Why batteries wear out

Two kinds of ageing

Batteries wear with use: every charge and discharge causes small, lasting changes inside, which is why makers count a battery’s life in cycles, each the equivalent of one full battery used. They also wear with time, which engineers call calendar ageing: a battery left in a drawer loses capacity too, and how fast depends on how warm it is and how full it is.

Heat does lasting damage

Apple’s battery advice is a clear summary. Its devices work across a wide range of temperatures, with 16 to 22 °C as the ideal comfort zone. Temperatures above 35 °C can permanently damage battery capacity, charging in high temperatures can damage it further, and even storing a battery somewhere hot can harm it for good. In the cold, by contrast, you may see shorter battery life, but it comes back once the battery warms up.

The usual hot spots are a car in summer, a sunny windowsill and a device that heats up while it charges: Apple suggests taking it out of its case first.

Full is not the best place to rest

Keil and colleagues stored three types of lithium-ion cells at 16 charge levels from empty to full, at different temperatures, and measured how much capacity each lost. The loss did not simply grow with the charge level: it came in plateaus, with similar losses across wide bands. The main cause during storage was a side reaction at the graphite electrode that speeds up when the cell is highly charged. Their conclusion: to make batteries last, avoid storing them at a high charge.

That is the idea behind the settings on many phones and laptops that stop charging at around 80%, or finish charging shortly before you usually unplug.

Empty is not good either

According to Apple, a device stored with a flat battery can slip into a deep discharge state in which the battery can no longer hold a charge, while one stored full for a long time may lose some capacity. For long storage, it recommends about 50% charge, a cool, dry place below 32 °C, and a top-up to 50% every six months.

Habits that help

• Keep it out of the heat. Cars in summer, direct sun, and charging inside a case that traps the heat.
• Do not keep it full for days if your device lets you cap the charge.
• Do not store it empty. For a long break, leave it about half full.
• Do not worry about the cold. Warm the device up; the lost battery life comes back.

Small batteries, same chemistry

A smart ring or an earbud carries a battery a small fraction of a phone’s, charged often and worn against warm skin, so the same rules apply and good charging software matters even more: smart ring battery life. For Lychnos Ring, our design target is 4 to 7 days per charge, still to be measured. Alkyon, our concept for a flying home companion, is designed so that its dock swaps a flat battery for a charged one: a dock that swaps the battery.

Sources, read 4 November 2026: Apple, batteries: maximizing performance; Keil and colleagues, calendar aging of lithium-ion batteries, Journal of The Electrochemical Society, 2016. General information.