Hi,
It seems as though the battery is failing and needs to be replaced.
What is the model number of the tablet?
Here’s a link to the ifixit [[Topic:Samsung Galaxy Tab A]] guides.
Hopefully your particular model is listed as a lot of them show how to replace the battery.
If your model is not listed or it doesn’t show how to replace the battery search on YouTube for ''Galaxy (insert model number i.e. SM-xxxx) battery replacement'' to see if there are any videos available online showing how to do it.
Once you have opened the tablet, search online for suppliers of the battery by using the model number of the battery (printed on the battery - usually starts with EB-)
If replacing the battery yourself seems too daunting, contact a reputable, professional mobile phone repair service and ask for a quote