How will I know when the batteries need to be replaced?

You should receive in-app, push and/or email notifications, depending on your settings, when the battery level reaches approximate values of 25% remaining (just a warning), 10% remaining (urgent), and 5% (critical), you will receive an in-app, push and/or email notification, depending on your settings. The detector starts beeping once every minute after reaching roughly 5% levels to remind you to replace batteries.