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From: Joseph Legris
X-Mailer: Mozilla 4.77 [en]C-SYMPA (Win95; U)
Subject: Re: How do I measure watch battery life?
References: <email@example.com> <3D7FC59B.A6E8520F@xympatico.ca>
Date: Thu, 12 Sep 2002 19:41:03 -0400
NNTP-Posting-Date: Thu, 12 Sep 2002 19:35:18 EDT
Organization: Bell Sympatico
Peter Nolan wrote:
> Hello Joseph,
> I've often wondered how this was done.
> It looks like from what you said one simply measures the voltage across the
> battery till it reaches some value as you say provided by the manufacturers.
> I had ideas along the lines that the battery was checked by placing a load
> across it(a resistor) at regular intervals determined by circuitry in the
> device and then looking at it's response.
> I guess it's more of a matter of knowing exactly how the voltage across a
> battery betrays it's charge capacity. I didn't think the two things were
> closely connected.
> Very interesting.
> I should enquire more instead of trying to figure everything out somehow.
> Peter Nolan.
> To email please remove x from firstname.lastname@example.org
> "Joseph Legris" wrote in message
> > 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.
> > That's the way to do it. Look up the battery specs on the manufacturer's
> > website. They should show a graph depicting the voltage discharge curve,
> > from which you can read off the end-of-charge voltage.
> > --
> > Best regards,
> > Joseph Legris
You can do it without a load, but that may not accurately detect
end-of-life for your application. One of the mechanisms of battery
wear-out is an increase in the internal resistance, so the greater the
load the greater the voltage decline. The manufacturer's data will
specify what load was used to plot the discharge curves, which you can
use as a guide to predict the performance in your application. Your test
load is normally the circuit that the battery is powering.
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