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I'm in the market for a cycle computer that lets me follow a preset route via GPX. I've tried using various apps on my phone but GPS+screen always on means my iPhone dies within a few hours, which isn't helpful.
The new Lezyne computers look just the thing, relatively cheap as they appear to off-load the actual navigation to a paired phone but as it doesn't need to have the screen - and hopefully the GPS - enabled the battery life should be much longer. However they're not available yet.
Anything from Garmin that isn't a bare tracker is 400 quid, so that's out.
Any other suggestions apart from taping a battery pack for my phone to the bike?
I have a Garmin Edge 800 for sale as I've just upgraded to an (awesome) Edge 1000. Works perfectly and had a fresh battery put in by Garmin a year ago so should have loads of life in it.
That'll tick all your boxes, let me know if you're interested, E-mail in profile.
Garmin Edge Touring, can be had for about £140, perfect for that.
Depends if you want maps. The Garmin 200, (old but still available and reliable) has a 'breadcrumb' feature, which is all I use. Take a paper map as backup though.
Garmin Etrex 20x will also do what you want for about £130 (less if you shop around). The 'Open Fiets Map' cycling map works particularly well with it around here (N Wales).
For the best battery life consider walking GPS units rather than cycling, with replaceable batteries. I have an older Garmin that uses AAs, and can get 10+ hours on NiMH rechargeables, and almost 24 hours on disposable Lithiums. As above, open source mapping is pretty good these days.
Etrex.
I use an edge touring for what you describe