CMOS Battery

Clarifications

The inherent difficulties with the Real Time Clock going haywire on each startup is usually sufficient reason to replace the battery 'poste-haste'. ;)

The unit could run forever but the real problem comes when it gets shut down. I've seen units run for years with a bad CMOS battery but it didn't present a problem until a power outage took it down and it refused to come back up until the CMOS battery was replaced and the BIOS set correctly. Unless you are imtimate with the BIOS settings, best to replace it before it fails. I have seen low batteries give some very strange and intermittent faults that defied diagnostics. These only apply to units that get shutdown as the battery is only used to keep the clock running and hold the BIOS settings when the unit is off.

It is very possible for a dead CMOS battery to shut down a system. It all depends on how the battery fails as it ages.

The first stage is for the battery to loose the ability to source enough current to run the Real Time Clock.

Next, the battery can no longer generate enough voltage to keep the CMOS memory state, and you loose your BIOS settings.

In the final stage, the battery can short or even reverse polarity. At this point, a fully dead battery can keep the BIOS settings in an indeterminate state that will prevent the computer from booting, and can even cause it to lock up and shut down.

We have systems that run 24/7, and about 3 or 4 times a year, one will go down for nothing other than a dead battery. These systems are not mission critical enough to warrant changing the CMOS batteries as part of scheduled PM. But, yes, a PC can shut down from a dead CMOS battery.

Chas