I've been looking at the Garmin Edge 500 which claims an 18 hour battery, though many have stated 12 hours. Garmin also sells a power pack which gives another 20 hours, reviews have stated about 16 hours. This should realistically provide a 28 hour runtime.
A number of people have said that using other, larger battery packs has resulted in the unit not recording as it thinks it is charging. I suspect in the $90 Garmin battery back, there is some intelligence that tells it that it has a remote battery, not that it is being charged. I could take one of those apart and use a Shimano or Schmidt dynohub to power the GPS, or, bring a few battery packs to cover rides.
I've written Garmin, but, their response was a bit vague and I'm waiting for clarification. I found one mention that it will hold 180 hours of tracks, but, I don't know if that means I could record a single 180 hour track. The Garmin 800 claims 750 hours of tracks.
Has anyone run across the maximum length of a track that can be recorded assuming that you can keep the unit powered? I will be running the cadence sensor and heartrate monitor and not running in SmartData mode. I will be tracking with 'every second' accuracy.