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?
4Life 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 hrsOne year has 24 x 365 hrs = 8760 hrs
Total battery life (from 3V -> 0V)
= 56250 hrs / 8760 hr per year = 6.42 years
8 Answers
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).
1I'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.
1As @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.
3When 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.
Here's a personal account:
Recently the Windows clock on my Asus K53 laptop started showing the wrong time. It was set to update automatically from a time server, but I had to manually sync it every hour or so.
When entering the BIOS, the clock would be frozen - no seconds moving, which signaled that the RTC (Real-Time Clock) was not being powered. And that's the job of the CMOS battery. When I disassembled the laptop and replaced the CR2032 3V battery, the Windows time got impeccable, and the BIOS clock got its seconds moving. So replacing the battery definitely fixed that.
So how long did the battery last? The Asus was purchased in 2011, used regularly every day for long hours, and the CMOS battery died in 2021 - so it lasted for 10 years. Its brand was KTS (made in Japan), now replaced with a Maxwell.
Unfortunately I'm on my phone I probably shouldn't chime in... Your math is not entirely accurate and what I mean by that is the milli-amp/hr is not I have X amps - Y hrs = 0 volts if you can fallow along I'm not trying to make this a history lesson but amp/hrs is 'HOW MUCH' current a battery is rated to put out, so you have to calculate how MUCH current is being used/drawn from the battery it should be considerably less.
To keep this short and rap it up you don't just take the 'CAR'S' horse power and multiply by how many days you drive a year by how many years; you calculate an average daily usage of HP and use that to calculate your HP for the year, that way your engine should last a whole lot longer than FULL THROTTLE!
1I have read more than one time that you should get on average about 5 yrs. out of a 2032 or any typical lithium watch battery. But then I am sure that would have to do with the efficiency of the circuit drawing the power from it and could fluctuate 10-20% either way. then you would have to factor in how many months (12-24 or more) that it sat in the store before purchased for use.
