At 09:40 PM 7/28/2003 -0500, fred jones wrote:

>Hi all,
>I hope there is an anolog person out there that can point me in the right
>direction.  I have a circuit where it is powered by a Nimh 24V battery.  The
>+ rail is the ground reference and the - terminal is the -24V rail with
>respect to ground.  I use a DC/DC converter to generate the +5V rail.  This
>of course is what is powering the PIC, 16F877, and I would like to be able
>to read the battery voltage to determine when its time to recharge.  Its a
>Nimh 24V pack, and at 20V I would like to display the need to recharge.

Your voltage should be 1V/cell when you terminate for max lifetime.
0.9V is acceptable.

You can use a zener, through an optocoupler, to give a pass/fail indication.
Otherwise, you're into something with an op-amp to scale and invert. A zener is still useful to trim off the bottom 15V or so, and expand your measurement range.

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