You can even replace most CMOS batteries on a running machine without causing any issues. The power from the system will keep the BIOS set, and it's a 1 minute job for many servers, slide out the rack, pop the top, thin phillips screwdriver to the CMOS battery to pop the old one out, pop the new one in and button her up.

Heck, you can buy 2 CMOS batteries for a buck at the dollar tree. They'll last a year or more with the light usage that would be.

I'm not sure what that engineer's reason specifically was, but I'll chock it up to laziness.

The CMOS batteries, themselves, have a shelf life and for whatever reason, we probably had none of the specific size required on hand. Being that the device was on battery backup in a rack, it probably led to the engineer thinking "it'll never be rebooted, a label will do"

Not all batteries on 486's are replaceable and serviceable.

Yes, we've soldered airplane batteries to 486's to keep them running longer