Thoughts on Tesla : Part 2-1, Mechanical Safety
There is clear structure and intent to the Tesla pack design – it’s relatively straightforward without overly complex physical structures that may complicate manufacturing and/or serve as restrictions to parts supply. It’s evident from its beginnings and I’d like to focus on the safety aspect of the design in this Part 2 discussion. Please consider that these are simply my opinion on this subject matter and may not be as detailed. However, this is not a fact-finding mission but rather an attempt to organize the importance of intent in engineering.
Every safety regulation becomes more stringent in passenger carrying vehicles compared to simple electronics. And when considering that compared to simple electronics, EVs hurtle down the freeway at over 100 kph and must function properly between extreme heat and cold… the task of designing an effective battery system is extremely difficult to say the least. Throw in the fact that there is a limited thermal, mechanical, electrochemical window in which the battery material shows optimal performance, the task seems near impossible unless serious design features are considered to accommodate the battery as an EV power train.
This seems to have been understood by Tesla as clearly evidenced by its two-track approach in its open patents – thermal safety and mechanical safety. Figure 1 is my attempt at grouping some Tesla patents to visually show the care put in Tesla’s pack design and shows that problems have been clearly defined from the start and my discussion will be based on investigating each patent listed below.
Figure 1 - brief summary of my understanding of Tesla’s approach to EV design
Overarching Foundation
US 7,671,565 B2 - BATTERY PACK AND METHOD FOR PROTECTING BATTERIES
This patent, which was filed in 2006, is simple but seems to be the foundation of cylindrical cell application in EVs. I’d like to point out three aspects from Figure 2 (Figure 1A of Patent).
Figure 2 - Figure 1A of Patent US 7,671,565 B2
118/120 – The Cylindrical Cell Holder
Fixes the cylindrical cell onto the battery pack while keeping each cell electrically isolated. Note how it holds only the top/bottom of the cell thereby exposing the battery side to enable efficient exchange of heat energy via air or liquid coolant cooling.
The batteries are physically separated from each other, which gives some free space crucial for crush abuse and thermal propagation tests.
To be fair, all youtube videos that show Tesla battery pack tear-down hints at the fact that the gap between the cells in the figure on the right is exaggerated but the intent is clear to be seen.
144 – Wiring
Acts as an electrical connector to the EV energy storage system that allows current to be charged/discharged when required.
In case of overcharge, the thin wire heats up to close to material melting point and severs its connection to the battery thereby protecting the battery from explosive events due to long term exposure to overcharge abuse conditions.
The reason why the wire can be thin is because there are thousands of cylindrical cells sharing the power load of the EV. With a large number of cells, each cell experiences several tens of Amps, even for high power throttles, which means the individual cell wiring can be designed to withstand only up to several tens of Amps.
This is another advantage of the cylindrical cell when it comes to deciding on form factors - but, this advantage is expected to reduce as cylindrical cell sizes become much larger.
140/150 – Current Collector Plate
Each cell is electrically wired to a current collector plate, which covers the entire cell brick and, in a larger sense, the entire battery pack.
From a thermal management point of view, this is efficient as the large surface area of the current collector plate will enable rapid cooling. From a manufacturing point of view the application of large parts that require less precision machining is efficient. There is no need for small parts.
From a cost and weight perspective this is not as efficient but remember that the design of an effective EV starts and ends with an effective battery pack design. And because Tesla effectively designs everything from battery to the finished EV, it can make strategic decisions of giving some leeway to battery packs and making up for the short-comings with other aspects of the EV.
US 9,065,103 B2 - BATTERY MOUNTING AND COOLING SYSTEM
This patent sort of summarizes the various schemes and design choices that have been implemented and considered in order to mass-produce a safe and effective EV. A lot to comment on but will only briefly touch on some points in this section.
FIG. 12B – Overall Pack Design
Cooling jackets are employed between cell groups with robust structural support ‘beams’ going through the entire battery pack.
