (2020) and other models will be equipped with NCM811 battery packs. Third, "Power batteries for electric vehicles" The "Safety Requirements" report has been submitted to the WTO and will be released soon. Mencius said that "fish and bear's paw can't have both". For lithium-ion power batteries that also use organic electrolytes, the high probability of battery energy density in the current technical background means the sacrifice of battery safety.

Battery safety is always relative and there is no absolute safety. Within the normal safety margin, the safety of battery use can be fully guaranteed; but under extreme abuse (such as overcharge, acupuncture, over discharge, heating, etc.), thermal runaway of the battery is a high probability event or even a necessity. . Therefore, a major responsibility of battery safety engineers in enterprises is to study the failure mechanism and performance of batteries under various extreme abuse conditions. Under the premise of considering battery performance and cost, the thermal runaway threshold under extreme battery abuse conditions should be improved as much as possible. , the possibility of thermal runaway is further reduced. In various abuse tests, overcharging is relatively demanding. Especially for the NCM811 system battery, it is common to carry out overcharge test according to GB/T31485.

 

Ahmed Abaza graduated from Warwick University in 2017. During his Ph.D., he worked with Jaguar Land Rover's battery development engineer Ronny Genieser on power battery abuse safety research. After graduating in 17 years, he joined Jaguar Land Rover as a high-voltage battery engineer. Dr. Abaza's research focuses on acupuncture, external short circuit, and overcharge. His doctoral thesis can be found online. Interested friends, especially those who have just started to contact power battery safety research, can download the link at the end of this article. In Ahmed Abaza's doctoral thesis, there is a chapter comparing the power battery over-temperature and adiabatic over-charge. Generally, the battery safety test overcharge is carried out at room temperature, and the adiabatic overcharge is relatively rare, so the result has certain reference value and is hereby shared.
Graphic analysis:
1. Basic Information
Table 1. Battery information used in the experiment.

Figure 1. Battery adiabatic overcharge test layout.
As shown in Table 1, the battery used in the experiment was a soft pack battery of 15 Ah LMO-NMC mixed positive electrode, and the normal voltage range of the battery was 2.7-4.2 V. The battery adiabatic overcharge is performed in the ARC adiabatic thermal instrument, as shown in Figure 1. The battery is not constrained for processing during overcharge testing. The cut-off condition for the overcharge test is that the battery voltage reaches 7.9 V.
2.Normal temperature overcharge

Figure 2. Comparison of battery normal temperature overcharge results: (a) 1.0 C overcharge; (b) 0.13 C overcharge.
As shown in Figure 2, the battery voltage of 1 C at room temperature overcharge for about 40 min reaches 7.9 V, while the overcharge at 0.13 C requires about 750 min to reach 7.9 V, 750 min/40 min=18.75>1 C/0.13 C = 7.69, indicating that overcharge has an important effect on the overcharge behavior of the battery, and the effect is not a simple linear relationship. From the surface temperature of the battery, the maximum temperature of the 1 C overcharged battery surface is 70 °C, and the maximum temperature of 0.13 C overcharge does not exceed 40 °C, and the battery does not have thermal runaway. In addition, it is worth noting that whether it is 1 C overcharge or 0.13 C overcharge, the voltage battery rises first and then appears similar to the platform at 5.0 V. Finally, the voltage rises rapidly to 7.9 V cutoff. Considering that the current value of the overcharge process is constant, the author believes that the platform area of about 5.0 V is the combined result of the increase in internal resistance and the short circuit of lithium induced by the gas bulging of the battery, and the subsequent step of the voltage to 7.9 V is the battery. The results of serious gas production and significant increase in internal resistance account for the main factors.

 


Figure 3. Comparison of battery normal temperature overcharge results.

 

 

 

Figure 4. (a) Photo of the battery after overcharge at 0.13 C; (b) Photograph of the battery after 0.33 C and 1.3 C overcharge.

Figure 5. Comparison of different rate adiabatic overcharge results for batteries.
Unlike normal temperature overcharge, the internal heat generated by the adiabatic overcharged battery cannot be completely dissipated, and the heat accumulation of the battery is more serious. As shown in Table 2 and Figure 5, the 1.3 C, 0.33 C, and 0.13 C overcharged battery voltages reached 7.9 V cutoff time of 58 min, 237 min, and 602 min, respectively, although the time was different but the excess capacity was calculated. The ones are very close (19-20 Ah), and the battery failure SOC is around 230% SOC, which is quite different from the non-linearity of the battery overcharge. In addition, in normal temperature overcharge, it is said that 0.13 C or 1 C overcharged batteries only bulged without thermal runaway, but under adiabatic overcharge conditions, 0.33 C and 1 C overcharged batteries both ignited during standing. The 0.13 C overcharged battery broke. This also indicates that the adiabatic overcharge test conditions are more severe and harsh than the normal temperature overcharge, and the battery is more susceptible to thermal runaway.

 

Figure 6. Comparison of different adiabatic overcharge results for batteries: (a) 0.13 C; (b) 0.33 C; (c) 1.3 C.
As shown in FIG. 6, the different-rate adiabatic overcharge batteries do not cause thermal runaway during the charging phase, but cause problems in the subsequent stationary process. 0.13 C, 0.33 C, and 1.3 C were adiabatic. Do not allow the battery to ignite or break at 14 min, 49 min, and 48 min after overcharge.

The overcharge test done by Dr. Ahmed Abaza gives us the following information:
(1) There are many factors affecting the final result of the overcharge test. The heat dissipation conditions, charging rate, battery chemistry system, etc. will all affect the results. The specific analysis should be analyzed in the test and analysis. Do not generalize.
(2) Normal temperature overcharge is not a simple linear relationship between battery performance and magnification due to heat dissipation. However, considering the scene of the power battery in practical applications, the normal temperature overcharge is closer to the real situation than the adiabatic overcharge.
(3) The failure SOC of adiabatic overcharged batteries is more consistent, and there is a certain linear relationship between the performance and the rate of the battery. This also indicates that the adiabatic overcharge can reflect some of the thermal characteristics of the battery.

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