I've just had a delivery (in the UK) this morning of a strip of 10 mic jacks from Mouser

This sounds like what has happened to 3 or my 5 devices. I was thinking that adding an external microphone might fix the problem and am intending to try that (even though I haven't used a soldering iron in years)

Thanks @Alex Rogers for the handy explaination of some of these settings. I'm fairly new to the bat recording lark and I've been stumbling around in the dark a bit myself, so this is really useful. I have managed to capture some bat calls (only really Common Pipistrelle, so far) - but once I've gotten the hang of the settings, I intend to make recordings away from the back garden. I've been uploading my recordings to the BTO Acoustic Pipeline, for automated processing with some success - as hacking about with hours of recordings in Audacity is taking up way too much of my time. The Acoustic Pipeline can identify 24 species of European bat, plus small mammals and some insects and can be found here https://app.bto.org/acoustic-pipeline/public/login.jsp

Thanks Alex. I think that I have got the hang of things now. I did see a repeat of the above behaviou on another evening, where the initial recordings were of the correct length and then there were further recordings of various lengths until the rechargeable batteries gave out. I agree with your analysis that the behaviour is consistent with the batteries needing to be recharged. I have now purchased some new rechargeable batteries and the Audiomoth is producing files of the anticipated length and size, providing I keep the batteries charged up. Thanks very much for taking the time to answer my query. 😀

OK, so having put this down in writing, I played around with the configuration and noticed that when I increase the sample rate, the frequency range that I can filter on, increases. Apologies if this was evident to other users, but it certainly wasn't to me ... and my physics degree was a long time ago. Doh! So that kinda answers my question on how I could use the AudioMoth to record Bat sounds. On the SD Card page of the 'Getting Started' section, I read this: From AudioMoth firmware version 1.2.2 onwards both FAT32 and exFAT file formats are fully supported. However, if you are using an earlier version of the AudioMoth firmware ensure that your SD card is reformatted as a MS-DOS (FAT32) card prior to use, or reflash you AudioMoth to use the newer firmware (see instructions here). As I have a new Audiomoth and Firmware version 1.5.0, I guess that my device can support cards formatted with exFAT and I presume also that this means that the 32GB card limit is also removed as I'm using a 64Gb card and at least managed to get some recordings generated. Re-configuring and putting the device out in the garden for another spin.