@Gwbw to start with don’t close the apps from the app switcher as this uses more battery power when starting the up again. When the apps are in the switcher they are in a suspended state using next to no power. If you go to Settings>Battery you can see what apps are using the most battery power. It might be worth allowing the battery to fully use up all its power and then leaving it a few hours ( over night is best ) then charge it up fully then run it down to nothing leave it again then charge it fully once more and see how it goes.
do you have a good cellular signal ? it might be worth a network reset because if you device is searching for a signal it can uses loads of battery power. You’ll need to put your WiFi passwords back on the device after this. If you go to Settings>Battery>Battery Health what does it say next to maximum capacity?
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