I think most iPhone users have discovered that their little "magical" friend has a propensity for chewing through battery life like high-schoolers chew through Doritos. Tips and tricks for extending life abound, but all the ones I've read amount to disabling functionality of the phone or applications.
I accept that keeping the GPS receiver on at all times will be a big drain; there isn't a lot of value of keeping it on when I spend the majority of my day inside and away from windows that could allow reception.
No, the things I don't want to disable are important functions to me. Things like push messaging; that's why I have a smartphone in the first place: to stay in touch and have messages at my fingertips.
So I husband my battery use, and most days am lucky to get through it with at least 30% still showing in the indicator.
But over the holiday weekend, I noticed something: I was regularly staying above 50% by the end of the day. Curious. I wasn't using it any less; if anything, I was on it more, taking pictures, checking social media, etc. What was the big difference?
I think I found it, and you may find it a surprise...
I use Bluetooth (BT) for in-car hands-free and for my Sony-Ericson HBH-IS800 headset. I normally leave the BT radio enabled so the phone automatically connects with the car; the way Apple buried the BT controls in the Settings app is pretty annoying for quick or frequent BT enable and disable, so I take the lazy way and leave it on. And I normally leave the HBH turned off to retain battery life, so the normal state for the iPhone is Bluetooth On/Unpaired.
And that seems to be a huge battery drain; an even bigger drain than having a paired device connected to it.
Here's what happened: this weekend, I kept my phone in my pocket, but didn't keep the HBH with me; I left it plugged into its charger until time came that I might want to use it. I noticed at one point during the day that my phone was paired up with it, however; I didn't realize it could/would do that while charging. I also noticed that the phone wasn't chewing through the battery like usual. So I did a qualitative experiment: One day, I would keep the phone paired with my HBH at all times; the next day I'd keep it un-paired, as usual; and the final day, I'd actively enable and disable the BT radio whenever I actually needed it.
Again; this was qualitative, not quantitative: there are too many other variables at play to put real numbers to it. However, by the end of the experiment, I was convinced: If you want to extend battery life without turning off the BT radio (by far, the best choice), make sure it's paired.
As I understand it, it goes a bit like this: the preferred state for the device—when BT is enabled—is to be paired. With anything. If it's unpaired, the radio throws extra power into trying to see if one of its partners are "out there" trying to reach it. When it's paired—and able to readily "stay in touch" with the partner—the radio draws less power than when it's "seeking."
Rule of thumb: when it comes to BT power consumption, radio off < radio paired < radio seeking