On power up the following message is displayed:

CMOS checksum error - Defaults loaded
CMOS battery failed

This error occurs when the PC is swithed off for several hours (overnight for example) but doesn't occur after the PC has been powered on for several hours (during the day for example).

I would normaly change the battery but there is no external battery on the motherboard.

My question is, what would be the best way to remedy this failure?