Few everyday objects are surrounded by as much contradictory advice as the battery in a phone. Much of it dates from the era of nickel-cadmium cells and no longer applies to lithium-ion.
What actually harms them, and what doesn't.
What ages a battery
Two processes run in parallel.
Calendar ageing. A battery loses capacity even sitting unused. Chemical processes at the electrodes continue. This ageing depends strongly on temperature and state of charge.
Cycle ageing. Every charge and discharge stresses the cell mechanically and chemically. Manufacturers often quote a number of full cycles after which a given percentage of original capacity remains.
Important: a full cycle isn't the same as plugging in once. Charging twice from 50 to 100 percent is one cycle, not two.
Heat is the main enemy
If you take one thing away, take this: temperature is by far the most important factor.
Chemical reaction rates rise with temperature, and that includes the unwanted side reactions that age a battery. A battery run consistently warm loses capacity considerably faster than one at room temperature.
The typical everyday heat sources: a phone on a car dashboard in the sun. Charging under a pillow or inside a thick case. Demanding games or navigation while charging simultaneously — heat from two sources at once.
Cold does less lasting damage but temporarily reduces available capacity substantially. A phone that shuts down at 20 percent in winter is usually not faulty.
State of charge
The second major factor. A battery held permanently at very high charge ages faster. Cell voltage is high, and high voltage accelerates degradation.
Very low charge over long periods is equally unfavourable, because the cell can over-discharge and then take permanent damage.
Which is why specialists recommend staying in the middle range day to day. Many devices now have functions that stop charging at 80 percent or schedule the final top-up to complete shortly before your usual wake time. These do exactly the right thing and should be switched on.
Myths that persist
"You should fully drain it regularly." Wrong, and with lithium-ion actively harmful. The memory effect applied to nickel-cadmium cells and doesn't exist here. Deep discharge is among the worse treatments.
"Charging overnight destroys the battery." Largely outdated. Modern charging electronics stop at 100 percent. The problem isn't the charging but the subsequent hours sitting at full charge — which is exactly what the functions mentioned above address.
"Only use original chargers." Overstated but not entirely wrong. Certified power supplies from known manufacturers are fine. With very cheap unbranded products there are genuine safety concerns, less about the battery than about inadequate isolation on the mains side.
"Fast charging ruins the battery." Fast charging generates more heat and therefore contributes to ageing. The effect is measurable and usually smaller than feared, because battery management reduces power as temperature rises. Given the choice, charge slowly overnight and quickly only when needed.
Wireless charging
A case of its own. Inductive charging is convenient and less efficient — some energy is released as heat, precisely where the battery sits.
For battery health that isn't ideal, particularly with the device in a case. Anyone leaving their phone permanently on a charging pad combines heat with high state of charge, the least favourable combination.
What to realistically expect
A word on long-term storage, since it comes up whenever somebody puts an old device in a drawer.
The worst thing you can do is store a phone either fully charged or fully flat. Full charge accelerates calendar ageing; empty risks the cell dropping into deep discharge, at which point the protection circuit may refuse to allow charging at all and the battery is effectively dead.
The recommendation for storing lithium-ion cells is somewhere around half charge, in a cool place, and to check every six months or so and top up if it has drifted low. A drawer in an unheated room is better than one next to a radiator.
This applies to anything with a battery you are not using — an old laptop, a camera, a power bank. Power banks in particular tend to be bought, used twice, and left flat in a bag for two years, at which point they are landfill. Charging them occasionally would have kept them working, and almost nobody does it because nobody mentions it.
After roughly two years of normal use, many devices sit somewhere between 80 and 90 percent of original capacity. That's normal and not a defect.
Most operating systems now display battery health. If that figure falls noticeably faster, it points to unfavourable conditions — usually heat.
And the most practically useful advice: a battery replacement costs considerably less than a new phone on most devices and extends usable life substantially. That it often doesn't happen has less to do with cost than with nobody thinking of it.