Battery health is one of the easiest things to overlook when buying a used laptop — and one of the most expensive to ignore, since a full battery replacement can cost a meaningful fraction of what you paid for the laptop itself.
1. Ask for the battery health percentage directly. On Windows, this is checked with a simple built-in battery report; on a MacBook, it's under About This Mac > System Report > Power. Any honest seller should be able to give you this number, or let you check it yourself before paying.
2. Watch how fast it drains during a demo. If a seller lets you use the laptop unplugged for ten minutes doing something normal (browsing, typing), a battery that drops more than roughly 8-10% in that short window is often already degraded.
3. Check if it holds a charge overnight. A battery in good health loses very little charge sitting idle. If it's noticeably lower the next morning after being fully charged and left untouched, that's a warning sign.
4. Look — and feel — for swelling. Gently check if the trackpad has any unusual lift on one side, or if the bottom panel doesn't sit perfectly flat. A swollen battery is a genuine safety issue, not just a performance one, and should be a hard pass.
5. Ask about charging behavior. Does it charge normally from 0-100%, or does it jump erratically (e.g. from 40% straight to 60%)? Erratic charging usually points to a battery the system can no longer read accurately.
Every laptop we list at ZagoTech has its battery health checked and recorded as part of our standard inspection before it ever reaches the shop — it's one of the specific line items in every listing's inspection report, not an afterthought.