Joined: Jan 2009
Posts: 44,172
Likes: 6,404
From: New York, NY, and High Falls, NY, USA
Bikes: 1962 Rudge Sports, 1971 Raleigh Super Course, 1971 Raleigh Pro Track, 1974 Raleigh International, 1975 Viscount Fixie, 1982 McLean, 1996 Lemond (Ti), 2002 Burley Zydeco tandem
I use an external battery to power my phone. It has enough capacity that I can keep the phone's display on all day and keep the battery topped up.
The downsides to using a phone are:
1. You need a really good mount. Don't cheap out. I use a Quad Lock.
2. Accuracy is not as good unless you use speed (and cadence?) sensors. I don't use them, because the GPS accuracy is good enough for me, but it's not for everyone.
3. Using a phone's screen is not always possible, even when it's right there, because fingers bounce, and I can't look at the phone for more than a second. A bike computer has tactile buttons.
You could use a dynamo-powered charger to charge a cache battery and run the phone off that battery. I haven't tried that, because I have a power outlet at the beginning and end of every ride. A good such charger is about $120.