# Solved: CMOS battery voltage



## Arcadion (Sep 2, 2004)

I get the odd old computer coming into my workshop with a flat CMOS battery, and rather than putting a new battery in I usually scavenge one out of a dead computer that isn't very old (mostly ones with dead motherboards that weren't economical to repair), since their CMOS batteries tend to be fine. A new battery has a voltage of 3V, what I'm wondering is, if the voltage is reading less than 3V on my multimeter (e.g. 2.93V) does that mean the battery is going flat and won't keep accurate time, or will the timekeeping only be affected if the voltage drops below a certain level, e.g. 2V? Most 2nd-hand batteries out of PCs only a few years old still read 3V.


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## MysticEyes (Mar 30, 2002)

Sure sounds like false economy to me, get a new battery. There is more to a battery than just keeping time.


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## Arcadion (Sep 2, 2004)

You didn't answer my question.


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## angel (Dec 2, 1998)

I can't answer your question definitively, but I did find another site that said:


> You should replace the battery when:
> * A 3-volt coin-shaped battery has less than 2.5 volts


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## kiwiguy (Aug 17, 2003)

The problem is that lithium batteries have a "flat discharge" curve, from the point of view of the terminal voltage.

They will maintain fairly constant voltage on low discharge (as in a PC) to within 5% of the battery life, so trying to judge the state of the battery by measuring it's voltage is not really possible.

I would expect that the battery would maintain the CMOS down to 2v as a guide, but by then it is all over anyway, you are on the last % or two of life.


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## Arcadion (Sep 2, 2004)

angel said:


> I can't answer your question definitively, but I did find another site that said:


Thankyou, that's exactly the info I was after :up:


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## Skivvywaver (Mar 18, 2001)

Arcadion said:


> I get the odd old computer coming into my workshop with a flat CMOS battery, and rather than putting a new battery in I usually scavenge one out of a dead computer that isn't very old (mostly ones with dead motherboards that weren't economical to repair), since their CMOS batteries tend to be fine. A new battery has a voltage of 3V, what I'm wondering is, if the voltage is reading less than 3V on my multimeter (e.g. 2.93V) does that mean the battery is going flat and won't keep accurate time, or will the timekeeping only be affected if the voltage drops below a certain level, e.g. 2V? Most 2nd-hand batteries out of PCs only a few years old still read 3V.


 That is kind of dishonest unless you are going to install the battery free. I mean come on a cmos battery is $2 and 1/2 hour of shop time is at least $25 and nothing gets fixed in less than 1/2 hour. Right?

Buy the person a new battery.


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## Arcadion (Sep 2, 2004)

kiwiguy said:


> The problem is that lithium batteries have a "flat discharge" curve, from the point of view of the terminal voltage.
> 
> They will maintain fairly constant voltage on low discharge (as in a PC) to within 5% of the battery life, so trying to judge the state of the battery by measuring it's voltage is not really possible.
> 
> I would expect that the battery would maintain the CMOS down to 2v as a guide, but by then it is all over anyway, you are on the last % or two of life.


I figured that that would be the case, that it would remain at 3V for most of its life and then drop below 3V as it started to go flat.


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## Arcadion (Sep 2, 2004)

Skivvywaver said:


> That is kind of dishonest unless you are going to install the battery free. I mean come on a cmos battery is $2 and 1/2 hour of shop time is at least $25 and nothing gets fixed in less than 1/2 hour. Right?
> 
> Buy the person a new battery.


I was after an explanation of the electrical properties of CMOS batteries, not a lecture on the morality of computer repair  I don't charge people full price for a half flat battery.


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