Boost battery life on smartphone: How to save battery using Standby apps feature on Android smartphones
www.gadgetsnow.com
Google added a host of new features and functionality with its latest version of the mobile operating system, Android 9 Pie. Over the layer of several other easily noticeable features, there’s one hidden feature called ‘Standby apps’ that can help users boost smartphones battery as well as performance.

I did not know of this before I found this article, this is bit more advance trick since you need to enable developer settings first which can be done by tapping about phone seven times in a row.