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.