Why my cell gets battery low while connecting to the sim network?

Why my cell gets battery low while connecting to the sim network?

My N70 set had been running well. when I came to change the housing of the set, battery consumption problem arrived. All the functions are working well but when I connect the set to the sim network battery gets low and set collapses. So, should you have any help for me to overcome such problem, I gracefully wait your reply. Please help me!