Agreed - the Nav needs to power down before removing the battery cable.
But, you don't need to wait 15 minutes for the whole car to go asleep, you only have to wait until the red LED light goes out on the Nav unit before disconnecting the battery.
Good time to think of updating to a MKIII or MKIV.
2000 740iS. MKIV, 16x9, DDEs controlled by Euro fog light switch, reversed steptronic +/- pattern, paddle shifters, clear corners, Basslink, iPod video with stereo audio, TV tuner, gauge rings, ///M pedals, backup camera with 'on demand' switch, quad brake lights, Dinan engine & tranny software, windows/sunroof close via remote.
2003 540iS. MKIV, 16x9, PDC, CWP, TV tuner, backup camera with 'on demand' switch, paddle shifters, windows/sunroof close via remote, iPod video & audio.