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From: tgg@otter.hpl.hp.com (Tom Gardner)
Newsgroups: sci.electronics
Subject: Re: Reply:  NiCads in Smoke Detectors
Message-ID: <72020001@otter.hpl.hp.com>
Date: 9 Mar 91 16:35:30 GMT
References: <1991Mar1.145532.7118@lonex.radc.af.mil>
Organization: Hewlett-Packard Laboratories, Bristol, UK.
Lines: 19

Many fire alarms specify that zinc-carbon batteries should be used, not
alkaline, not nicad. What follows is informed speculation.

For nicads and, to a lesser extent, alkaline cells the voltage remains
more-or-less constant until the cell is exhausted. Then it falls very rapidly.
The voltage of zinc carbon batteries falls constantly during the cell's life
and the voltagecan be used as a crude indicator of the remaining life.

In many fire alarms the battery voltage is monitored and as the voltage falls
below a certain level the alarm emits a plused shriek to indicate that the 
battery need replacing. Thie shrieking draws more current and thus the
battery is drained faster, but hopefully not before the battery is completely
discharged.

It can be seen that the zinc carbon battery will thus shriek for a longer
period before it is completely useless, thus increasing the chances that 
someone will notice it. It would be a great shame if the (nicad) battery
took 1 day to die completely, and the owner was away for that day. ZnC
cells will continue to shriek for more than a week.