Did it die after talking to his girlfriend?
That would be extremely convenient timing, but it’s possible he talked it to death. But then the GPS records would exist for the day in question (up to the time the battery died) that the prosecution is stonewalling. Another possibility this brings to mind is the gal pal could be lying from start to finish when she claims to have had a conversation with St. Skittles.
I don’t think the battery was dead all day. He wouldn’t have been carrying it if it was. I think the dead battery is a ruse the prosecution is using to justify not giving up the legally requested GPS records for the day of the event. His traced path must be pretty damning for them to do this, unless they are simply stalling to run Zimmerman out of money and has to go with an incompetent Public Defender.