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Guide

Laptop battery health percentage: what it means and when to replace

Battery health is the one hardware number almost everyone understands, and one of the few that a laptop reports honestly. Here is where it comes from, how to read it on any operating system, and how to decide when 45% is a problem and when it isn't.

By Eugene Trotsan · · 4 min read

What the percentage is

Every battery reports two capacities. The design capacity is what it could hold when new, printed on the label in watt-hours. The full charge capacity is what it can hold now, re-measured by the battery's own gauge each time it is charged to 100%. Health is simply the ratio:

health = full charge capacity ÷ design capacity
       = 25.8 Wh ÷ 57 Wh ≈ 45%

A battery at 45% health is not "45% charged". It is a battery that, when full, holds 45% of the energy it was built to hold. Runtime scales with it: a laptop that ran for eight hours new runs for about three and a half.

The second number worth knowing is the cycle count. One cycle is 100% of capacity discharged, cumulatively; going from 100% to 50% twice is one cycle, not two. Lithium-ion cells are typically specified to keep about 80% of their capacity for somewhere between 300 and 1,000 cycles depending on the maker and the chemistry, and to decline faster after that.

Reading it on each system

Linux

cat /sys/class/power_supply/BAT0/energy_full_design   # µWh when new
cat /sys/class/power_supply/BAT0/energy_full          # µWh now
cat /sys/class/power_supply/BAT0/cycle_count          # if the firmware reports it

# or, friendlier:
upower -i $(upower -e | grep BAT)

Some batteries report charge_full and charge_full_design in microamp-hours instead of energy; the ratio works the same way. upower prints energy-full, energy-full-design and a capacity percentage that is exactly the health figure.

Windows

powercfg /batteryreport

This writes battery-report.html to the current folder. The "Installed batteries" table shows DESIGN CAPACITY, FULL CHARGE CAPACITY and CYCLE COUNT; the "Battery capacity history" table shows how full charge capacity has drifted over months, which is the most useful view of all.

macOS

System Settings → Battery → Battery Health shows Maximum Capacity as a percentage and a Condition of Normal or Service Recommended. For the cycle count, open System Information → Power (or run system_profiler SPPowerDataType).

When to replace

There is no health percentage at which a battery becomes unsafe or the laptop stops working; a worn battery just holds less. So the decision is about how you use the machine:

  • Below about 60%, if you use it unplugged. This is the line Innards draws in its report. Under it, runtime is less than two thirds of the original and falling; a replacement is the cheapest way to get hours back.
  • Any health, if it is always plugged in. A laptop that lives on a desk or works as a home server does not need runtime. A 45% battery still rides through a power blip, which is a useful thing for a server to have.
  • Immediately, if it is swelling. A trackpad that has lifted, a case that no longer sits flat, or a bottom cover bulging out is a swollen cell. Stop using the battery, do not puncture it, and have it replaced; this is the one battery symptom that is a safety matter rather than a convenience one.
  • Soon, if it shuts down at 20–30%. An old battery's gauge and its cells disagree; the laptop believes it has a quarter left and the cell voltage collapses. Sometimes a full discharge-and-charge recalibrates the gauge for a while; it does not restore the cells.

The cost is usually modest. For the 2017 ThinkPad X1 Carbon in Innards' own report, the upgrade advisor estimates a battery at $40–$120, replaces it under "Service" rather than "Upgrade", and adds that it is "usually a 10-minute job with a screwdriver": that machine has a bottom cover held by captive screws and an internal battery on a single connector. Check a service manual or a teardown video for your model first; the same job on a glued-in ultrabook is a different afternoon.

Making the next one last

Battery wear is driven by heat, by time spent at 100%, and by full cycles. You cannot change the chemistry, but you can change those three:

  • Cap the charge if the machine mostly lives plugged in. ThinkPads, many Dells and ASUS laptops and most recent designs let you stop charging at 80% (on Linux, /sys/class/power_supply/BAT0/charge_control_end_threshold on machines whose firmware supports it, or via TLP or the GNOME power settings; Windows exposes the same setting through the vendor's utility; macOS does it automatically with Optimized Battery Charging). Innards suggests this whenever a laptop is being used as an always-on server.
  • Keep it cool. A laptop on a duvet, or one whose fan is clogged, cooks its battery along with its processor.
  • Do not chase small top-ups. Plugging in at 80% to reach 100% before leaving the house is fine; keeping it pinned at 100% for a year is what ages it.

None of this makes a battery last forever. A well-treated pack in a laptop used daily reaches the 80% line somewhere around its third or fourth year; the one in the ThinkPad above reached 45% after 1,729 cycles, which is a battery that did its job.

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