hardware//lithium-ion battery

A lithium-ion battery is a rechargeable cell that stores energy by moving lithium ions between a graphite anode and a metal-oxide cathode, and it powers almost every laptop, phone and electric vehicle. As a component it is described by its capacity in watt-hours, its voltage window and how fast that capacity fades with use and with time.

A lithium-ion battery is a rechargeable cell that stores energy by moving lithium ions between a graphite anode and a metal-oxide cathode, and it powers almost every laptop, phone and electric vehicle. As a component it is described by its capacity in watt-hours, its voltage window and how fast that capacity fades with use and with time.

The capacity fades through two routes. Cycle aging comes from charging and discharging, and grows with depth of discharge and charging current. Calendar aging happens even when the battery sits unused, and is driven by two conditions: high state of charge, which keeps the cell at a high voltage where side reactions eat the electrolyte faster, and high temperature. A battery held at 100% in a warm laptop ages faster than one held at 50% on a shelf, without doing any work.

A laptop that stays plugged in is the worst case for calendar aging: always full, often warm. A charge threshold (for instance stop at 80%, resume below 75%, the default on Lenovo machines) keeps the cell out of the top of its voltage range; the cost is a fifth less runtime when unplugged.

Capacity is reported against the design value, and a battery with a few dozen cycles normally sits at 95 to 100% of it. The figure the system shows is itself an estimate (state of charge estimation), corrected now and then by a full calibration cycle.

Above the threshold the battery is not drained to reach it; it waits until it is used. Setting a threshold on a full battery changes nothing until the next discharge.

For energy accounting the battery also works as a meter: a laptop's design capacity in watt-hours, multiplied by the fraction drained over a timed interval on battery, gives its average draw over that interval (standby power).