Why phone batteries wear out

When your phone was new, the battery lasted all day. A few years later, it runs low by the afternoon, because the battery has worn out.

Phone batteries are lithium ion batteries. Inside, tiny charged bits of lithium, called ions, move to one side when you charge, and back when you use the phone.

Each trip wears the battery a little. Some lithium gets trapped in a thin layer that slowly grows inside. Trapped lithium can no longer carry charge.

Heat speeds up this wear. A phone left in a hot car, or one that gets hot while it charges, ages faster.

Sitting at a full charge for many hours also adds wear. That is why some phones wait to finish charging until just before you usually unplug them.

All of this adds up over the years. The battery holds less charge, so a full charge runs out sooner.

Keeping your phone cool helps it last longer. Every charge wears the battery a little, and heat and long hours at full charge make it wear faster.