Some years ago I had an IBM ThinkPad that one day failed to boot, and every subsystem diagnostic that ran at power-up (keyboard, memory, disk controller, ...) reported a problem.  On a whim I put in a new clock battery and everything was fine.  Now any time a machine suddenly goes flakey, the clock battery is the first thing that gets replaced.

On Fri, May 26, 2017 at 21:22:17 +0930,Tim wrote:
On Thu, 2017-05-25 at 12:47 -0700, Rick Stevens wrote:
> Otherwise, with a weak battery the BIOS will usually revert to default
> settings which are generally considered conservative and "safe".

I'm not so sure that's the case.  In many PCs, the BIOS clock, BIOS
memory, and perhaps other BIOS hardware, are powered solely by the
battery (even when the computer is running off mains power).  So, with
failing power you could have all manner of random things happen.