1) I was inserting the wrong charger in my phone (is this sentence correct or any better word other than insert)
2) My cell phone battery is low or It should be cell phone's battey(why phone's )
1. I was plugging the wrong charger into my phone.
2. My cell phone battery is low. When it's a description of something "belonging" to an inanimate object, we don't use the apostrophe+s construction very often. (Cell phone battery; table leg; TV remote control)