When you put your phone in sleep mode and you leave your iphone with Safari still launched and on a say a news page, which updates frequently, even though the phone is asleep, the battery discharges as the news page is updated. There is, apparently, no way to turn off background refresh for Safari specifically, unless perhaps you turn off background refresh totally. (We like to leave one specific app in background refresh mode in case of contact from our daughter.) Personally I always shutdown each app as I finish with it to stop background refresh, unlike our other iphone which often has recent apps still launched. Last night the other battery discharged to almost empty and the battery info suggested that only Safari had been launched. This may also happen with a similar page on a Safari tab. (The above is for two iPhone 8's; so may or may not be true for other models.) Edit: 2 minor points omitted: a) I am using ios 13.2.2 on both iPhones b) News pages have images and it is these that seem to take a lot of processing -hence battery discharge.