Why won't my tablet turn on
I plug it in and there is an empty battery graphic.
So I charge it for a few hours and it keeps happening.
This has been going on for months
Try replacing the battery and then check what happens.
At the moment you don’t know whether it is a faulty battery that is the cause or something else. You need start testing with a known good power source i.e. the battery, otherwise you may be spending a lot of time looking for another problem that is not there.
Here’s the ifixit Samsung Galaxy Tab Pro 12.2 Battery Replacement guide that will help you to do this.
ifixit sells replacement batteries for the tablet, click on Buy in the Parts section of the guide. (note the shipping restrictions when looking at the part).
Alternatively search for SM-T900 battery to find suppliers that suit you.