dell tablet 7840 issue

first of all, i purchased this tablet january in 2015.

i found this device turns off by itself at battery 22% while using it(about a month ago) and now,  it turns off at 37% over and over.

in order to power on, i have to plug in.  if not, there is no way to run it. 

plus, one of my acquaintance's happened the same issue since used this tablet about six months ago from now on.(so he did factory reset, but his still the same now.)  i guess this tablet has technical difficulty.  

i'm living in korea and asked for a chat service on the dell official website, but they just gave me an international call number.  it seems not like solving this problem.