Right, I've now given up with NiMH batteries, you think you've got a good charged set, and then you look away and they've faded. Having got fed up with checking the D500x and finding out that it recorded only two nights out of the four as the batteries died, I've gone back to environmentally unfriendly alkalines. Having done that, I've managed to get seven nights of recording from one set of four batteries (3 second files at 500 kHz) of 2500 files and still had the batteries at 4.7 V, so probably fine for another night or two. I've got an external battery box that will take four C cells, so I would imagine that with good quality alkalines in those I should get up to two weeks recording. I shall check that when we get to August.