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From: tomb@hplsla.HP.COM (Tom Bruhns)
Newsgroups: sci.electronics
Subject: Re: Reply:  NiCads in Smoke Detectors
Message-ID: <5170118@hplsla.HP.COM>
Date: 7 Mar 91 18:16:24 GMT
References: <1991Mar1.145532.7118@lonex.radc.af.mil>
Organization: HP Lake Stevens, WA
Lines: 18

rpw3@rigden.wpd.sgi.com (Rob Warnock) writes:

(Good notes about lithium batteries deleted...)

>p.s.  A 9v lithium cell's internal resistance is about 16 ohm, compared
>to an alkaline's 0.82 ohm. While this would not matter at all in the above
>fire-detector application, it does affect lithium's use in high-power pulse
>applications. In those, you may need a hefty electrolytic capacitor across
>the battery...

Hmmm.  This worries me a little.  The smoke detector must idle along at
very low current for (hopefully ;-) a very long time, but when it must
do what it was put there for, will the lithium battery hack it?  I confess
I don't know how much current the alarm part draws, but assume it must be
in the hundred milliamp range.  Perhaps I can check this out, but I don't
think it's safe to assume that the higher resistance would be
inconsequential in a smoke alarm!  (One alarm I have has two batteries, one
for the alarm and one for a rather bright light.)