Tesla Model S battery degradation data

Meanwhile, the 2006 Prius I bought two years ago just hit 110K miles (purchased with 80K miles), cost $10K, and has needed nothing more than new tires and a replacement cooling block on the engine (which was deferred by the previous owner). I plan on running this thing until they no longer can service it, a full swap of the hybrid transaxle is relatively cheap as is replacing the engine, but with regular maintenance they’ll probably last 500K or more and I’ll only have to replace the battery at some point.

Always do research on the reliability and cost of repairs on a car before you buy it, this alone was why I picked the Prius over other cars on the lot (there’s very little that can need serious repairs because of how simple the ICE design is, and the electric drivetrain components are pretty long lived as well).

But the reasons they need to be replaced often is more about 1. Not keeping them at/above 40% charge, and 2. Laptop batteries don't balance the cells. Most laptop batteries can get 80% capacity back just by replacing one or two crap cells.

Li-Ion battery degradation is generally modeled as two (roughly additive) components, called calendar and cycle aging. Calendar aging is what you're talking about in your comment - it's basically determined by the temperature and state of charge. Both of these impact the rate of chemical processes that lead to loss of lithium and active material. This is what happens when the battery is sitting on the shelf.

Cycle aging, on the other hand, happens when you charge or discharge the battery and is driven mostly by the actual volume change in the anode and cathode when they get lithiated (it's quite significant, up to 15% or so in some materials). This introduces mechanical stress, which can break the protective film that forms between the electrodes and electrolyte and allow chemical degradation to proceed at a faster rate. The degree of mechanical stress is mostly determined by the depth of discharge, although rate of charge and discharge is also believed to be important. Cycle degradation is also impacted by the temperature at which the battery is cycled, in a similar way to calendar aging.

With high temperature variance (low temperatures will wreck your batteries too, and I can't recommend keeping them at 5C), calendar aging is the dominant mode. This is absolutely the case with cell phone batteries. However, electric vehicles, especially higher end ones like the Tesla, have an active cooling system that keeps the battery at a constant temperature, even when the car is "off". This is why many EVB warranties will be voided if you let them run out of charge for more than 14 days - at that point, the cooling system isn't working and your poor battery is at the mercy of the elements.

Under closely controlled temperatures, cycle aging is the most important factor, and it's dominated by depth of discharge. If you drain the battery as far as it will allow you every day, it's going to be hosed. If you only go down to about 80% of the allowed charge every other day, it'll last for quite a while. These numbers are roughly correlated, obviously, with the mileage on the car, but the relationship is not simple, and I'd caution against considering mileage to be a good determinant of battery degradation.

Some good sources if you want to learn more:

A fairly simple model of degradation (ignores charge/discharge rate): https://www.researchgate.net/publication/303890624_Modeling_...

A thesis on the subject, goes into great detail and has an excellent bibliography: http://webfiles.portal.chalmers.se/et/Lic/JensGroot.pdf

There are also some (rather weak) reports that if you take the individual 18650 cells out of a tesla battery they last significantly longer than the NCA 18650s you can actually buy from Panasonic, which indicates that they might made to better manufacturing tolerances or something, but it's really all about Tesla's treating the batteries much better during use.