RTC in BIOS

I Just installed a refurbished MOBO in my C640, the first time I powered it up I had to set the RTC and everything worked smoothly. Then I powered it off and unplugged AC supply (no batteries in either bay), waited about 30 mins, plugged AC back in and booted up computer. I got the same message to set RTC. Thought it may be the back up battery so I changed it with one that I know is good I still get the same thing. It only happens when I remove AC power cord and take the main battery out of its bay. Is there a re-flash I need to do or is it a faulty MOBO ? My bios version is A10.

Thanks.