How to Check a Second-Hand Laptop Before You Pay
The three things that decide whether a used laptop is worth buying — battery wear, screen defects and keyboard faults — are all invisible in a photograph and all testable in about ten minutes. Sellers are rarely hiding them deliberately. Most genuinely do not know.
Check the battery first, because it is the one you cannot renegotiate
Everything else on a laptop is either obvious or cheap. A battery is neither. It degrades invisibly, it is the most common reason a cheap laptop turns out to be expensive, and once you have paid you have no argument left.
Two numbers matter. Cycle count is how many full charge-discharge cycles the battery has been through; most laptop batteries are rated for 500 to 1000. Design capacity versus current full-charge capacity tells you how much of the original battery is left. A battery at 70% of design capacity will give you roughly 70% of the advertised runtime, today, and less every month after.
A browser cannot read either number — that is an operating-system call. On Windows, open Command Prompt and run powercfg /batteryreport, then open the HTML file it writes. On a Mac, hold Option, click the Apple menu, choose System Information and look under Power. Our guide on finding your battery cycle count walks through both. What the browser can tell you instantly is whether the battery is charging and what level it reports — run the battery test while the charger is plugged in and then unplugged.
If the seller will not let you run a battery report, that is your answer.
The ten-minute checklist
Run these in order. Each takes under a minute and needs nothing installed.
- Screen. Open the dead pixel test and cycle through the solid colours full-screen. A dot that stays black on every colour is a dead pixel. A dot that lights on some colours but not others is a failed subpixel. Look for uneven brightness at the edges too — that is backlight bleed, and on a used machine it usually means the panel has been opened or dropped.
- Keyboard. Open the keyboard test and press every key. Dead keys are obvious. Watch also for keys that register twice from one press — that is switch chatter, and it gets worse, not better.
- Trackpad and touchscreen. If it has a touchscreen, the touchscreen test finds dead zones that a quick swipe will miss.
- Webcam. The webcam test shows the live feed and the resolution the camera actually reports. A camera that works but reports a lower resolution than advertised is worth knowing about.
- Microphone and speakers. Run the microphone test and the speaker test. On laptops, one dead speaker is common and almost never mentioned.
- Ports. Bring a USB stick and a pair of headphones. Try every port, including the ones you do not plan to use.
Everything in that list runs in the browser on the machine you are inspecting, which is the point — you are testing the actual hardware, not taking anyone's word for it.
What a seller can hide, and what they cannot
A freshly reinstalled operating system hides software problems and tells you nothing about hardware. A clean exterior tells you nothing about the battery. A short demonstration hides thermal problems, which only appear under sustained load.
What cannot be hidden: dead pixels under a full-screen colour, a key that does not register, a speaker that produces nothing, a battery report. That asymmetry is why a ten-minute test is worth more than an hour of conversation.
Thermal behaviour is the one real gap in a quick inspection. If you can, leave something demanding running for ten minutes and put your hand near the exhaust. A laptop that is uncomfortably hot and loud at idle has either a dust problem or a failing fan.
Red flags that should end the deal
- The charger is missing or is not the original. Replacing it is cheap; the reason it is missing often is not.
- The laptop will not power on without the charger connected. That is a battery at end of life, whatever the seller says.
- A BIOS or firmware password you are not given. This can make the machine unusable and sometimes indicates a device that was not the seller's to sell.
- Visible repair marks around the hinges or base with no explanation offered.
- Any refusal to let you run tests. A seller with nothing to hide has no reason to object to five minutes in a browser.
What is worth fixing and what is not
A worn battery on a model with a user-replaceable battery is a negotiating point, not a dealbreaker. A single dead pixel on a large screen is cosmetic — annoying, and worth a discount, but not a fault that spreads. A dead key on a laptop usually means replacing the whole keyboard assembly, which on many modern designs means significant disassembly.
A failing screen, a failing fan, or a battery that will not hold charge on a sealed machine are all repairs that can cost a meaningful fraction of what you are paying. Price accordingly, or walk.
Sources: Microsoft — powercfg command-line options · Apple — About battery health management
Frequently asked questions
Can I check battery health in a browser?
Not fully. A browser can read the current charge level and whether the device is charging, but cycle count and design capacity are operating-system data. Use powercfg /batteryreport on Windows or System Information on a Mac — our battery cycle count guide covers both.
How many dead pixels are acceptable on a used laptop?
There is no standard for private sales. One dead pixel away from the centre is cosmetic and worth a small discount. A cluster, or anything in the middle of the screen, is worth walking away from — panels do not repair themselves and replacement is expensive.
What cycle count is too high?
Most laptop batteries are rated for 500 to 1000 cycles. Above roughly 80% of the rating, expect noticeably reduced runtime. The percentage of design capacity remaining matters more than the cycle count on its own.
Is a reinstalled operating system a red flag?
Not by itself — many sellers wipe a machine as a courtesy. It does mean you learn nothing about how the machine behaved before, so it raises the value of testing the hardware directly.
How long should I spend testing?
Ten minutes covers screen, keyboard, trackpad, camera, microphone, speakers and ports. Add ten more under load if you can, to check thermals.
Do these tests work on a machine I do not own?
Yes. Everything here runs in the browser on the laptop you are inspecting, needs no installation and no account, and nothing you test is uploaded anywhere.