I don't use backpacks so I can't comment on that.
I do use my iPhone, along with Cyclemeter, as a cycling compute. It's paired with a Wahoo TICKR heart rate monitor, a Wahoo BlueSC speed/cadence sensor and a Wahoo RFLKT display device. A 60 mile ride this morning left me with 68% battery life left and a century ride I did, in June, left me with 31% battery life left. I don't use portable battery chargers on my rides, but I do take one along on rides over 80 miles, just in case.
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HCFR Cycling Team
Ride Safe ... Ride Hard ... Ride Daily
2017 Colnago C-RS
2012 Colnago Ace
2010 Giant Cypress