2015 Tesla Model S 85

2015 Tesla Model S 85

We know that Tesla has squeezed more cells into the same physical size pack in their new P100D battery pack that was just announced August 23, 2016 in a conference call with journalists.

Tesla CEO Elon Musk & CTO JB Straubel talk all things battery and Gigafactory last month

Tesla CEO Elon Musk & CTO JB Straubel talk all things battery and Gigafactory last month

“The cell is the same – but the module and pack architecture has changed significantly in order to achieve adequate cooling of the cells in a more energy dense pack, and to make sure that we don’t have cell to cell combustion propagation” – Tesla CEO Elon Musk from the conference call on the new 100 kWh options.

So obviously there was room for improvement in the old 85 kwh pack design. If it was as good as it could be they never would have improved it –yes?

But how much improvement: Again we know the answer. One hundred kwh’s divided by 90 kwh is an 11% improvement.

Let’s put more numbers to it by looking at the old 85 kwh pack and applying some simple geometric calculations.

First, look at a photo taken of the old 85 kwh pack, and using power point let’s draw some circles around the cells. You can see the edge of the cells in the photo so one can draw the circles fairly accurately. More photos in this TMC forum post.

Tessa 85 kWh battery pack with circles laid over the cells. Photo courtesy Tesla Motors Club Forum poster wk047.

Tessa 85 kWh battery pack with circles laid over the cells. Photo courtesy Tesla Motors Club Forum poster wk047.

Now eliminate the photo underneath and just leave the circles and it is easy to visualize how closely the cells are packed. We can draw a box around the cell group and calculate the ratio of the cells divided by the total area. The ratio is  64%.

Model S 85 kwh cell density

Model S 85 kwh cell density

You can also see that the gaps between the cells where the cooling snake goes is larger than when there is no cooling snake. The ratio of the area of the cooling snake goes to the total area is 5% as shown below.

Ratio of area for cooling snake to total area is 5%.

Ratio of area for cooling snake to total area is 5%.

So you can see from this simple geometric analysis why there definitely was room in Tesla’s old pack design for improvement….and improvements in packaging have been achieved in the new P100D pack.

….but what has Tesla actually done to increase packing density? A conceptual design is proposed in this article.