Default Losing charge whilst turned off

Hey guys,

I recently upgraded to the Nexus 5 but kept my N4 to sell on to make up for what I spent, however, it has recently started to lose charge whilst turned off. I have had the phone sent off once and nothing was fixed, they just flashed and older version of android and then when I got the phone back it update again and just carried on happening.

Has anyone else had this problem? I'm thinking I will have to send it off once again but I just wanted to know if someone else had the problem and there was an easy fix.