Neighbours Windows computer started losing some Cmos defaults and would not boot up Windows.
The Cmos date and time was ok so I didn't think the battery was a problem.
Reseting the default parameters fixed the problem for a few days.
Eventually I removed the battery and a test showed 1.8v - well down from the usual 3v.
A repacement battery fixed the bootup problem.
With the low battery I was surprised that the default settings dropped out and yet the Cmos clock remained ok.