Hi,
All batteries have a life span, rechargeable batteries are no different. Their life depends on how often they have been recharged, what levels of discharge they have been subjected to etc.
The solution for you is to replace the battery.
Here is a link that shows how to dis-assemble your tablet. It is handy in that it shows how to open the unit and once it is open it is easy access to the battery, which unplugs (seen 26 seconds into the video). You should not have to disconnect anything else.
https://www.youtube.com/watch?v=DsTQFV2Xq4E
The battery is available on Ebay and other sources. Just search online for ''Acer Iconia A1-830 battery'' to get the results.