when it start's the battery get low

@gulrejkhan what you are describing sounds like a power issue. I suggest that you start off with replacing the battery. Then re-evaluate and let us know if that helped.

Since iFixit does not yet have a guide for this, use something like this video as your guide.

While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has the same problem as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.

Repair is War on Entropy!