Try these Windows steps in order:
- Make sure the PC is fully shutting down, not going to sleep.
- Select Start > Power > Shut down.
- A laptop that enters sleep or suspend can still use battery while it looks off.
- Restart the PC once.
- Select Start > Power > Restart.
- This can fix temporary power-management problems.
- Run the Power troubleshooter.
- Go to Start > Settings > System > Troubleshoot > Other troubleshooters.
- Select Run next to Power.
- Review battery and power settings.
- Go to Start > Settings > System > Power & battery.
- Check battery level and how it changes through the day.
- Open Energy recommendations and select Apply all, or adjust settings such as screen brightness, screen saver, and sleep timing.
- Turn on Battery saver.
- Go to Start > Settings > System > Power & battery > Battery saver.
- Turn it on now, or set it to turn on automatically at a chosen battery level.
- Reduce background power use before shutting down.
- Turn off Bluetooth and other wireless connections when not needed.
- If Internet is not needed, turn on Airplane mode from Start > Settings > Network & internet.
- Check what the power button does.
- Select Search on the taskbar, enter control panel, then open Control Panel.
- Go to Power options > Choose what the power buttons do.
- Confirm the power button is not set to a sleep state when a full shutdown is expected.
If the battery still drains heavily while the laptop is off after these steps, that points to either the device not fully powering down or a battery health/hardware issue.