Because of the following reasons:
1. Elon said they will stop at 100 kWh (
source). I'm pretty sure he doesn't mean forever but at least for a while, maybe until the end of 2018.
2. In conference calls, there is too much talk about profit margins. Keeping the pack at 100 kWh would lower costs and increase profit margins.
3. The Model S battery pack has a hump section with two modules stacked on top of each other. Keeping the capacity at 100 kWh would get rid of the hump section. All modules would be at a single level. This reduces production time and costs of the battery pack casing.
4. If they stay at 100 kWh for a while and increase the capacity of Model S/X and 3 all at the same time, the range numbers will be more balanced. The Model S 100D is expected to have 343-350 miles EPA rated range. The Model 3 75D would have 304 mi and Model 3 80D 325 mi EPA rated range. These numbers look balanced. I don't know if the largest battery of Model 3 will be 75 or 80 kWh but in either case, the range will be less than S100D.
Therefore it makes sense to increase all three car models at once to keep the numbers more balanced. You might say, why wouldn't they just release the Model 3 with an 85 kWh battery and increase the Model S to 105/110? I agree that would keep the numbers balanced as well but financially it doesn't make sense to push the Model 3 battery capacity during the first year. First-year profit margins will be low anyway. Pushing the limits of Model 3 battery capacity will lower the profit margins even further.
Here is the ideal scenario I can think of:
- Model 3 launches with 55 and 75 kWh versions in Q3 2017.
- Model S/X stays at 100 kWh until late 2018.
- At the end of 2018, Tesla increases Model 3 battery capacity from 75 to 85 and Model S capacity from 100 to 110 kWh.