Earlier quoted context omitted.
All phone battery longevities are probably within a factor of 2x (for reputable brands). An iPhone official battery replacement is $100 installed. Is longevity really that material for a component that is 1/8 the price of the phone?
Apple wouldn't replace my ipad battery and is bullshitting me saying it is at 100% health. The device is 4 years old and I can tell it's only lasting half to 2/3 of the original battery life (even with only safari open). I'd be happy to pay $100.
One, do a wipe and re-install from backups. If the device doesn't really have much of importance on it, just wipe it and start fresh.
Two, discharge the iPad until it shuts off, then fully charge it and leave it plugged in for several hours after that. That will update the battery capacity gauge.
You can see the internal battery stats by plugging the iPad into a Mac and running any of a couple of different utilities - one free one is coconutbattery.