Iphone 5 heating up and battery problems

Hello this is my 3rd iPhone my 1st(original) had a broken lock button and got replaced my 2nd and my latest (3rd) both have the same problem heating up and draining battery.apples customer service are by far the best but i had one faulty phone and they send out another phone with the same fault I'm just tired of keep waiting for a replacement and then that gets bust straight away I've looked around and seen that by closing multi task and that it should help but it doesn't i use the phone from 8am to 4pm and the battery's dead yes its 8hours but in them 8 hours I'm only on it for 45 minutes if even that as i be busy so the remaining time its on standby.when i rang apple they said battery shouldn't be like that so why send another refurbished phone out with the same problems thanks for reading and reply if you can.thanks