Why do batteries degrade?
Every charge cycle drives slow side reactions that permanently consume lithium and thicken an insulating film on the electrodes, so a little capacity is lost each time and never comes back.
Simple intuition
The plain reason, in everyday words
A rechargeable battery works by shuttling lithium back and forth between two electrodes. Charging pushes lithium one way, using it pulls lithium back. In an ideal world the same lithium would travel forever. In reality, on every trip, a small amount gets stuck. It reacts with the liquid the battery is filled with and forms a solid crust on the electrode surface, and once it is locked into that crust it can never carry charge again. So each cycle leaves a fraction less lithium available than the cycle before. The battery is not getting tired or losing its memory — it has literally lost some of the material it needed. That is why the loss is permanent, and why a battery at eighty per cent capacity will never return to a hundred no matter how you charge it.
You should fully discharge a lithium battery occasionally to recalibrate it.
Deep discharge accelerates ageing. What a full cycle occasionally recalibrates is the fuel gauge's estimate, not the battery itself, and modern gauges rarely need it.
Leaving a device plugged in overcharges the battery.
Charging circuits stop at the target voltage. The real harm is different: the cell then sits at high state of charge, often warm, which is precisely the condition that accelerates calendar ageing.
Batteries have a memory effect that you must avoid by cycling fully.
Memory effect is a nickel-cadmium phenomenon. Lithium-ion cells prefer shallow partial cycles, which is the opposite advice.
Fast charging always destroys batteries.
The risk is lithium plating, which depends on rate, temperature, and state of charge together. Well-designed fast charging tapers hard above about sixty per cent and refuses to run fast when cold, which is why the first half of a charge is so much quicker than the second.
Batteries are now the limiting component in phones, cars, tools, and grid storage, and the difference between careless and careful use is easily a factor of two in service life. It also explains a policy question that keeps coming up: why electric car batteries are warrantied on capacity retention rather than on failure, and why second-life stationary storage is a real industry rather than an environmental gesture.
Who worked it out
The intercalation concept came from M. Stanley Whittingham in the 1970s, John Goodenough's group identified the layered oxide cathode in 1980, and Akira Yoshino built the first practically safe cell in 1985. The three shared the 2019 Nobel Prize in Chemistry.
What problem forced it
The 1970s oil crises drove the search for rechargeable energy storage, and the immediate commercial pull in the late 1980s was portable electronics — camcorders and early mobile phones needed far more energy per kilogram than nickel chemistry could give.
How it changed since
Sony commercialised lithium-ion in 1991. Since then energy density has roughly quintupled and cost has fallen by well over ninety per cent, but the degradation mechanisms have proven stubborn — most progress has come from managing them, through charge algorithms, thermal management, and electrolyte additives that build a better SEI, rather than eliminating them.
Why SSDs wear out
Another everyday technology where ordinary use is physically destructive at the microscopic scale.
How electric car battery management systems work
The engineering built specifically to slow the mechanisms described here.
Written for Curio rather than collected from a forum — it is part of the curated corpus that ships with the platform. The references it draws on are listed under Sources.