Battery discharge issues Lumia 920

[Mr_Grinch]

Hey everyone. I recently got my sister a factory refurbished unlocked At&t Lumia 920 and it appears to completely discharge from 100% to 0% within 3 hours without doing anything. I myself have a Lumia 920 and can easily get 20+ hours on an average day of usage so the abysmal battery life is really weird. Does anyone know of any apps that can diagnose the state of the battery in the phone? I'm thinking that the phone might have a really old battery and it simply can't hold much charge so it discharges really quickly. I could simply just order a replacement battery and replace the old battery (not too hard if you know how to properly open up the phone). The phone runs 8.1 with cyan (not 8.1.1 since my sister isn't in the DP program), so everything is up to date. The phone originally came with like Amber or a really old version of WP8 and even then it discharged within a matter of hours so I don't think it's a matter of software and a factory reset won't appear to do much. Any help, advise or apps that's could diagnose the state of the battery?