18650 compared to a 20700, both 3D printed. 1:18th Scale Diecast Model S for scale - photo via Aaron Cocker |

Back in May, I wrote a story that said the Tesla Model 3 would have half the number of battery cells of the Tesla Model S. This was based on many assumptions. Some of these assumptions have now been clarified.

During the 2016 Annual Shareholder Meeting, at the 1 hour, 48 minute mark, Elon Musk and JB Straubel talk about the motivations to build the Gigafactory and the innovations that they are putting into both the batteries and the factory itself.

Elon Musk confirmed that the battery cells made at the Gigafactory will be bigger than the standard 18650. He also gave a precise size. Tesla's battery will be 20mm in diameter and 70mm in height. The 18650 is so named because it is 18 mm in diameter by 65 mm height. Using the same naming convention, the new Tesla battery cell would be a 20700 battery cell.

18650 compared to 20700 via Seeking Alpha |

Elon clarified that Tesla does not use commodity laptop 18650 cells. They have been using cells specifically designed for their needs for some time now. It just so happens that they have continued to package these in the same 18650 form factor.

This change in size to 20700 was based on a first principles analysis. This new size allows for increased energy per cell while still allowing for thermal management and fire control within the pack.

On a related note, Quartz reports that Tesla has hired Jeff Dahn, a leading battery researcher who teaches at Dalhousie University in Nova Scotia. Dahn is tasked with doing “whatever it takes” to improve the company’s battery performance.

Tesla Model 3 will use a larger 20700 cell with a better chemistry than the current Model S.

#### How Many Cells?

The original article assumed the new cell would be 20% bigger in each dimension. With such a size increase, it was easy to have half the number of cells. That is, however, not correct. The increase is ~10% in each dimension. Given this update, let's guess at how many of the new 20700 cells will be in the Model 3.According to teslamotors.wiki, there are 7,104 cells (16 modules of 444 cells) in Model S. For Model 3, we'll be able to reduce that number with several factors. First, as discussed above, the Model 3 will use a larger cell. Second, the Model 3 will be smaller, lighter, and more aerodynamic. Third is the range, the base Model 3 will not go as far as the base Model S. The final factor is the battery chemistry will be one or two generations ahead of the 2015 chemistry that is in the Model S and X today. Let's look at each of these in detail.

The first one is easy, the cells are 33% larger. This means a 25% reduction in the number of cells needed for a given energy level. (75%)

The second item is the smaller, lighter, and more aerodynamic Model 3. The final specs for the Model 3 are not released yet, but we'll make an assumption and update this as data is available. Model S has an energy efficiency of 89 MPGe. Looking at other EVs we can guess at Model 3's efficiency. Looking at two examples, the 2016 Leaf has a 114 MPGe rating and the BMW i3 has a 124 MPGe rating. Neither of these carry as large of a battery pack as Model 3 will, but Model 3 will be more aerodynamic than either of these. So, assuming Model 3 will be in this range, that would be a 70 to 80% reduction in needed capacity. (75%)

The third factor is range. The only range known today about Model 3 is that the base model will be at least 215 miles. Assuming Tesla follows the Model S offerings, there would be one battery pack upgrade option, a dual motor upgrade, and performance options. Since we only have an idea about the base model, let's compare the base option 215 miles (worst case) to the base Model S 70. Model 3's 215 mile range is 92% of the 234 mile range of the Model S 70. (92%)

The final factor is the battery technology improvements. The Model S 90 kWh pack was first offered in July of 2015. The Model 3 is expected to start shipping in the 4th quarter of 2017. Battery advancements happen in fits and starts, there is no telling what, if any, improvements there will be above and beyond the July 2015 offering, but let's just take the trend on face value and assume a 5% improvement. (95%)

Putting these factors together and we have an estimate of ~50% of the cells of Model S. That is approximately 3550 of the new 20700 cells in Model 3. This is more than the previous estimate, but still less than half of the 7104 cells in the current Model S / X.

Tesla Model 3 will have approximately 3550 battery cells.

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