Handling Thermal Runaway at the Pack Level
The content below by 朱玉龙 (Zhu Yulong) has been translated from 汽车电子设计 with permission.
Intro
Luxin’s (Net_Green) “How to Improve Energy Density While Ensuring Safety and Reliability? — Analysis of the ER6 Battery System” mentions several approaches to handling cell thermal runaway. We first take a look at these approaches, and then explore how to deal specifically with modules composed of NCM811 cells. As discussed with my friend Li a while back, the 811’s 51 Ah cell is not easy to deal with. In order for the 234 Ah and 156 Ah cells to meet the national standard, further sustained development is necessary.
ER6 Solution
Basically, the Roewe ER6 pack design adopts three solutions:
1. A complete fireproof cover within the pack
As shown in the figure below, this design uses a complete fireproof cover over all of the battery modules. The main purpose is to prevent flames from spreading when a single cell undergoes thermal runaway. This prevents the upper cover from being punctured and allowing oxygen to diffuse in during the process.
The fireproof cover consists of two layers:
- One layer is a silicone-based composite material (which hardens into a semi-inorganic material with excellent thermal insulation properties at high temperatures).
- The other layer is a very thin fiberglass material (to enhance the overall strength of the ceramic layer and prevent the material from falling apart while it is being formed).
An upper cover over the fire proof cover is constructed from an aluminum alloy sheet. The combination of a fireproof ceramic and a sheet metal alloy prevent the battery cover from melting during the high temperature and high pressure eruption caused by a cell thermal runaway.
2. Exhaust gas isolation and explosion-proof valves
The fire proof cover allows the hot exhaust gas to pass through the exhaust channels in the battery pack and exit through the explosion-proof valves on the body. Four one-way spring-loaded explosion-proof valves (see image) are placed on the outer edges of the pack, which enables large quantities of gas and heat to be exhausted in the event of a thermal runaway. A thin layer of steel is added to the inside of the explosion-proof valves to protect them from direct exposure to the high-temperature exhaust. The high-temperature gas passes through the exhaust channels and is blocked by a protective steel sheet before reaching the exhaust valve, which allows the temperature of the exhaust gas to be controlled to a certain extent.
3. Additional heat dissipation for cell thermal runaway
When the battery management system’s (BMS) real-time monitoring detects that a thermal runaway may be occurring, the battery pump will be run at full speed. With the aid of thermally conductive materials, the excess heat will be quickly transferred from near the thermal runaway cell to other locations in the battery pack, which effectively reduces the temperature.
“It should take longer for prismatic cells to transition to 811, in order to address the cell thermal runaway problem.”
How to Handle NCM811
While the methods for handling 523 and 811 are similar, doing so is still very difficult. Previously, Xiaoyu showed that 150 Ah and 234 Ah lithium batteries have different responses to the nail penetration test. I personally feel that, in addition to the above measures, it is necessary to increase the thickness of the thermal layer between the cells and delay the start of the next cell thermal runaway as much as possible. The reason being that, if the rate of the thermal runaway of the first cell varies widely, the entire safety system becomes very difficult to implement. It’s a bit like running a DOE experiment with a spread of results. Pouch cells handle this relatively well, and the pressure is released quickly. Of course, the thermal isolation will be more difficult, so it should take longer for prismatic cells to transition to 811, in order to address the cell thermal runaway problem. After all, we are not just targeting the national standard of 5 minutes, but trying as much as possible to prevent a single cell thermal runaway from spreading through the entire pack.
This kind of development requires a lot of simulations and experiments. It is really not easy to do well. Bravo to the researchers ^_^
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