Tuesday, November 11, 2014

31. A PEEK INSIDE THE BATTERY OF A TESLA MODEL S                           

A single lithium-ion 18650 cell is relatively small in size and in capacity. So how does Tesla pack 85,000 W.h in the battery pack of the Tesla Model S? The answer is very carefully.

The battery pack in a Tesla S is a very sophisticated assembly of several thousands of small individual 18650 cells connected electrically in a series* and parallel combination. A colleague alerted me recently to an outstanding teardown activity that an owner of a Model S is performing on his battery pack. This offers a great peek into how Tesla designed this battery pack.

First, it is very important to note that 85 kWh is a huge amount of energy. The voltages at the terminals of the pack are high, and the currents can also be dangerously high. In other words, safety is of paramount importance in the design, assembly, and equally the teardown of a large battery pack of this size.

The photographs that this owner has published are very telling and provide a great insight into the design of this pack. The first photograph shows the entire pack with its top cover removed.

Photograph of the battery pack for a Tesla Model S electric vehicle. Courtesy: Tesla Motors club user [wk057]. 

This photograph shows a total of 16 sections, or modules, electrically connected together. A closer inspection of one individual module shows that it contains a number of 18650 cells, all sitting next to each other in a vertical position. One can diligently count a total of 432 individual cells in one single module. 

Therefore, the first observation we can make is that there are a total of 16 x 432 cells = 6,912 cells. The capacity of each cell is 85,000 / 6,912 = 12.29 W.h., or equivalently, 3.4 A.h. The individual cells are of the 18650 type, manufactured by Panasonic. They use an anode made of graphite, and the cathode is made of NCA (nickel-cobalt-aluminum alloy). The NCA-graphite architecture has a lower nominal voltage than the cobalt-oxide alloy commonly used in mobile devices. The nominal voltage of a NCA-based cell is 3.6V.  The owner measured the module voltage to be 19.63V when the battery was virtually dead. A dead (i.e., empty) cell has a voltage near 3.2V. 

Photograph of one individual module from the pack.
Therefore, our second observation is that each module contains 19.63V/3.2V = 6 cells in series. Consequently, the module is configured as 72 parallel legs, each containing 6 cells in series (abbreviated as 6s x 72p).

The energy of one single module is 85,000 / 16 = 5,312 W.h. This is equivalent to the energy contained in about 100 (yes, one hundred) laptop PCs. A closer examination of the module assembly shows that each cell is wired to the main bus (the primary electrical path) through little fuses...this is an outstanding safety feature that will disconnect an individual cell that may have shorted with time.

Photograph showing the fuse wires connecting individual 18650 cells to the main bus.
Our third observation is that the entire pack consists of 16 modules connected in series, therefore the overall architecture is 96s x 72p. The stack voltage is nominally 96 x 3.6 = 345 V, but would be as low as 310V when the pack is nearly empty, and 403 V if the battery is at 100% full (but Tesla does not recommend that you charge the battery to 100%).

Our fourth observation is about weight. Panasonic specifies a weight of 46 g for each 18650. The weight of all 6,912 cells comes out to be 318 kg or about 700 lbs. The weight of the entire battery pack is estimated by various sources to be 1,323 lb. So the 18650 account for approximately 53% of the weight of the pack -- the rest is due to electronics, cooling systems, wiring and safety.

Judging from my earlier post on cost trends, the estimated cost is about $1.50 for each 18650 cell. I am assuming that the Tesla sourcing team is very influential in demanding attractive pricing from the cell manufacturer, Panasonic. This equates to a cost of approximately $10,000 for the cells used in the pack. Given a delivery rate of about 35,000 cars for 2014, that equates to nearly $350 million that Panasonic will collect this year from selling cells to Tesla...and the number may grow to $1B in 2015.

Adding another estimated $5,000 for the cost of the electronic battery management systems, and one has a preliminary material (BOM) cost of $15,000 for a pack used in the Tesla Model S. That equates to less than $200 for each kWh of stored energy. It also works out to about $50 of battery cost for each mile of driving range. It's amazing what one can derive from a handful of photographs!

* A series configuration means the positive terminal of the first cell is connected to the negative terminal of the second cell. The voltage at the free terminals is now the sum of the voltages at each cell. The series combination allows raising the voltage of the battery pack to much higher voltages.

† A parallel configuration means electrically connecting the positive terminal of the first cell to the positive terminal of the other, and the negative terminal of the first gets wired to the negative terminal of the other cell. The voltage at the terminal of one cell is identical to the voltage at the terminal of its sister cell. A parallel configuration allows the addition of capacity without raising the voltage.

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