Why does my battery drain so fast on my Lumia 520 with Windows 10 Technical Preview?

[Windows Central Question]

using lumia 520 with windows 10 technical preview and its battery draining in just 2-4 hrs

hi
i am using windows 10 technical preview (insider fast) on lumia 520 ..its laggy , bt acceptable but main problem is that its BATTERY SAVER is still not working and my lumia 520, eating its full battery in just 2-4 hrs without hardcore use
please suggest me any way so i can update it

thank you