I have experienced what I believe is a battery life issue, but I hope to run this situation by you all and see what you think or suggest.

Main question I have is, does anyone recommend a rechargeable AA battery, either Lithium or Ni-MH for field applications in Audiomoth ARU?

I know the going recommendation is to use the Energizer Ultimate Lithium non-rechargeable batteries (I know these well as I use the in other applications, but they are a little pricey, and cannot be reused...), as mentioned in this, very similar post here:

Audiomoths stopped recording early - battery issue? | openacousticdevices

Here is the background story:

In 2020, we purchased a number of Audiomoth units for a new project, and we decided to go with using rechargeable batteries, to decrease our waste production.

We considered the OneLoop batteries, but since they were a little expensive, we went with another high rated battery, the EBL 2300 mAh Ni-MH batteries. We purchased two 8 bay chargers (XTAR VC8) and charged all the batteries before loading each ARU and deploying around 60 of them in the field in the 2021 field season.

Our field locations are spread along Western Oregon and Washington, ponds and streams just east of and west of the Cascade range, all around 2000-4000 ft in elevation, from around May through September.

We experienced no major issues in 2021, although we decided to swap out batteries in about 40 of the units mid year, just to be on the safe side.

Units not swapped out lasted an average of 3 months, with some expected variation in battery life.

In 2022, we saw no reason to change, so we recharged all batteries again, and redeployed our units, same basic approach as spring of 2021.

We decided to not swap any batteries this year, since we had such good life in 2021, understanding that ~3 months was plenty to reach our goal.

All units are clones, programed the same with the same sandisk 64G micro SD cards, etc.

Below is a configuration file from one of our units, in 2022.

Basically, all units are programmed to turn on at 7pm PST, record for 5 minutes every half hour, and off at 2am, every day, repeat, etc.

Note that some variables in the programming were not available during the 2021 deployment due to older firmware.

The only option we turned on in 2022 that was not in the 2021 deployment is the Enable Energy Saver Mode.

Device ID : 247475025E471DC6

Firmware : AudioMoth-Firmware-Basic (1.7.1)

Time zone : UTC

Sample rate (Hz) : 16000

Gain : Medium

Sleep duration (s) : 1500

Recording duration (s) : 300

Active recording periods : 1

Recording period 1 : 02:00 - 09:00 (UTC)

Earliest recording time : ---------- --:--:--

Latest recording time : ---------- --:--:--

Filter : Band-pass (0.3kHz - 8.0kHz)

Amplitude threshold : -

Minimum trigger duration (s) : -

Enable LED : Yes

Enable low-voltage cut-off : Yes

Enable battery level indication : Yes (NiMH/LiPo voltage range)

Always require acoustic chime : No

Disable 48Hz DC blocking filter : No

Enable energy saver mode : Yes

Enable low gain range : No

Enable magnetic switch : No

Enable GPS time setting : No

As I download files off of these 2022 units (about half way through), I find almost all failed (stopped recording) after 1-2 months, many even sooner, like a week or two after deployment.

Checking the batteries, I find many throw an err message on our charger, and testing them with my voltmeter they show 0.01 volts, or less. So those look like they are done.

In almost ever single unit, at least 1-2 batteries show err messages (0.01 volts) and 1-2 will show ~1.22 volts.

In spring of 2022, when recharging the batteries from 2021, I did see a few batteries with err messages, about 10 or so.

So given our results, any thoughts on what happened?

Any thoughts on what we should do?

It is odd to me that we had different results between the two years, but I can't think of a significant change, other than the batteries being "old", in their second season, between years...

Did we just get lucky in 2021, with fresh rechargeable batteries, and by 2022, they were (mostly) shot?

And other than switching to the recommend non-rechargeable Energizer Lithium's, I am not sure how to replicate the great 2021 battery life results in 2023. Or at least get closer.

I read about the recommendation of using Lithium for any "more intensive deployment", would our situation qualify?

I did send an inquiry to EBL, but have not heard back from them.

I do have a friend who has used Oneloop batteries for over 10 yrs, and has high praise for their life, so I have that one anecdotal data point.

And please share any thoughts or questions on things I might have missed as I investigate and try to trouble shoot this; it's possible I missed something obvious. Or if this has already been posted/answered, please point me to that post.

Thank you for your help.