We have been using the device for a good 2 years and are very satisfied. The baby part is always plugged in and the parent part almost always. Normal alkaline batteries are used. Since a few weeks, the parent part beeps again and again, as if the batteries were empty. When it is unplugged, it continues until the batteries are empty after about 3-4 hours. The game repeats itself after about 1 week. This was not the case before (we have already tried different battery brands). Is it possible to operate the device without batteries because it is always plugged in? Does it make sense to use rechargeable batteries?