Why the battery matters more than the year
A battery is the one component in a laptop or phone that degrades with use no matter how carefully the device was treated. Two identical machines from the same year can be in very different condition depending on how many times each was charged.
Batteries are rated in charge cycles — one cycle is a full charge's worth of use, not one plug-in. Capacity fades gradually as cycles accumulate, and the useful question is always “how much capacity is left?”
On a Mac
Open System Settings, then Battery, and click the ⓘ next to Battery Health. macOS reports a condition and the cycle count.
For the raw numbers, open Terminal and run:
system_profiler SPPowerDataType
That prints cycle count, condition and the maximum capacity the battery can still hold.
On a Windows laptop
Open Command Prompt and run:
powercfg /batteryreport
Windows writes an HTML report to your user folder. Open it and compare DESIGN CAPACITY with FULL CHARGE CAPACITY: the gap is the wear. The report also shows recent usage and a rough estimate of remaining runtime.
On an iPhone or iPad
Settings → Battery → Battery Health & Charging shows Maximum Capacity as a percentage of new. Anything in the high 80s and above is healthy for a used phone; the lower it goes, the sooner you will want a replacement battery.
The same screen tells you whether the battery has been replaced with a non-genuine part, which is worth knowing before you pay.
Reading the number honestly
Capacity percentages are estimates, not certificates. A battery at 85% is not broken — it simply holds 85% of what it once did, which may be plenty for a desk-bound laptop and not enough for a day of travel.
Factor a replacement into the price if the number is low. On many laptops the battery is a reasonably priced part, and on phones it is one of the cheapest repairs there is.