So yes unplug charger and disconnect the battery connector,
Plug in just the charger and see if it powers up.
* If yes then time to replace battery (how old is it?).
* If no then remove the RAM memory, HDD/SSD hard drive and any cards like WIFI.
If it powers up now then add one component at a time until it doesn't power up. That is you defective component.
Tell us how it goes.