I have seen quite a few threads about CMOS batteries needing to be changed.
I have eight computers which I use for training purposes and have never, ever had to change/renew a CMOS battery.
I also have a very old 486 dx40 computer with an 850mb (yes 850mb) hard drive running Windows for Workgroups 3.11 and it is still up to date with the date and time. In fact, today, it has just updated it to British Summer Time.
Am called on when there's trouble among family and friends, but if I include my own PC it still only adds up to 5 "regulars".
Even so, with all the snags which have arisen, including several other's PCs from the past, there have been perhaps two occasions where the CMOS battery needed changing.
Apart from a ridiculous system date, I can't say truthfully that there was any other real upset to using that PC with a duff battery. Probably so few problems because nearly all used the default BIOS settings?
You can actually activate your pc to warn you of a low battery from within the BIOS I think. They don't last for ever as they don't in watches or any other electrical gizmo. If they do run out in a computer you risk corrupted data or at worst a complete pc breakdown.
The CMOS battery is a backup device when the PC is on then it isn't being drained.
Computers built over the last 6-7 years also use the mains to hold the CMOS setting even when the PC is switched off. so battery only used when unplugged from mains. This also means there is a trickle of power through the motherboard when plugged in. So the old advice to leave plugged in when renewing components (using the earth lead of the mains cable) is now dangerous advice. The machine should be fully isolated when "opening the box"
I am constantly re-furbishing computers and I check the state of the battery with a meter.Most of the batteries need replacing.Wise to do so anyway when you consider they cost 30p.
On some motherboards I have found that they will not boot up if the battey is really dead.Watch out for this if you are handing your oldish pc into a repair shop!!..."£75 please sir_you will find it's like new now".
I've built perhaps 20-30 PCs as well as helped to repair dozens, replaced a battery once in a cheap PC Chips motherboard, 6 years later its still going strong on the same battery. I can only assume the original battery was duff.