The current collector plates mentioned above can be seen in a macroscopic scale and the structural support it could bring is clear to see - although questions on potential short-circuiting risks as a result of vehicle impact should be asked.
1018 – Integrated Mounts for Individual Cells
Injection molded holes were placed adjacent to individual battery cells in the pack design that provides six cooling channels for each of the inserted cell in case liquid cooling jackets are not employed.
The integrated mounts underline the notion that Tesla understood the importance of thermal management in LIBs and tried to integrate this into its pack design and both air cooling and liquid cooling were considered.
Figure 3 - Figures 12B and 10A from US 9,065,103 B2
Mechanical Safety
US 8,702,161 B2 - SYSTEM FOR ABSORBING AND DISTRIBUTING SIDE IMPACT ENERGY UTILIZING AN INTEGRATED BATTERY PACK AND SIDE SILL ASSEMBLY
This particular patent is probably old news for vehicle designers and engineers but it shows how to work around the large battery pack when designing a robust EV system. After all the Tesla’s skateboard battery pack design takes up the entirety of the EV area and therefore conventional structural support designs cannot be directly implemented.
Figure 4 - Figure 1 from US 8,702,161 B2
FIG. 1, 105 & 107 – Pack Side Sill & Front Beam Design
Shows the overall structural design points that are implemented in absorbing and distributing side impact energy to prevent damage to the battery pack. Therefore hollow and corrugated structures that are designed to absorb impact in order to cause minimal battery pack deformation.
For side impact, the key is the internal hollow, cross-beam structure of 105, the transverse support provided by 103 and 101, and the curvature of 109 and 113 providing impact absorption. For front impact, the key is 107 as it provides direct protection and 109 that provides the same curvature for impact absorption.
Figure 5 - Figure 15 from US 8,702,161 B2
FIG. 15 – Pack Side Sill Structure
Red box outlines the classic cross beam structure that is implemented as the side sill. This is analogous to common architectural and metal parts designs that has been tried and tested throughout. A frame by frame collapse of the side sill structure upon impact is shown via simulation in the patent, which show that the battery pack itself is not damaged during collision.
Blue box outlines the plate design that houses the battery pack and the corrugated design that also adds structural support element against impact force.
US 8,361,649 B2 - METHOD AND APPARATUS FOR MAINTAINING CELL WALL INTEGRITY USING A HIGH YIELD STRENGTH OUTER CASING
This patent is quite simple in concept – add additional layer of structural support to help maintain cell wall integrity during thermal runaway thereby preventing thermal propagation and abnormal abuse scenarios that may impact adjacent cells. No doubt the addition of the secondary can will help the structural robustness of the battery system itself as preparing for high temperature abuse pretty much safely covers room temperature abuse.
The key points are better encapsulated in the summary of the patent rather than the figures:
Recognition that material properties changes in high temperature / high pressure environment.
(This seems to be material science 101 - phase diagrams)The secondary can needs to be a high yield strength material comprised of single layer or multi-layer materials.
(Multi-layer materials are the core research topic of Ph.D. Advisor - it may be expensive and hard to produce but are a great choice when it comes to harnessing the best of several materials)All materials must be above 250 MPa of yield strength and ideally even at temperatures as high as 1000℃.
Please remember all these points were considered for the 18650 form factor, which I believe highlights Tesla’s attempt at incorporating electrochemistry into the automobile industry. In addition, these points will become ever more important with larger cell designs including the new 4860 cylindrical cell. However, the addition of a secondary can - and other structural support designs - cannot be simply applied as the added parts at the cell level may lead to unintended changes in mechanical shock and vibration behavior.
As I repeat again and again - the big picture impact must be considered.
A short summary - an investigation was performed on several of the fundamental Tesla patents that revealed the careful considerations made during preliminary stages of EV design and several mechanical safety related Tesla patents that shed light on the thought processes on marrying vehicle design to battery system design.
The discussion on thermal safety will be carried on in Part 2-2.
