Why is my iphone battery dying on standby?

This is driving me nuts and by the end of my shift and work my phone is dead, even though I hardly use it as I'm at work?! I know iPhones are know for having pathetic battery life but it's never been this bad.
Can anyone suggest a reason it's draining so fast?

What version of iPhone do you have because older models have less battery size and therefore don't last as long as the newer ones. Also if you get a text or something the timeout time to shut the screen off again might be set to never and then the battery would die. Also the battery is still being used up by being on standby.

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