Join Date: Aug 2008
Location: Ann Arbor, MI
Bikes: '15 Kinesis Racelight 4S, '76 Motebecane Gran Jubilée
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Bluetooth is a power hog, and your phone will not last long, probably about an hour from a full charge, displaying HR info and communicating via BT.
I don't know what the ANT+ dongle uses powerwise, but I'd still think going ANT+ is better so that you've got connectivity in the very likely event you want to get a dedicated cyclometer (and leave the phone for calling), or add power and cadence info to your training.
The firm iBike make an iPhone case that mounts to the stem and has space for a backup battery, so that may be worth looking into. The also have an app that allows split screen nav and biometric readings.