Originally Posted by IntrepidRunner
This wrist case would not easily allow a PowerLet cord, so the iPhone must run on batteries. How long will an iPhone battery last when receiving GPS signals?
With the screen on and at full brightness to combat direct sunlight, around 2 hours.
With the screen off, I've done a 6 hour ride and still had a bit of juice left, maybe 8 hours max?
This was using MotionX GPS set to it's most power hungry (and most accurate) track recording mode. Different apps will probably drain the battery at different speeds.
Also, as the battery gets older and depending on your usage/charge cycles and the luck-of-the-draw physical properties of your individual battery... you will notice the battery life go downhill over time. A new device gets much longer battery life than a two year old one.
If you've got a strong phone signal, your battery will last longer. My 6 hour test was mostly in poor or no signal areas. Removing the sim card (assuming it's a GSM iPhone) will probably give significantly better battery life, but I haven't tested. Airplane mode disables the GPS receiver unfortunately, which is stupid in my opinion.
You can buy battery packs that double the battery life of the phone, according to (possibly biased) claims of the companies that sell them. And I reckon you could charge the phone while riding... keep in mind, a flat lithium-ion battery charges *much* faster than an almost full one. So if you're down around 10% and charge it for 5 minutes while having a break from riding, it will actually get quite a lot of charge in. Pay attention to how many watts the charger puts out. Look for "iPad compatible" if you can, they're beefier and will charge an iPhone quicker than an "iPhone compatible" charger.