Fred,

The voltage you measure from a 9 Volt alkaline battery near the end of its life
depends upon the load placed on the battery at the time you are measuring it.
You can take an alkaline battery that is near the end of it's life and allow it
to sit on a shelf for a while and measure it's voltage.  The voltage will read
pretty high.  Draw some current from the battery and the voltage will drop
rapidly.  The more you draw, the sharper the drop.  Let the battery rest again
and the voltage will pop back up.

With that said, I think you will need to experiment with different usage
scenarios for your product.  If you're lucky you can determine voltage readings
that give you the warning duration you want to provide under each of the
scenarios.

-Mike
-----Original Message-----
From: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU] On
Behalf Of fred jones
Sent: Thursday, May 27, 2004 8:15 AM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [PIC:] ADC source impedence

<snip>

I have a question.  At what voltage do you read from the 9v battery do you
determine that it should be changed?  I am building a device using some 9v
alkaline batteries and I am putting a low battery indicator circuit to light an
LED but not really sure at what point you should have it trip and light the LED
to tell the customer it's time to change the battery. Thanks, FJ

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