How long does CMOS battery (3V) on a motherboard last?

Just wondering usually how long does CMOS battery last on a regular motherboard?

I read that motherboard leakage current of about 3 - 6 micro ampere. I did a calculation below but I believe that i may have missed something. What is the minimal voltage required for a motherboard's date/time function to work correctly?

Life of battery (down to 0V):

Normal Case (leakage current = 4 µA).
Battery capacity = 225 mAH / 4 µA
= (225 x 10^(-3)) / (4 x 10^(-6)) hr = 56250 hrs

One year has 24 x 365 hrs = 8760 hrs

Total battery life (from 3V -> 0V)
= 56250 hrs / 8760 hr per year = 6.42 years

Unfortunately, the number of variables involved is large:

• the standby current of the motherboard
• the actual capacity of the battery (every battery is a little different, and differences between manufacturers can significantly change run time)
• how long is the PC is on vs. off every day (when it's on, the battery isn't doing anything, so the more it's on, the less the battery is being used)
• variations in storage conditions before the machine was bought may also affect the battery lifetime

In short, there's no good, right answer for all situations. I've generally gotten at least 3-4 years out of batteries (in other words, I replace the motherboard or whole PC before the battery goes).

I've personally fired up computers 10+ years with no use and still had cmos information stored by battery. I wouldn't count on that as the norm but I have seen it

My experience shows that the CMOS battery lasts about 5-7 years before it needs to be replaced.

When the CMOS battery in my father's Dell failed, it read about 1.5 volts on my multimeter, well below the 3-volt nominal voltage, but not quite zero volts. A battery generally has virtually no energy remaining this far below the nominal voltage, so you shouldn't expect the battery to be usable all the way to zero volts.

As @Michael Kohne stated, there are many variables affecting the life of CMOS battery, so it is not easy to predict. In addition to his answer, I want to add:

The battery's output voltage need not to fall to zero before it becomes useless. When the voltage produced by the battery is less than the minimum required supply voltage for the chip, it already becomes useless.

When I managed a medium-size population of workers, I instituted a cyclical replacement program. Each fiscal year I would replace 25% with brand-new systems. The entire population would be upgraded in 4 years. Never once did I encounter an internal battery problem. We simply replaced the PCs in a timely manner. The oldest PCs got repurposed or sold off to sister agencies. All the PCs were from Dell.

To answer your question: It certainly is more than 4 years.