A faulty battery can prevent the system from powering up. Power off, remove the base cover and disconnect the battery from the system board.  Hold the power button for 30 sec.  Remove and reinstall the memory modules.  Verify that the LED on the AC adapter or DC plug is on before you plug it into the system.  If it's not, replace the adapter with an OEM Dell unit. Once you have verified the adapter works, plug it in to the system and see if the system will POST.  If it will, shut down and reconnect the battery and try again.  If the POST halts with the battery connected but not without it, the battery is bad. If the system still won't POST without the battery, the mainboard is bad.  
this morning I found that battery cannot be charged any more. I plugged in power adaptor, the battery icon changed to charging icon. but battery is not charged. Then the laptop can not turn on anymore, pressed the power button, it will be on 2-3 seconds, then turned off. the LED on power adaptor is on and I measure the voltage of adaptor, it is about 20V. so I think power adaptor is OK. but I do not understand why laptop can not turn on if battery is dead.
XPS 15 9560 can not turn on