[NickM]

> Lithium batteries degrade. 15-20% capacity loss per year. 2-5% per hundred cycles.

This is a gross overgeneralization. There are many different lithium-based battery chemistries available, with wildly differing lifespans.

The voltage at which you stop charging also makes a huge difference in lifespan. Charge a li-ion battery 10% less full and it can make an order of magnitude difference in its lifespan. When you charge a li-ion battery to full, it's not really truly "full"; the charging system just stops you from charging any further past a certain point, in order to stop you from degrading the battery too much. It's up to the device manufacturer to decide exactly when the battery is "full", based on what kind of tradeoffs they're willing to make on capacity vs lifespan.

High temperatures can also really increase the degradation rate for li-ion batteries, and Tesla employs a sophisticated cooling system to prevent the batteries from getting too hot while they're charging or discharging. The original Nissan LEAF had no active cooling, and their batteries degraded quite quickly, especially in hot climates. This was partially addressed in later model years, but it's now clear that this makes a huge difference.

Like most people, you're probably used to seeing heavy degradation over time in your cell phone or laptop battery. Just bear in mind that it's much easier for companies like Apple or Samsung to advertise "our phone gets 20% longer battery life!" than "our battery capacity degrades 5% less per year than our competitor!" Add to the fact that many people replace their cell phone every few years anyway, and the incentives are clearly aligned to keep pushing higher charge voltages, leading to greater short-term capacity at the expense of longevity. It's also a hell of a lot harder to fit a good battery cooling system in a smartphone than a car.