Samsung Tab 3 battery draining fast but going up if left on standby

[Android Central Question]

Have had a cheap Samsung Tab 3 for just under 3 years. Last year the battery suddenly started draining very quickly, then remaining on 1% for hours. I've also noticed that the "charge" percentage actually increases if the tablet is left on standby for a couple of hours. Have tried various "solutions" (such as battery calibration apps), nothing seems to work. Any ideas?

 

[Rukbat]

First, 3 years of draining the battery below 30% is amazingly long life.

Second, after 3 years it's probably time to have the battery replaced. Having it "regain power after sitting is another indication that it's at the end of its life.