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From: "Spehro Pefhany"
Subject: Re: How do I measure watch battery life?
User-Agent: tin/pre-1.4-19990216 ("Styrofoam") (UNIX) (Linux/2.2.14 (i586))
Date: Thu, 12 Sep 2002 23:10:38 GMT
NNTP-Posting-Date: Thu, 12 Sep 2002 19:10:38 EDT
The renowned John wrote:
> Hello.. I am designing a device that runs off of a 3V watch battery,
> and need some way to determine the life of the battery (and output
> this in terms of logic). I was thinking of a simple comparator, but I
> don't know what to use as a reference voltage. Any help would be
> greatly appreciated.
It's non-trivial to design circuitry that will measure the voltage of a
watch battery against against a reference (included) and not significantly
shorten the life of the battery just from the current it draws. High
impedance circuitry with closely matched parts doesn't lend itself easily
to simple discrete analog solutions.
I suggest looking at canned "chip" solutions. The reset chips I usually
use for microcontrollers draw about 45uA, which is probably too much for
your application. If money is no object, just slap in an LTC1540, which
you can probably get down to < 1uA. $1.65 in 100's.
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