Why Your Phone Battery Wears Out, and How to Make It Last Longer

When your phone was new, it easily lasted from breakfast to bedtime. Now it needs a top-up by mid-afternoon. You haven’t imagined it: the battery really does hold less charge than it used to. The good news is that a few simple habits can slow the decline.

How a phone battery works

Almost every phone uses a lithium-ion battery. Inside are two layers, usually a graphite anode and a metal oxide cathode, with a liquid electrolyte between them.

When you charge your phone, lithium ions move from the cathode to the anode and are stored there. When you use your phone, they flow back to the cathode, and that movement releases the electricity that powers the screen and chips.

In a perfect battery, this could go on for ever. In a real one, each trip back and forth comes with small losses.

Why batteries lose capacity

The main causes of wear are well understood:

• A growing film on the anode. From the first charge, a thin layer forms on the anode’s surface. It protects the battery, but it keeps growing slowly, and every bit of growth traps lithium that can no longer carry charge.
• Wear on the electrodes. As lithium moves in and out, the materials swell and shrink slightly. Over hundreds of cycles, tiny cracks and structural changes reduce how much lithium they can hold.
• Rising internal resistance. As the battery ages, it becomes harder for it to deliver power quickly. That’s why an old phone can suddenly switch off at 15% on a cold day, or slow itself down to avoid that happening.

These processes happen faster under three conditions: heat, a very high charge level held for a long time, and to a lesser extent very deep discharges.

Heat is the biggest enemy of a phone battery, more than any charging habit.

What a “charge cycle” is

Phone makers measure battery life in charge cycles. One cycle means using 100% of the battery’s capacity in total, but not necessarily all at once. Using 60% today and 40% tomorrow adds up to one cycle.

Manufacturers design batteries to keep most of their capacity for a set number of cycles. Apple, for example, says iPhone batteries up to the iPhone 14 are designed to keep 80% of their original capacity after 500 complete cycles, and the iPhone 15 and later models after 1,000 cycles, under ideal conditions. Since June 2025, the European Union has required smartphones sold there to have batteries that keep at least 80% of their capacity after 800 cycles.

What actually helps

1. Keep it cool. Don’t leave your phone in a hot car, on a sunny windowsill or under your pillow while it charges. If it gets hot while gaming or charging, take it out of a thick case.
2. Avoid sitting at 100% for hours. Turn on your phone’s battery-saving charge setting. iPhones have Optimised Battery Charging, which learns your routine and holds the charge at 80% until shortly before you usually unplug, and newer models also offer a fixed 80% limit. Many Android phones offer similar options, such as stopping at 80% or 85%.
3. Don’t make a habit of running it flat. Regularly letting the battery drop to 0% adds stress. Charging whenever it’s convenient is fine.
4. Use a decent charger. A reputable charger and cable from the phone maker or a known brand is safer and avoids overheating. Fast charging generates more heat, but phones manage this by slowing down as the battery fills.
5. Store it half charged. If you put a phone away for months, charge it to about 50% first, and switch it off. A battery stored completely empty can fall too low to be revived, and one stored full ages faster.

Myths you can ignore

• “Run your battery right down before charging.” This advice comes from older nickel-cadmium batteries, which suffered from a “memory effect”. Lithium-ion batteries don’t, and deep discharges actually add wear.
• “Charging overnight ruins the battery.” Modern phones stop charging when full, and features like Optimised Battery Charging reduce the time spent at 100%. Heat is the bigger risk, so don’t charge under a pillow.
• “Closing all your apps saves battery.” On modern phones, apps in the background are mostly paused. Reopening them from scratch can use more energy than leaving them.
• “Cold kills batteries.” Cold makes a battery perform worse temporarily, so it drains faster in freezing weather, but it recovers when it warms up. Lasting damage comes mainly from heat.

When to replace the battery

Most phones show battery health in their settings. On an iPhone, go to Settings > Battery > Battery Health & Charging. When maximum capacity falls to around 80%, or the phone shuts down unexpectedly, a replacement battery can make an old phone feel new again, and it costs far less than a new phone. Use the manufacturer or an authorised repairer, since cheap batteries can be unsafe.

If your battery is swollen, so the screen or back is lifting, stop using and charging the phone and get it to a repairer straight away. A damaged lithium-ion battery can catch fire.