Ambient Display as far as I know is a feature that allows the user to check specific information (time, date, notifications, etc) on the screen while the phone is locked, with the less possible battery consumption. Please note that I said "less possible" and not "cero". Pixels on the screen have to be lighten up in order for you to be abe to see these information and these turned on pixels will consume battery. But not that much (and thats because of OLED technology). See it for yourself: 7hrs 30mins of usage , consumed just a 3% of your battery. Thats absolutely normal (and expectable).