In order to optimise performance and autonomy of your external battery, it is programmed to automatically go into sleep mode.
This is called “sleep mode” and is set by default to occur after one hour.
You can customise this sleep mode, according to your needs, from your Angell app (version 1.4.3 onwards).
To do so, go to settings in the mobile app at the bottom right, select "Preferences" then "Battery Sleep" and select the duration that suits you.