Weird Smoke Detector Problem

This is what I got:
FIREX brand detectors. 120 VAC hardwired with battery backup.
Detectors are relayed to each other (total of 6), so if one goes off, they all go off.
One detector started giving the signal its battery backup was low. Replaced the battery, it still beeping and diode light indicated battery was low.
Replaced battery again. Still low battery.
Replaced detector with a spare. It indicated low battery
Pulled one detector (that was fine) from another room and installed it. It began to indicate a low battery.
I checked the wiring, its good. The smoke indicated it is receiving power.
If I test the detector, it sounds and will set off the other detectors as well.
What the hell is causing the low battery warning then? Not enough voltage from the 120 VAC source? Bad neutral? Evil spirits?
thanks in advance...
Matt
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How old are these units? The americium sensor does not last for ever. I had to replace the HW units in my house after about 7 years (with battery-only since there were then frequent electricity supply outages.)
-- Don Phillipson Carlsbad Springs (Ottawa, Canada)
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I replaced it with a new detector (same brand and compatible with the existing system). Detectors ONLY malfunction when put in this particular location. If I take the same detector and put it in a different room, its fine. I checked the wiring already. The only thing I haven;t done is hook up a volt meter. I am just try to find out if there are any power supply problems that could cause this to happen. It would make it easier to find the cause.
Don Phillipson wrote:

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