New new phone, old old problem

My new RAZR is having the same trouble holding a charge that the old one had. I can't possibly have gotten three bad batteries in a row! Or two bad phones in a row. But what else could it be?

• My settings are fundamentally the same as Siobhan's identical phone, which is a year and a half old and still holds a charge at least a day, usually more.
  
• I've tried several chargers. (Next experiment though is to use her charger.)
  
• I barely use the phone. I haven't had a call or even a text message in days. Siobhan uses hers a few times a day on average.
  
• Could it matter where I am, which cell towers I'm communicating with? Could my office somehow drain it more?
  
• I have more pictures on mine than she has on hers, but we both have an MP3 ringtone.
  
• Hers connects by Bluetooth to something for an hour and a half every day, but mine rarely connects to anything by Bluetooth, even though it's been bonded to more things in the past. But the extra things it's been bonded to don't even have Bluetooth turned on.
  
• If all else fails maybe I'll try swapping batteries with her phone and see if the problem stays with the battery. I can't imagine it will since this is the third battery I've had.