Warped Factor,
The internal clocks on motherboards are extremely cheap. They are not known for keeping accurate time. (One of the chief reasons Microsoft added Internet Atomic Clock Time Checking feature into WinXP.)
If your mobo loses time while you have the computer turned off but plugged into the wall, it is not the battery. The motherboard battery is only used when the computer is off and not plugged into the wall. (or when the Power Supply is switched off by the switch on the back, if it has one.)
Take Care,
Richard