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From: Troy Schultz
User-Agent: Mozilla/5.0 (X11; U; Linux 2.4.17-acpi i686; en-US; m18) Gecko/20010110 Netscape6/6.5
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Subject: Re: How do I measure watch battery life?
Date: Thu, 12 Sep 2002 11:45:50 GMT
NNTP-Posting-Date: Thu, 12 Sep 2002 07:45:50 EDT
> 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.
Various battery types all have their own tell-tale typical discharge
curve. This curve will change somewhat depending on the rate of
discharge and this is why you will often see several curves published.
You can then measure the battery voltage and compare this to the point
on the curve where the battery has very "usefull" energy left. With
non-rechargeable single use batteries this point is much easier as you
usually pick a voltage where your application's cutoff voltage is. If
the battery is a rechargeable type then you must be carefull to ensure
you pick a voltage point where the battery will not be damaged by an
For a battery low indicator make sure you pick a voltage a bit higher
than the actual cutoff to ensure that there is still some usefull life
left in the battery. Also make sure to take into consideration
variations between batteries as well as componenet variations.
Batteries are wonderfull little devices, but they are far from the
simple little things that consumers think they are when they pick them
up from their local grocery store. Each battery chemistry has its own
quirks and I have yet to see a perfect battery chemistry type.
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