Mighty Mouse Update

Re:  Wireless Mighty Mouse: Cause for concern?

The other day I had posted about something I noticed in my System Preferences.  My battery power was draining as normal, but then it was at full power again.  Someone mentioned that the wireless Mighty Mouse only used one battery at a time and then when the first was drained it would kick over to the second battery.  This may have been what I witnessed in the System Preferences.

If it took several weeks to drain the first battery I would assume the second would last just as long.  However this morning my batteries died on me so I had to replace them with some junk ones until I go into town later today.

What could have caused the issue that I witnessed if it was infact not the batteries switching over?  Could this be a defective mouse?