The 4680 Cell - A Deeper Dive
Less resistance = more efficient
The 4680 form factor is not just a larger cell that can pack more energy. If it were that simple, other battery manufacturing suppliers would most likely be pursuing those cells. Tesla has relied on leaders such as Panasonic, LG Chem, and CATL for years, so why wouldn’t they just partner with them?
The new cell form factor has a novel internal design which requires fewer electrical connections and lesser needs for thermal management (cooling).
A typical cylindrical cell is made via a jelly roll winding process around a spindle. Conductive tabs are then laser welded to the anode and cathode so that they can be connected into the circuit. Tesla will change the game by introducing a tabless cell made by laser patterning conductive foils in a shingled format as seen below. This format allows for dozens of electrical connections within the active material.
The “dozens” of connections mentioned during Battery Day is how the cell will maintain and actually improve its efficiency versus the smaller 18650 and 21700 cells. The electrical path length will reduce from 250mm to 50mm, and increase the power to weight ratio of the cell. There will be far less energy loss, because electrons will have a shorter distance to travel.
Pack Design
When arranging the final cell pack in the center of the vehicle there will also be more connection points for the proper parallel/series configuration. The connections could be within the cell rather than just on the top/bottom caps likely resulting into a shorter path length between cells.
This will ultimately make the battery pack and vehicle more efficient. Electrons that have to travel long distances will experience resistance, which in turn generates heat. Heat has a negative impact on a battery’s cycle life as shown below. In a Li-ion battery containing liquid electrolyte, heat can facilitate crystallization of lithium salts that limit ion mobility. It is critical that electric vehicles have heating and cooling mechanisms to maintain the battery temperature at or near 25C.
If one were to try to increase the current in order to enable those electrons to move faster this would only increase resistance/heat and cause the battery life to decay faster.
For context think about a standard 15 amp, 110v outlet in your home. It is most likely supplied by 14 gauge copper wire that carries the electricity from your electrical panel. If you wanted to try to increase the current, it would be more difficult to squeeze more electrons through the same size wire. Resistance tends to generate heat, and this is the basic concept behind how you dial in the heat setting with appliances such as an oven or space heater.
Thermal Management
In order to combat the potential for heat generation in 21700 cells in the Model 3, battery pack design engineers developed a thermal management system to keep the individual cells cool. A tube containing glycol is run through a string of cells. During operation the cells can get warm, but the constant flow of glycol through the system helps keep them cool and efficient.
If the new 4680 cells do not generate nearly as much heat as the current 21700 cells in the Model 3 is there even a need for a thermal management (cooling) system? Imagine the module above without any cooling lines, pumps, or liquid glycol. How much weight saving could that contribute?
The 4680 cell presents opportunities for more than just range improvement. The architecture and complete rethinking of the Li-on battery cell could shape a complete internal redesign that results in an even more efficient and cost effective electric vehicle.
Ps. In case you were wondering, Tesla did the work and determined that 46mm was the optimal height for maximum range and cost reduction.
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