Why is the asymptotic curve not realistic?
We all accept that any mechanical system wears over time right? Regardless of how gently it is used, a gear box, an ICE engine or a clutch will eventually wear to a level that it becomes unusable. We all have experienced this with over a century of ICE cars.
Chemical systems such as our battery packs are just the same. Chemical reactions take place at the beginning of the life cycle of a battery in a nice and clean environment. As the electrolyte gets older, ions will move less smoothly and in smaller quantities as worn material aggregates in sensitive areas of the electrolyte as well as on the anode and the cathode.
So, in that view, an asymptotic curve with an initial 5-10% degradation and then stabilization forever is simply not realistic. The battery
will see its capacity degrading over time until it becomes realistically unusable. Same goes for instant power BTW. I am sure that the instant power I use to enjoy in the early days of my EV driving (not any more as I became a gentle EV drive ;-) with a new pack is no more available after 300.000km and 5 years. A 0-100km/h test would be an good evidence: no way I can make it 4,2 sec anymore. Actually, one could see that initial 5-10% degradation as a sort of break-in period of time, a bit like a new ICE engine needs to run a few hundred hours before it can deliver its full potential. Except that, for batteries, things seems to go towards an initial (limited) degradation rather than (temporary) improvement as observed on an ICE.
There are many variables to be taken into account in analyzing battery aging: time life, stored and operating temperature, number of charge/discharge cycles (whether full 0-100% cycles or micro cycles such as acceleration/regen cycles repeatedly applied to the pack during normal driving), frequency in approaching 0% or 100% SoC, aggregate time with pack closed to 0% or 100%, etc.). Each of these factors individually will affect battery aging and, most importantly, will affect it even harder when combined together. So it is the combination of all these factors that will accelerate the capacity degradation at some stage, to a level that it is no more usable (some will consider such level to be 70%, other will still be happy with 50%, it’s all up to the user). But at the end, one thing is for sure: the battery WILL die at some stage and faster than historical data would have predicted. Based on my 300.000km and 5-year experience, it is simply not realistic to predict 80% capacity after 800.000km as stated in the
article and suggested in our
GoogleSheet.
