Garmin died mid-ride or mid-run: how to merge the two recordings

The battery goes on a long run, or the bike computer locks up, and you end up with two files: the first half, and the second half after you charged it or grabbed a backup. Neither Strava nor Garmin Connect can join them. You can merge the two files into one and upload that instead.

Get both files

From Garmin Connect. Open each activity, click the gear icon, and choose Export Original. You get a zip with the .fit file inside.

From Strava. Open the activity, click the three-dot menu, and choose Export Original.

From the device. Plug it in and look in the Garmin/Activity folder (Garmin) or the equivalent for your brand. The files are named by date and time.

If the watch died hard and the first file never got saved, it may be truncated. That's usually fine: the editor recovers what it can, and the file won't upload guide covers what to expect.

Merge them

Open the first file in FIT File Editor.
Press the plus button in the editor and add the second file. It lines up after the first on the same timeline, in the order the two were recorded.
Check the map. The route should be continuous, with a small gap where the device was off.
Press Export. You get a single .fit file with one session, the distance carrying on across the join instead of starting again from zero.

You can add more than two files. A ride that died twice is three files, and they all go on the same timeline.

Upload the merged file

Delete the two original activities on Strava, or on Garmin Connect, or both. Otherwise the new one overlaps them in time and gets rejected as a duplicate.
Upload the merged file.
Check the total distance and time against what you expected.

What the gap looks like

The time the device was off shows up as a pause. Elapsed time includes it, moving time doesn't, the same as if you had stopped at a cafe. Distance isn't invented for the gap, so if you rode a kilometre while the unit was dead, that kilometre is not in the file. If you need it, the honest fix is to accept the shortfall, or to draw the missing section as a route in another tool and merge that in.

When the files won't merge

They overlap in time. Two recordings that were both running at the same moment can't be combined into one stream. This happens when a backup device was started before the first one died. Crop the overlap off one of them first, then merge.
They're from different sports. The merged file takes its sport from the first file. Change it afterwards in the details panel if needed.

The short version of the tool is on the Merge FIT files page.