Thoughts on Tesla : Part 2-2, Thermal Safety
Continuing from Part 2-1, Part 2-2 will be overviewing the Tesla open source patents dealing with controlling thermal runaway of its cylindrical cells and battery packs.
Thermal Safety
US 7,433,794 B1 - MITIGATION OF PROPAGATION OF THERMAL RUNAWAY IN A BATTERY PACK
This patent is an overview of the mechanisms and design features that has been and is no doubt still is the bedrock for pack and management system design.
Figure 5 & 6 – Consideration for Feature Pack Design
Figure 1 - Figures 5 & 6 from Patent US 7,433,794 B1
There is consideration for in-plane and out-of-plane cylindrical cell configurations when clearly the strategy thus far has been centered around a single layer of cylindrical cells (Figure 1 of text).
Two Levels of Temperature Control
Everything is centered around the ‘sheets’, which are comprised of several cylindrical cell ‘blocks’ that are connected in series. This seems to enable effective monitoring of cylindrical cells in groups rather than having to monitor each one of the thousands of cells.
Figure 2 - Figure 1 from Patent US 7,433,794 B1
Figure 1 of patent (Figure 2 of text) strategically located thermal mass that transfers locally generated heat throughout the entire ‘sheet’. This seems to hint at the foam that encapsulates each individual cylindrical cell that numerous tear-down analysis youtube videos showed.
Use of cooling tubes that are arranged uniformly into each of the ‘sheets’ that are specifically noted to be primarily – but not limited to – aluminum tubes with propylene glycol and/or ethylene glycol mixed with 50% or more of water.
The patent specifically mentions that the purpose of the cooling tube is to ‘… keep the temperature uniform along the length of the cooling tube thus keeping the temperature of the adjacent cells uniform such that a zero gradient is formed’.
Sensors
Humidity sensor : This is a factor that I had not considered at all but the consideration for dew point of the environment is important so that condensation does not build up within the pack itself.
Smoke sensor : Detects combustion during thermal runaway and the patent specifically mentions that the placement of the sensors must be so that it is able to detect the thermal runaway in a matter of seconds or milliseconds.
Voltage detection : This is conducted in groups of bricks where any deviation from the ‘norm’ voltage is corrected. In a rudimentary case, the high voltage brick is discharged to meet the average voltage, but in a more complex case, the high voltage brick is discharged to charge the other bricks.
Additional Points
Figure 3 - Figure 3a from Patent US 7,433,794 B1
28 – Collector Plates : Provide additional thermal mass and conduction pathway top and bottom of the cylindrical cells
18 – Packing of Cylindrical Cells : the hexagonal packing structure is the most efficient and was therefore selected.
“Thus, the combination of geometric spacing and thermal properties of the Surrounding materials is engineered in the present invention to optimize for best propagation prevention, the lowest weight for the entire energy storage system 12 and the most reasonable and Smallest packing efficiency for use in the electric vehicle. Therefore, transverse packing of the cells 18 and the associated spacing is optimized in the present invention.”Fuses are placed at the pack level, module level, and twice at the cell level.
The patent specifically mentions that “… the battery pack will not charge at temperatures below 0 °C.” though it will still allow the vehicle to drive in conditions as low as -20°C. To preserve chemical integrity the cells 18 must be heated above approximately 0°C. to 5°C. before charging will begin, once the ESS 12 has fallen below 0°C.”
US 7820,319 B2 - CELL THERMAL RUNAWAY PROPAGATION RESISTANT BATTERY PACK & US 8,647,763 B2 - BATTERY COOLANT JACKET
These patents provide a rough outline of what the battery pack will need to be to withstand thermal runaway. However, the key take away that I haven’t mentioned yet is Fig. 9 from the former – notice the surface area and the cooling of the greatest possible surface area possible. Although, due to the cylindrical nature of the cell, it is only point contact with the cooling tubes, this idea of ‘zero temperature gradient’ needs to be remembered.
Figure 4 - Figures 9 from Patent US 7820,319 B2 and Figure 14 from Patent US 8,647,763 B2
And from the latter take note of Fig. 14 – notice how the cooling tube was changed intentionally to be a cooling jacket to provide greater surface area for cell cooling.
US 7,736,799 B1 - METHOD AND APPARATUS FOR MANTAINING CELL WALL INTEGRITY DURING THERMAL RUNAWAY USING AN OUTER LAYER OF INTUMESCENT MATERAL
This patent is similar to the one introduced in Part 2-1 where the mechanical integrity of the cell wall is strengthened to prevent thermal runaway from affecting adjacent cells. The purpose is to prevent perforations from occurring and even if perforations are formed, the design attempts to inhibit flow of hot, pressurized gas out from the battery.
Figure 10 – Final Sleeve Design
Figure 5 - Figures 4 & 10 from Patent US 7,736,799 B1
Tesla performed other conventional improvement methods like cell wall thickness increase. The conclusion was that the thickness of the cell casing must be significantly increased to have visible improvements. And “if the conventional approach only adds 4 grams per cell, for a battery pack with 10,000 cells, this increase adds up toe 40kg”.
Therefore, a new approach was thought of by Tesla.The sleeve design is said to have several advantages including : i) only the inner casing will be perforated and leave the outer casing intact, ii) the sleeve redirects that hot gas escaping from the perforated inner casing to the cell ends, iii) achieves this with a significantly less added mass, iv) because the casing is added as a post-production step, the casing does not inhibit the range of suppliers that can be chosen, and v) there is not limitations to cell outer casing material as it does not have to be nonreactive with electrolyte, cell materials, high voltage environment, etc.
The preferred mechanical strength the sleeve must exhibit is 250~500MPa at room temperature and ideally 250~500MPa at high temperatures.
Tesla found that using three 25 micron thick layers of SUS was as effective with a single 100 micron thick layer due to the thermal contact resistance between layers.
A short summary - an investigation was performed on several of the fundamental Tesla patents dealing with several thermal safety related Tesla patents. It is clear to see - based on Parts 2-1 and 2-2 - that there has been carefully thought out intent behind each of the design choices. This is the definition of disrupting the market.
Part 3 will probably will be in two parts and will discuss batter management (more of a study rather than an investigation as it’s not my forte).
