How to check the battery of a used phone or laptop

The battery is the part that wears out fastest: after two or three years a phone battery typically holds about 80% of its original capacity. A worn battery isn't a reason to skip a device, because a battery is far cheaper to replace than a screen. But you need to know its condition to pay the right price, and to spot dangerous faults such as a swollen battery.

iPhone

Go to Settings › Battery › Battery Health & Charging:

• Maximum Capacity of 85% or more is good; 80–85% is fine but worth a lower price; under 80% needs replacing soon.
• A "Service" message, or a note that performance management is on, means the battery is significantly degraded.
• iPhone 15 and later show the cycle count in Settings › General › About.
• A third-party battery shows as "Unknown Part" under About › Parts and Service History, and often shows no health percentage.

Samsung

Open Samsung Members › Diagnostics › Battery status. Normal is fine; Weak or Bad means a worn battery. Some recent Galaxy models also show remaining capacity as a percentage.

Pixel and other Android phones

• Recent Pixels and many Android 14+ phones: Settings › Battery › Battery health.
• Without that option, rely on the drain rate under load (below), and ask the seller whether the battery has been replaced.

Windows laptops

Open Command Prompt, type powercfg /batteryreport, and open the battery-report.html it creates (the path is shown in the window). Under Installed batteries:

• Design Capacity: the capacity when new.
• Full Charge Capacity: the capacity now.

Divide Full Charge by Design: over 80% is good, 60–80% is moderately worn, under 60% needs replacing. Cycle Count (if shown) is the number of charge cycles.

MacBooks

• System Settings › Battery, click ⓘ next to Battery Health: it should say Normal; Apple silicon Macs also show maximum capacity (%).
• System Information › Power, Cycle Count: most MacBooks are designed for about 1,000 cycles.

Measuring the drain rate

The health figure doesn't tell the whole story: a battery can report 90% and still drain unusually fast. A simple test: unplug, open Soi Máy's battery test, then run the CPU test a few times over 5–10 minutes. The battery test logs how long each 1% drop takes.

A healthy device usually takes more than a minute per 1% under heavy load. If it drops 1% every few dozen seconds, or jumps suddenly from 30% to 5%, the battery is very weak.

A loose charging port

Plug in the charger, unplug and replug a few times, and gently wiggle the cable at the port. The battery test reacts instantly to each plug and unplug. If charging flickers on and off as the cable moves, the port is loose or full of lint: cleaning sometimes helps, but it often needs replacing.

Swollen batteries: a safety issue

A swollen lithium battery can catch fire. Signs:

• Phones: the screen lifting up, a gap between the screen and frame, or a back cover coming away.
• Laptops: a bulging bottom case, a raised touchpad that is hard to click, or a laptop that rocks on a flat table.

Only buy a device with a swollen battery if the price already covers a replacement, and replace it immediately.

Should you buy a device with a worn battery?

Yes, at the right price. Find out the cost of a genuine battery replacement for that model and take it off the price. Prefer a genuine battery when replacing: cheap ones wear out faster and on iPhone are flagged as "unknown".

See also: how to check a used iPhone, how to check a used laptop, and the manual checklist.