OK. So, I tried the battery test with music playing only from the SD card. This time the battery lasted for 23 hours.
Not as good as from the main flash memory, but I kind of expected that.
This isn't a big deal for me, as I usually only put movies on the SD card. But it does tell me that the battery won't last as long on videos either, so perhaps I should put my movies on the flash memory of the device and not the SD card. Using the SD card will use up battery power quicker than the flash memory.