Battery health: what the percentage really tells you

The percentage on your screen describes the charge relative to the battery's current capacity, not its original one. A battery that has aged can still show 100 percent while holding noticeably less energy than when it was new.

Lithium-ion cells wear with charge cycles, and also with heat and with sitting at a full charge for long periods. Many laptops and phones now offer a charge limit around 80 percent, which is useful if the device stays plugged in most of the time. Avoid leaving devices in hot cars or under pillows while charging.