All macs since 15 years autostart if you plug valid power supply with if no battery is present.[br]
by valid i mean the one with enough power rating, in your case 60W or more. [br]
this will not damage the motherboard. you can unplug battery flex cable, plug power supply and carefully plug battery back in. however, Trackpad / Lid Power trigger should work unless there is some other issue with the device.