Galaxy S5 SM-G900f draining battery


hi, i need help from experts, my Samsung Galaxy S5 SM-G900f draining battery continuously 1% in 2 minutes and some times it automaticaly restarting and show battery 0% and after another restart it shows 30% and different. it was perfectly working 1 week back. battery is perfect but screen is using 40% but i dont know how. it is on OS 4.4.2. thank you in advance.