A laptop that used to last six hours and now dies in two is not always a dying battery. Often something is working the machine hard in the background. Ten minutes of checking will tell you which one it is.
1. Get the battery report
Windows keeps a detailed battery history. Open an administrator Command Prompt and run powercfg /batteryreport. It saves an HTML file — open it and look at two things: Design Capacity versus Full Charge Capacity (if full charge is under 70% of design, the battery itself is worn), and the recent usage graph, which shows when the drain started.
2. Check what is using power right now
Open Settings > System > Power & battery and click Battery usage. It lists apps by power use over the last 24 hours. A browser with twenty tabs, a cloud sync app stuck uploading, or a game launcher running in the background are the usual suspects. Close or limit the worst one and watch the estimate recover.
3. Look at the screen and the power plan
The screen is the biggest power draw on most laptops. Dropping brightness from 100% to 60% can add an hour by itself. Then check the power mode: Settings > System > Power & battery > Power mode should be on Balanced or Best power efficiency when you are away from the charger, not Best performance.
4. Stop the silent drain: sync, search and updates
Three background jobs drain batteries quietly: cloud storage re-syncing a large folder, Windows Search indexing a new drive, and Windows Update downloading in the background. Pause cloud sync for an hour, check Task Manager for SearchIndexer or TiWorker using high CPU, and let updates finish while plugged in.
5. When it really is the battery
If the battery report shows full charge capacity far below design, no software tweak will bring the hours back. A worn battery also swells sometimes — if the trackpad feels raised or the case does not sit flat, stop using the laptop and get the battery replaced; a swollen battery is a safety issue, not a performance one. For most laptops a replacement battery is a straightforward job and far cheaper than a new machine.