shut's down for no reason?

First of all i would like to say thanks to who ever is taking the time to look at this question. Well i have a nobis tablet (model no NB7850 S) that keeps shuting down for no reason. I mean its fully charge to 100 percent and i wait about 5 minutes and then it shuts down. what i did notice is that it wont shut down when its charging . But as soon as i pull the plug it wait's about five minutes and then it shuts off just like that. Iam thinking about buying a new battier for it or maybe installing a app that would tell me whats wrong . I have a lot of options but i rather get some info from the guys that know best. thank you