Why do rechargeable batteries wear out?

Rechargeable batteries wear out because charging is never a perfect reversal of discharging. Every cycle, and even every day of sitting around, triggers small side reactions that use up some of the active material for good. A little less can be stored each time, and the battery gets worse at delivering it. Over their lifespan, batteries degrade gradually, losing the charge they can cycle and gaining internal resistance.

In a lithium-ion battery, the main culprit is a film that grows on the negative electrode. The electrolyte reacts there and builds a layer called the solid electrolyte interface, and each time that happens some lithium ions are irreversibly trapped in it. Lithium that is trapped can no longer carry charge. Manufacturers usually define a battery's cycle life as the number of cycles until its capacity falls to 80 percent of its rated value. And you do not need to use a battery to age it: simply storing a lithium-ion battery in a charged state reduces its capacity.

Heat and extremes make everything worse. Degradation is minimal at room temperature but increases when a battery is stored or used at high or low temperatures. Charging often above 90 percent and discharging below 10 percent may also hasten capacity loss, while keeping the charge between about 60 and 80 percent can reduce it.

Other chemistries have their own ways to die. In lead–acid car batteries, some lead sulfate turns into a stable crystal that no longer dissolves when recharged, so usable material slowly declines. Different chemistry, same story: part of the active material gets stranded where the recharge cannot reach it.