(Cross posting from E&L, did not get any response there.)

This should be a basic fact, but I can't seem to find the answer in the owner's manual, or in Google.

In the "maximum data" mode (1 datapoint per second, heart rate, cadence, power), what is the longest continuous ride that I can record with Edge 800, and is there a limit at all, other than the amount of internal storage?

I used to ride with eTrex 30. It had relatively huge internal storage (compared to Edge), ~2 GB, and running out of space simply by recording rides was not a problem. It had a much smaller memory buffer of undisclosed size for the current ride, and it would run out of memory after about 5 hours of continuous recording at 1 point per second. However, data beyond 5 hours wasn't lost, it was simply dumped into internal storage and I could pull it out and stitch with the current ride. (Or I could increase the recording interval to 2-4 seconds, and raise the maximum ride length accordingly.)

Edge 800 seems to have only 100 MB of internal storage and the manual says that it will stick to 1 point/second when it detects the power meter. So I'm wondering what will happen if I go on a really long ride.

I think the answer is it really doesn't matter for a single ride.

My largest .fit file from my Edge 500 is 1.8MB, and that's from a century. I always use 1/sec data recording with heart rate, cadence, power, gps, etc...

Do you think it can record a whole _double_ century at once at 1/sec?

Without a doubt. I archive the files off to my computer about every 3 months or so because the load times from Strava get too long as it searches through all the files. You would run out of battery many times before you run out of storage.

I believe the internal memory is capable of storing about 1,500 hrs so unless you're going very slowly it should be sufficient. If that's not long enough you can store to a 2GB memory card and probably record a years worth of data.

As far as recording a long single ride wouldn't it run out of battery first?