iphone 5s battery not holding charge

I have been have problems with my Iphone5S battery life recently, I have followed the steps to save as much battery life as possible and I went to a genius bar as I thought the battery may be on its way out but they ran tests and told me its still a healthy battery, in the 90% mark and anything over 80% is good especially when I have had over 500 charges. They told me software errors and crashes have occurred a lot and to complete a fresh install of IOS which I'm hoping i factory settings, which I have just completed yesterday and so the software is completely new. I have found that the battery has already decreased by 10% in 20 minutes without hardly any use. Does anyone know whats causing this issue? thanks