My computer clock loses approx 15 minutes or so per day
With a newly installed battery.

Any ideas ?

What do you mean by "computer clock"? Do you mean the actual clock on your computer or a digital clock? I'm assuming digital because you said you installed a battery..

Installed new battery on motherboard (approx size of nickel)

Computer clock still loses time.

Is this a computer on a domain or a stand alone computer?
With most Windows computers you have the option of syncing your clock with an external clock. Double click on the time in the system tray and it's under there somewhere.
Do some searching on tucows, there are utilities out there that will keep your clock in sync.