Battery question

[Bentoix]

So the battery will charge to max 81%, even after overnight charge, when it reaches ~20% it will start to free fall, no matter what the use, it can only be charged when the phone is off, when it's on it actually acceleratingly starts to go down.
Then it's definitely empty, you can't turn the phone on. I tried calibrating and switching charger and cable but it doesn't change.
I think I've got a faulty battery but can't be sure if it's not software problem and I'd like some second opinions as I'm new to android.

I have a 2800mAh at 3.7V, 5" screen at min. brightness,3g off,no mobile internet and apps,android 4.2

https://www.dropbox.com/s/751bjyft9klvnh9/Screenshot_2014-06-30-10-02-28.png ,rising and flat is charging while off
https://www.dropbox.com/s/ev4v14ttm94r74t/Screenshot_2014-06-30-10-02-35.png ,voltage
https://www.dropbox.com/s/6xdbsmy8uxkb8on/Screenshot_2014-06-30-10-01-28.png ,android's usage statistics

Thank you for any help

 

[Jeffrey]

Welcome to the Forum.

What phone do you have?

 

[Bentoix]

Doogee dg500c. I think its the bad battery, because whenever the percentages get screwed up the voltage drops and the battery never reaches its maximum voltage so I'm confident it's not phone or software problem. Is this right? I'm going to try new battery.

 

[promo87]

Well, do one thing remove your battery put it over the flat surface i.e. Put it upside down over the flat surface and then try rotating it ! If it rotates freely then I guess your battery needs a replacement which I am sure is going to be your case.