iPad Battery Depletion

About a month ago, the battery on my 2 year-old iPad Air, using iOS 11.2.6, started depleting overnight while not in use and now is depleting from 100% to as little as 3%. I have deleted little used apps, disabled location services for many apps, cleared out Safari history and website data but still the battery depletes while the iPad is not in use. Under Settings - Battery Usage, Home & Lock Screen shows 96% usage over the last 24 hours and 70% usage over the last 7 days. In the last 15 minutes or so while I have been composing this question on my iMac, the iPad battery has reduced from 100% to 96% while sitting beside me, not in use. Any suggestions as to what is wrong and/or what else can I do?