Batteries will decrease in time as they are charged and discharged @Nigel63 How old is it?
Can you check in the battery stats to see what's using your battery the most.
You could try putting it in Safe Mode which will turn off all the downloaded apps and leaves the stock ones alone which can then help you by highlighting if its a rogue app.
Sometimes clearing the system fik caches can help. This is achieved by turning the device Off and then via a set series of button pressess that takes you into the phones file system and clear the system cache.
Or a back up and factory reset can help.
Finally a Samsung Experience Store or Samsung Service Centre location can run a diagnostic on the phone.
Current Phone ~ S8 Model: SM-G950F. CsC:BTU.