I just picked up Bluetooth headset for my iPhone and I'm wondering how using it will effect the iPhone's battery life. I will probably only use it when driving, so I leave Bluetooth turned off on the phone during this time? Or will just turning the head set off be plenty?
I think BT only seriously drains battery when it's actually doing something. But being connected to a headset may involve some sort of chatty keep-alive traffic, so that might count as actually doing something. For example, the PlayStation 3 has a BT media remote that's on and connected all the time the PS3 is on, runs on a couple disposable batteries, but the batteries last a long time, as it only uses power when you hit a button to send a signal to the PS3. Same with the BT controllers. They're rechargeable, but they don't turn off automatically after a period of no use. Still, let them just sit there turned on without actively using them and the battery doesn't ever noticeably drain.
I don't talk on the phone enough to use my BT headset on it, so I keep BT off. Otherwise, I've found the only thing that really causes much battery drain on an iPhone is actually *talking* on it -- like any cell phone. I'll listen to music for hours while working with airplane mode switched on, so nobody can call me and interrupt me, and a the battery gauge won't even budge. Leaving both cell on in standby and WiFI on, the battery barely drains, too.