Hello, we deployed AMs with 64GB cards (formatted to FAT32 with a Mac) and 3 x eneloop 1900mAh AA batteries. Calculated they should last ~8-9 weeks at our recording schedule. Have just found that the first 8 we retrieved all ceased recording within a day of each other, 4 weeks after deployment. The cards only have 19GB of data each, so probably not a card issue. Individual testing of AA batteries pre- and post-deployment showed average voltage of 1.34 each prior, and now each AM had at least one battery with voltage as low as 0.06-0.21V (but others up to 1.14V). Stumped, as a colleague had the same set- up with a slightly longer daily recording schedule and longer deployment, with no issues.
Am I right in thinking:
1) there is some critical minimum threshold in battery voltage, below which the AMs won’t work, despite our calculations they should have lasted twice as long?
2) if we decrease the amount of data collected per day they would last longer, and
3) does ticking or unticking the ‘enable low battery cut-off’ box in the configuration app make any difference to this?
4) is there a large error around the mAh drawdown prediction on the app that could explain our predicted vs actual data collection period?
Dear Alex-san
Thank you for explaining about the battery voltage. I had same problem.
You showed us the comments field of the WAV file on Mac. I found another way to check the battery voltage in the AudioMoth Play Web Service. ( Cos my main PC is windows. )
https://play.openacousticdevices.info/
Hi, Are these eneloop NiMH rechargeable batteries? If so, the problem is the low cell voltage of such batteries. AudioMoth needs about 3.6V input to guarantee 3.3V to the SD card. Typically the cards will continue to work at lower voltages but they will eventually stop writing successfully.
The Energizer Ultimate lithium batteries that we recommend for any deployments that are likely to be power intensive - https://www.openacousticdevices.info/batteries - have an initial voltage of 1.65V and will be pretty much fully discharge before they reach 1.2V so they give the full quoted lifetime. Alkaline batteries aren't quite so good but stay above 1.2V for about 2/3 - 3/4 of the their life.
A NiMH cell has a much lower voltage and is quite close to the voltage threshold for the majority of its life so it's much harder estimate the actual capacity that you will achieve. Some people report that they work well but I think it is probably a bit marginal and depends whether the recording schedule gives the battery time to recover a little between writing to the SD card and also which SD card you use as they also have different characteristics.
The 'enable low battery cut-off' box tells the AudioMoth to actively check the battery voltage and stop recording when is reaches about 3.3V (this measurement isn't very precise). You should typically use this all the time as it ensures that the attempts to record are not made when the voltage is too low. We make is selectable as some people power AudioMoth through the USB socket without batteries.
The battery voltage is written into the comments field of the WAV file so you can track the dropping battery voltage in the recording that were made.