Battery health is one of the few things a browser genuinely cannot measure. There’s no web API for cell capacity, so this one is on you, but it’s a five-minute job and it produces an actual number rather than an impression.
That number matters twice: on a new machine, where a bad cell is a defect worth returning; and on a used one, where it’s a replacement cost worth negotiating.
What “health” actually measures
Three figures do all the work.
Design capacity is what the cell held when it was manufactured, in watt-hours or milliamp-hours. It never changes; it’s the specification.
Full charge capacity is what it holds today, fully charged. This declines with age and use. Health is just the second divided by the first: a battery with a design capacity of 60 Wh that now holds 54 Wh is at 90%.
Cycle count is how many full discharge-equivalents it’s been through. Two half-discharges count as one cycle. Manufacturers typically rate laptop batteries to retain around 80% of design capacity after 1,000 cycles, though the exact promise varies.
What health does not measure is how long the machine lasts on a charge. That depends on screen brightness, what you’re running, and background activity far more than on the battery. Confusing the two is the source of most “my battery is broken” conclusions that turn out to be fine.
Reading the numbers
Windows. Open a terminal and run:
powercfg /batteryreportIt writes an HTML file and tells you where. Open it and look for Installed batteries, which holds design capacity and full charge capacity, then Battery capacity history, which shows the decline over time. Some manufacturers fill in the cycle count field and some leave it blank.
macOS. Hold Option and click the Apple menu → System Information → Power. Cycle count, condition, and capacity figures are all on that page. System Settings → Battery → Battery Health gives a friendlier summary with a maximum-capacity percentage.
Linux. upower -i /org/freedesktop/UPower/devices/battery_BAT0 reports energy-full against energy-full-design, or read the raw values under /sys/class/power_supply/BAT0/.
Whichever platform, write down design capacity, full charge capacity, and cycle count. Those three lines are the whole record.
What’s normal on a new machine
A cycle count in the low single digits. A brand-new machine should be under about five. Factory testing accounts for one or two. Twenty cycles on a supposedly sealed machine means it isn’t new, and that’s worth a conversation with the retailer.
Health at or just under 100%. Slightly below is normal; cells lose a little in storage between manufacture and sale.
Battery figures on day one that look terrible. Ignore them. A new machine spends its first days indexing files, installing updates, and running preinstalled software. Wait until it settles, then measure.
When it’s a return, a warranty claim, or neither
A return. Health noticeably below 100% on a new machine, or a cycle count that says the machine has been used. Both are grounds inside the window, and inside the window you don’t have to argue the point.
A warranty claim. A battery that fails prematurely: capacity falling off a cliff in months rather than years, or a machine that shuts down at 30% charge. Manufacturers cover defective cells, though usually with a capacity threshold attached.
Immediate, regardless of policy: physical swelling. A trackpad that’s lifting, a case that no longer sits flat, a lid that won’t close properly. Stop using the machine and contact the manufacturer. This is a safety issue, not a performance one.
Neither. Gradual decline with use. A battery at 85% after two years is behaving exactly as designed. So is one that lasts four hours when the specification claimed ten. That figure comes from a specific benchmark at a specific brightness, and real work never matches it.
The number to keep
If you record one thing, record full charge capacity against design capacity, with the date.
On a new machine that’s your baseline. If capacity drops unusually fast over the following months, you have a before-and-after rather than a feeling, and that’s the difference between a warranty claim that goes somewhere and one that doesn’t.
On a used machine it’s the most useful figure for deciding what the laptop is worth. A replacement battery is a real cost, and a pack at 70% of design capacity has that cost already baked in.
ReturnCheck can’t read any of this; a browser has no access to it. All this page can do is show you where to look. The figures stay on your own machine, unread by us. If you plan to compare them later, note the date you checked alongside them.