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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

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@OP: To be considered also is the low voltage threshold defined by the Motherboard manufacturer. Its not necessary that the battery be completely dead (i.e at 0V) before it starts acting dead. –  Bhargav Bhat Mar 2 '12 at 3:44
    
@Moab: The battery doesn't seem to be a rechargeable type, wikipedia lists it as a common button cell that is used in calculators and watches etc. –  Bhargav Bhat Mar 2 '12 at 3:46
    
@Moab: Li-ion technology allows for rechargeable batteries, but the CR2032 is not a rechargeable battery. –  Bhargav Bhat Mar 2 '12 at 4:15
    
@Moab: I should have been more clear in my previous post. The CR2032 is not a lithium-ion battery. It's simply called a lithium battery because it uses lithium metal or lithium compounds in the anode. See this wikipedia page for details. –  Bhargav Bhat Mar 2 '12 at 5:22
    
@BhargavBhat thanks for the clarification. –  Moab Mar 2 '12 at 16:16

4 Answers 4

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).

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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!

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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.

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I 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.

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