Why phone batteries get worse, and the habits that slow it down

A phone battery that held a full day when new struggles to reach the evening after two years. Nothing is broken. The cell is doing what lithium-ion cells do, and a couple of ordinary habits make a measurable difference to how fast it happens.

What is going on inside

A lithium-ion cell stores energy by moving lithium ions between two electrodes. Charging drives them one way, discharging lets them travel back. Nothing is consumed; the ions shuttle back and forth.

Capacity is lost because some of those ions stop taking part. A layer forms on the negative electrode where electrolyte reacts with it, called the solid electrolyte interphase. A thin version of this layer is necessary and forms when the cell is first used. The problem is that it keeps growing, slowly, and every bit of growth consumes lithium that is then locked out of circulation for good.

Alongside that, the electrode materials themselves gradually degrade, and internal resistance rises, which is why an older phone can shut down abruptly under load even with charge apparently left.

Two kinds of ageing, and one matters more than people think

Cycle ageing comes from use. Every charge and discharge causes small physical changes in the electrodes. This is the one everybody knows about.

Calendar ageing happens whether or not you use the phone. The chemical reactions building that interphase layer proceed on their own, driven by time, temperature and the state of charge.

This is why a spare phone left in a drawer for two years at full charge comes out with noticeably less capacity than it went in with, despite not having been used at all. Ageing is not a usage counter.

Heat is the biggest single factor

Chemical reactions run faster at higher temperature, and the reactions degrading the cell are no exception. Heat accelerates calendar ageing substantially.

In practice that means a few specific situations do real damage: a phone on a car dashboard in the sun, a phone charging under a pillow, a phone in a thick case running a demanding game while fast charging, a phone left in direct sunlight on a table outside.

If you change one thing, change this one. It matters more than any charging routine.

Being full is a stressed state

A fully charged lithium-ion cell is held at a higher voltage, and that higher voltage speeds up the unwanted side reactions. Sitting at 100 per cent for hours every night is therefore worse for the cell than sitting at 60 per cent.

The same is true at the other end. A deeply discharged cell is also stressed, and leaving one flat for a long period can damage it permanently.

The comfortable middle is roughly 20 to 80 per cent. This is exactly why phone makers have added features that pause charging at 80 per cent and finish shortly before your alarm, and why electric cars ask you to set a daily charge limit below full. Both are the same idea.

What a cycle actually counts

Ratings are quoted in charge cycles, and people assume a cycle means plugging in once. It does not. A cycle is one full battery’s worth of energy, however it is accumulated.

Using 40 per cent today and 60 per cent tomorrow is one cycle, not two. Topping up from 70 to 90 five times is also one cycle. This is why frequent small top-ups are not the harm they are sometimes described as, and it is another reason the old advice about full discharges is wrong.

What is worth doing

• Keep it cool. Out of the sun, out from under bedding, out of a hot car. The largest single win.
• Turn on optimised or limited charging if your phone offers it. It is doing the 80 per cent trick for you.
• Do not leave it flat for days. Store an unused device around half charged.
• Top up freely. Small charges are fine and do not waste cycles.
• Use slow charging when you have time. Less heat, and no downside overnight.
• Take the case off if the phone is getting hot while charging.

None of this stops the process. Degradation is inherent to the chemistry and a battery is a consumable part. What these habits change is whether you are replacing it in year two or year four.

More everyday machinery

How a refrigerator actually works and why knives go blunt apply the same treatment to two other things that quietly wear out.