Same problem here, except it seems mainly to happen when I have WiFi turned on. Just now it went from 100% to 5% in about 10 seconds when I turned on the WiFi. I have iPhone 6SE.
BeetsyG
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I just updated my iPhone 6 SE to iOS15 and found the same issue. Just now it was at 100%, I turned WiFi on, and it drained the battery to 5% in about 10 seconds. Turning WiFi off keeps the battery alive, but as other posters have noted, without WiFi or Bluetooth, there’s a lot less I can do with the phone.