The new D500x firmware version 2.2.2 has been released. The trigger sensitivity option now allows for better rejection of noise events and there is a new timer option that can start and stop the detector relative to sunrise and sunset when you put in your latitude and longitude. Both are very welcome additions.
One of the most useful features I've found though is that the log file created during recording events shows the battery voltage through time. This really helps in choosing the right batteries for the duration the detector is used. I trialled it with an external battery case with four NiMh 'C' cells which gave out 4 x 1.2v = 4.8 V. You could see the voltage change through time until the detector got to 4.2 V when the detector stopped working. I'm sure with four alkaline C cells at a total of 4 x 1.5v = 6 V it would have been fine.
While we all want to be responsible and use rechargeables, it often just does not work with devices like this when you need them to be reliable and are not checking on the batteries all the time. Most of my NiMh AA cells are unusable after two seasons as they suddenly lose charge. I think the best power source is still a 6V lead-acid battery for long periods in the field.
I've had some lovely long-eared recordings from my D500x from a church recently, plus a load of very varied but wonderful social calls. Long-eard bats are certainly very talkative.