The cathode in Li ion batteries forms defects (microcracks) due to stress caused by expansion and contraction relative to neighboring materials. It is simply bigger or smaller depending on whether it's charged or discharged. The more you discharge it, the more it changes size. And the more it changes size, the more microcracks it gets. These microscopic cracks lower the battery's capacity. That's one reason why smaller cycles, more often, contribute to longer battery life. That's why you should plug it in every night.
Heat aggravates the microcracking process, so keeping your battery cool contributes to longer battery life. And guess what? Your battery heats up more when used at a lower SOC because it requires more amps to keep your car going 65mph than it does at a higher SOC. That's why you should charge it every night.
Capacity fade also comes from the build-up of non-soluble deposits on the anode and cathode. This chemical process happens faster when the battery is warmer. It also happens faster when at a high SOC. But the process slows to a crawl when you drop the SOC to 80 or 90%, and slows only a tiny bit more at 50%. So if you are going to drive your car, keeping it charged in std mode has less impact on battery life (lower amps, less heat) than driving at a lower SOC. But if you're not going to drive your car for a few days, there are no amps or heat to worry about. That's when Tesla recommends putting it in storage mode, which keeps it at a lower SOC.
end of science lesson.
(But 70 in the daytime.) For me, unexpected trips are always expected, so I can charge extra if I need to. But once a week works great 95% of the time and I rarely get below 50% SOC.
