CO detector lifespan

I have a Kidde model KN-COB-B battery-powered carbon monoxide detector that I bought in January, 2006. The manual says that the life of the detector is 7 years from first use, and that it will begin generating an alarm beep every 30 minutes when it reaches 7 years of life.
However, I only use the detector during furnace season - about 6 months of the year - and take the batteries out the rest of the year. So I'm wondering if the lifespan is a function of the total time it is actually running, or just the passage of time.
What component in a CO detector would have a limited lifespan? Would such a component's lifespan be a function the total time it is on and running, or just the passage of time whether running or not?
I assume the device itself calculates total time while it is running, so it can't know how much time has actually passed in my case - unless it has a lithium battery built in, or a WWV receiver. No evidence of either.
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Seven years is about the life of lithium batteries.
The sensor may also accumulate filth.
I bought an ac detector, and after not installing it, something made me throw it away after it was well past expiration date. Not sure if that was the right move.
Greg
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Some of the chemical type sensors just go bad with time. I think I had a plug in model that used heated element, 10 year life. I opened the garage door once while testing an onan generator near the garage. Didnt take long for sensor to sound. Generator rally was stinky.
Greg
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Peabody wrote:

It happened to me. Same brand CO detector started beeping at intervals non-stop. I thought battery needed repacing but new battery did the same thing then I realized it's time was up. My wild guess is the sensitivity of sensor decreases over time.        
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On 12/17/2012 3:35 PM, Peabody wrote:

material from one electrode to the second electrode when a voltage is applied. The active life of this device set when it is manufactured for about 7 years. This is also the average active life of the CO detector device.
So, a continuously used detector will know when 7 years has elapsed and begin to beep. When used only part of a year, the CO detector will most likely quit working, but you will not know it until it's too late.
The first home CO detectors used to tell you all this in the instructions. The ones I had to buy 2 years ago didn't.
So, to answer directly, once activated, the detecting device begins to deteriorate, no matter what you do.
Paul
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get a source of CO and see if it works.
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