The CMOS battery is dead -- it will need to be replaced (and note that without a main battery, the reserve battery won't last as long as it would with a main battery installed, as it will be needed anytime the system is unplugged). This is the part you will need to replace [URL]  
Hi! I have Dell Latitude 5480. It doesn't have a battery and when I plug it in the power switch turns on for a few seconds but the screen remains black then the laptop switches off. When I press the power button again it takes several tries till it stays on. Then I'm met with the "No bootable device" message. I entered the bios and changed the boot sequence from UEFI to Legacy and the SATA operation from RAID to ACHI and set the date right and it booted. Any idea what's wrong with the laptop? Thank you!
Dell Lattitude 5480 power and booting issues