How to get one activity back
- Charge the device and look for what it kept. Switch it on and open its list of activities. If the part before the battery gave out is there, get its file the usual way; the export guide has a section for each kind of device. If it isn't there, and doesn't show up in the device's app either, there is no device part to join, and your phone recording stands on its own.
- Get the phone recording as a file. If you finished with the Strava app, the file comes from strava.com, not from the app. The steps are in the Strava guide, and the export guide shows the menu.
- Drop both files above. A common pair is a FIT from the device and a GPX from the phone, and that works: the FIT guide and the GPX guide cover the two formats. Order doesn't matter.
- Check the map and the totals, then save. Each part has its own colour on the map. Leave the format on FIT unless you need another, and download the file.
Both recordings are read and joined in this tab, so they stay on your device.
When the device's file stops in the middle
A file cut off by the battery still opens here, and the page warns you about it. How that works is in the FIT guide. What matters for you is the result: the totals cover only the part that was readable, and a stop the device logged before it died isn't counted as moving time. We checked this on a file cut at 60 percent and on a ride with a nine-minute stop before the cut.
What happens to the time in between
Between the moment the device died and the moment your phone started, nobody recorded anything. In the merged activity that stretch is a pause. The minutes you spent swapping devices don't drag your pace down.
Where you went during the gap is not recorded either. In the options box, Count elevation change across gaps is off. Switch it on and the difference in height between the last point of the first part and the first point of the second part goes into ascent or descent.
If your phone was already recording
Some people start the phone as a backup when the battery warning comes up. Then the last minutes of the device file and the first minutes of the phone file cover the same time. The page turns the pair away and names both files with overlap in time. The FIT guide answers this in its questions.
The page can't trim a file, so remove the overlapping minutes from one part first. If the phone part is on Strava, its Crop tool cuts time off the start or the end of an activity. After cropping, choose Export GPX: Strava's crop article says Export Original brings back the original file, so that one still has the minutes you cut. Strava doesn't say whether the GPX keeps the crop, so drop it here and let the overlap message tell you.
After the merge
Uploading the result is the same as for any merged activity. For Strava, delete the two parts first; the Strava guide explains why. For Garmin Connect, use the Garmin Connect guide.
Two small habits help next time. Charge the device before a long day, and look at its battery before you set off. If you start a second recording while the first is still running, expect the overlap above.
Questions
My watch saved nothing. Can the phone file fill the gap?
No. A file the device never wrote can't be recovered here, and FitMerge only joins recordings that exist. Your phone recording is then the whole record of what it saw, and you can upload it as it is.
The watch part is on Garmin Connect and the phone part is on Strava. Where does the merged activity go?
On whichever of the two you want to keep it. Get each part from the platform it sits on (Garmin Connect, Strava), merge them here, and download the FIT. Then follow the guide for the platform you chose, Strava or Garmin Connect, for deleting the parts and uploading the merged file.
The page says the phone file starts 4 km from where the device file ends. What does that mean?
It is a warning, not an error. If you were driven or walked somewhere before you started the phone, that is what you are seeing, and the merge goes ahead. If you weren't, check that you added the right files.
Will the total distance include the stretch I walked between?
Not in FitMerge's totals or in a FIT download: nothing recorded it, so they cover only what the two files hold. A GPX is different, because Strava adds a straight line across the gap when it reads one; the GPX guide explains.