Why batteries wear out at all
Nearly all modern phones and tablets use lithium-ion batteries, or a close variant. Inside, lithium ions move between two electrodes as the battery charges and discharges. Each trip is slightly imperfect. Small chemical changes build up over time, which reduces the amount of charge the battery can hold and makes it harder for it to deliver power quickly.
This wear is gradual and normal. A battery is a consumable part, much like the tyres on a car, and a few years of use will usually leave it noticeably weaker than when it was new. The aim of good habits is not to stop ageing, which cannot be done, but to avoid speeding it up.
Charge cycles
Manufacturers often describe battery life in charge cycles. One cycle means using an amount of energy equal to a full battery, not necessarily going from zero to one hundred in a single go. If you use half the battery on Monday, recharge, and use another half on Tuesday, that adds up to roughly one cycle.
Batteries are typically designed to keep a large share of their original capacity after a few hundred cycles, with the exact figure depending on the model and the maker. A phone charged once a day therefore passes its first hundred cycles in a few months and might reach several hundred in a year or two. Those numbers are approximate, and you should check your manufacturer’s published figure rather than rely on a general rule.
Heat is the biggest everyday enemy
Of the factors you can influence, heat usually matters most. Higher temperatures speed up the unwanted chemical reactions inside the cell. Common sources of heat include:
- leaving the phone in a car or in direct sun
- gaming or video recording for long stretches
- charging under a pillow or inside a thick case
- fast charging, which generates extra warmth, particularly when combined with the points above
Phones have protections and will often slow down, dim the screen or pause charging when they get too hot. That is a safety feature, not a fault. Cold weather is different: it temporarily reduces available capacity and can cause sudden shutdowns, but it generally does not cause the same lasting damage as heat, and normal behaviour tends to return once the phone warms up.
Charging habits, sorted into sense and myth
Some advice is well supported, and some is a leftover from older battery types.
- Staying at very high or very low charge for long periods puts more stress on the battery. Moderate levels are gentler, which is why many phones offer an optional setting that holds charge back at a limit or delays the last part of charging until near the time you wake.
- Deep discharges to zero are best avoided. Occasional ones do no harm, but frequent ones add stress.
- Plugging in overnight is generally fine. Modern phones stop drawing power once full, though sitting at full charge for hours is not ideal, hence the smart charging options.
- Using a certified charger and cable is wise. Poor-quality accessories can deliver unstable power.
- You do not need to drain the battery fully before recharging. That advice belongs to older battery chemistries.
The same logic applies to other portable devices, and the habits are closely related to those in our article on laptop battery care.
What the battery health number means
Many phones show a battery health or maximum capacity percentage. It compares the battery’s current estimated capacity with its capacity when new. A figure in the nineties suggests little wear, while a lower figure suggests more.
It is useful, but it has limits.
- It is an estimate, and different makers calculate it differently, so figures are not directly comparable between brands.
- It reports capacity, not how the phone performs overall. A phone at a lower percentage might still last the day for a light user, while a heavy user could struggle with a higher one.
- It does not reflect how fast a battery drains in a particular session. Poor signal, a bright screen, a misbehaving app or a recent software update can all make the battery seem worse without any change in health.
- A single reading on a single day is less meaningful than a trend over months.
Some devices also reduce peak performance if an old battery cannot deliver power reliably, one of several causes behind phone slowdown. Your settings may tell you if that has happened.
When ageing becomes a problem
If your phone no longer lasts through a normal day, there are a few steps worth taking before replacing it:
- Check which apps use the most power, and restrict background activity for those you rarely open.
- Lower screen brightness or turn on adaptive brightness, since the display is often the largest consumer, as explained in our look at how phone screens work.
- Install any available software updates, which sometimes improve power management.
- Consider a battery replacement through the manufacturer or a reputable repairer. Replacing a worn battery can give an older phone several more years, provided it still receives security updates.
A swollen battery, a case that bulges or a phone that becomes unusually hot even when idle is a different matter. Stop using and charging it and seek professional help, since a damaged battery can be a safety risk.
Realistic expectations
Batteries age at different speeds depending on use, climate and chance, so no habit guarantees a particular result. The helpful middle ground is simple: keep the phone cool, avoid extremes of charge, use decent accessories and accept that some decline over the years is part of owning a battery-powered device.