Our organization recently deployed 21 AudioMoths for recording birds, bats and grasshoppers. They were set to record 3 times 2 hours per day during one week, and encased in the waterproof case. Batteries are no older than one year and were fully charged a couple of days before deployment. The AMs sat in the fields for maybe 3-4 days before recording periods started.
After getting the AMs back from the field, I noticed that about only a third of the devices recorded correctly. 14 AMs either recorded no files at all (3 devices), stopped recording after a few days (2) or recorded increasingly short periods during the week (6).
I updated the AMs to the newest firmware before deployment (version 1.7.1). We recorded using the same settings last year and got much longer recorded periods.
Is it possible the batteries are two weak? I inspected the .wav metadata of the files recorded on one device using exiftool, and battery voltage seemed to have been fine.
Temperatures got to around 0°C and at the beginning of the week there was some strong rain. I don't think this should pose a problem?
However, the decreasing file size over time suggests some kind of voltage problem.
What else can I do to find the cause of these problems and prevent them from happening in the future?