In this particular case it's my use of NiMH batteries, which have a different discharge slope than normal Alkaline batteries. So it's not really a "feature" of the device but rather a limitation of the type of batteries used. With my alarm clock the signs when to charge the batteries are very obvious (the LCD starts fading away so it becomes hard to read, but the device is still functioning perfectly for some more weeks even then - it just doesn't much power at all).
>>TonyTr+(OP)
curious: why do alarm clocks run on batteries? Seems counter intuitive for a device that is just going to sit in one place to not be plugged in?