ChromeOS doesn’t yet provide the ability to customize battery percentage alerts, but should in a future release. However, if you would like to have this ability, download and install the Charge Status extension from the Chrome Web Store.
Extension page: Charge Status
After you install the extension it will take a bit, but after 10 seconds or so your initial battery charge status will display…
You can also go to the Profile tab and customize your Low and Critical Alert Thresholds for which you’ll receive notifications.
To access these pages later, just tap or click the Charge Status icon in the Chrome web browser Extensions list.
For even deeper reporting on the Chromebook’s battery life, you can use the battery_test command in the ChromeOS Developer Shell.