My XPS 9550 won't power on unless the battery is unplugged and it is not plugged in to the AC adapter, I hold down the power button for 20 secs, then plug in the AC adapter. At this point it will automatically boot up and run diagnostics. If I shut it down to reconnect the battery it won't boot back up, from battery or from ac adapter alone, only when I hold the power button for 20 sec. It will, however, restart with just the AC adapter plugged in.
As a potentially important side note, I had the whole bloated battery problem that dell barely recognizes and replaced the battery, so maybe that has something to do with it?