A no frills heart rate logger from bluetooth low-energy heart rate devices implementing heart rate service.

FEATURES:
Look for bluetooth devices advertising heart rate service
Select device to connect to using a spinner drop-down menu
Show heart rate on screen and in the notification bar (the logger runs as a foreground service)
Save heart rate to SD card/dataLogger/hrLog as a tab-separated text file (the folder will get created)
Save R-R intervals to SD card/dataLogger/rrLog as a tab-separated text file (the folder will get created)
Save Zephyr HxM custom data (activity count and maximum acceleration) to SD card/dataLogger/actLog as a tab-separated text file (has no effect on other devices as they will not respond to the request. The request will be made nevertheless).

OF NOTE:
I wrote the app for Zephyr HxM Smart and have only tested with ZTE blade version 2 running Android 5.1. The app crashes even on my own device on occasion particularly if I'm running other apps simultaneously. Typically I have been able to log around 30 min before a crash (which is why I have not been able to fix this bug). Please let me know how you go (how long did it log, what else you had going on simultaneously).