Bluetooth is at a layer beneath outside tinkering. This is why almost anything to do with it just creates shortcuts, toggles, or spoofs devices (mono headset receiving music). If the car has an older BT profile (1.0, 2.0), it may not be as clear. You'll have to either find a dongle for the car that may have better reception or connect using a different method (ie: earphone). Worst case scenario, the BT hardware in the phone itself is faulty.
Battery life... That's a loaded question. The S4 is the most temperamental device I've seen when it comes to battery life.
- Some people have good batteries and no software hiccups, giving the phone life for an average ~18 hrs with moderate usage and everything still enabled. Longest so far is up to 5 days (all standby).
- Some have phones where the Google Ecosystem has gone on a rampage, killing batteries in as quick as 6 hours, forcing the disabling of several Google services and features to get through the day.
- Then there are the ones with just plain bad batteries.
Depending on where you fall in this list and your daily usage, battery life will either be a moot point or in need of task killers and power savers. You also have the option of going with another ROM (depending on your software version) that may have better battery life.
So far the only way to unroot the S4's are to use triangle away to reset the download counters and then reinstall the stock image using Odin. Again, what you can do with flashing depends on your software version.
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