cmos keeps going every 4 or 5 months

[_Jimlad_]

I have a little shuttle and the bugger is going through cmos batteries every 4 or 5 months, i have a slight over clock, but seriously minimal, when i shut the computer down at night i kill all power by turning it off at the wall. Of late i turn the pc on, the bios settings remain intact but the clock has lost a few hours. Just tested the battery and its very week. i have used diffirent makes of battery too,

Any ideas on this, or just a fault with the shuttle