The shorter journey to empty
A phone that once lasted all day now asks for power before dinner. It still charges to 100 per cent, so what has gone missing? The number on the screen is a percentage of the battery's current usable capacity. If that capacity has shrunk, a full charge stores less energy than it did when the cell was new.
Most portable electronics use lithium-ion batteries, so they are the clearest example. During use, lithium ions move through an electrolyte between two electrodes while electrons travel through the external circuit. Charging drives the main process back the other way. That reversibility makes the battery reusable, but it is not perfect. Small side reactions accompany the useful one, and the materials do not return to precisely their original state after every journey.¹
The result is usually gradual rather than a sudden failure. A cell can still work while offering less capacity or becoming less able to deliver power quickly. Those two kinds of decline are related but not identical: an ageing battery may have both fewer usable energy stores and greater internal resistance.²
The protective layer that keeps growing
One important change occurs where the electrolyte touches the negative electrode, often made of graphite. Some electrolyte reacts and forms a thin layer called the solid-electrolyte interphase, or SEI. A stable SEI is useful: it helps prevent further unwanted reactions while allowing lithium ions to pass. But forming and repairing that layer consumes some of the lithium that could otherwise shuttle between the electrodes.²
The layer can keep changing over the battery's life. As it thickens or reforms, more usable lithium can be tied up and the path for ions can become less easy. Researchers call this loss of lithium inventory. Other mechanisms remove active material from an electrode or disrupt electrical contact. Together, they reduce the amount of charge the cell can store or deliver.²
Cycling adds mechanical wear. Electrode particles expand and contract slightly as ions enter and leave. Repeated changes can strain them, create cracks and expose fresh surfaces for more side reactions. Under some conditions, particularly when charging is too fast or cold for the ions to enter the negative electrode smoothly, lithium can plate onto its surface instead. That can further reduce the useful supply. The balance of these processes varies with the cell's design and how it is used.²,³
Capacity and power can also fade at different rates. A cell may still hold a useful amount of energy but struggle with a sudden heavy demand because its internal resistance has risen. This helps explain why battery health is more complicated than measuring how long one charge lasts under a single set of conditions.²
Ageing even when nothing is plugged in
It is tempting to treat battery life as a simple counter: after a fixed number of charges, capacity disappears. Researchers separate cycle ageing from calendar ageing. Side reactions can continue while a battery sits on a shelf. Time, temperature and state of charge all influence that quieter decline. A battery stored hot and nearly full may age differently from one stored cooler at a moderate charge.²,⁴
This is why heat is such an important enemy. Higher temperature can speed unwanted chemistry, though the precise effect depends on the cell. Maintaining a high state of charge for long periods is also a significant source of capacity fade in some lithium-ion applications, according to the US Department of Energy. That does not mean a single overnight charge ruins a phone. Battery ageing is cumulative, and modern devices manage charging in different ways.⁴
There is no universal instruction to keep every battery at one magic percentage. A laptop, electric vehicle and cordless drill have different cell chemistries, control systems and priorities. The broad principles are steadier than the folklore: avoid unnecessary heat, and recognise that high voltage and harsh cycling can add stress. For a specific device, its maker's charging guidance is more useful than a rule copied from another battery type.
Why a new charge cannot restore an old cell
Charging moves ions back, but it does not simply dissolve every unwanted layer, reconnect every cracked particle or replace lithium that has become unavailable to the useful reaction. Think of a well-used rechargeable battery as a route with a gradually shrinking number of working lanes. Traffic still goes both ways, but less can complete the journey efficiently.
Scientists are improving electrode materials, electrolytes and charging controls to slow this decline. There are also many rechargeable chemistries besides lithium-ion; nickel-based and other cells have different failure mechanisms. The common principle is that “rechargeable” describes the main energy-storing reaction, not a promise that all microscopic changes are reversible.¹,²
So the 100 per cent on an old phone is not lying. It means the battery has reached the top of the capacity it has now. The quiet chemistry of time and use has made that container smaller.