I've got one I can answer right, finally. I've had the same problem on
several battery-operated pieces.

The answer is to do it in reverse. Instead of measuring the battery voltage
against a known reference, measure a reference against the internal battery!

The way I usually do it to use a 2.7V to 3.3V zener, LOW KNEE
(100mw-150mw). The Anode (Neg) lead of the zener is driven to ground, while
the cathode (BAR) is connected to the analog input and a pullup resistor.
It will be a high current pulse (i.e. about 2-3ma) so only turn it on for a
split second.

The value you measure will then be a ratio (x/256) of the VCC battery
voltage. You can grind out a calculation, if you need to... but I just
build a table taken from actual battery tests.

The accuracy will be the accuracy of the zener itself. Unfortunately, only
a small difference exists between a LOW battery and a HI one, and that
tolerance MIGHT not be enough. So I usually calibrate the offset at the
battery midpoint voltage. Not too much production work for that...

--Bobn


At 05:58 PM 8/9/2003 -0400, you wrote:
>Has anyone had any experience using a PIC's internal A/D to monitor its own
>battery supply?
>
>My application will use one of the little PICs 12F675 or 16f676, but
>information about others would be useful.
>
>The part I'm having a hard time wrapping my noodle around is, what do I use
>for a reference voltage?
>
>This could be a high volume device, so cost is a concern. But I don't need a
>high degree of precision, 4 or 5 bits would be fine. I just need an
>approximation of battery health.
>
>  Thanks
>  Denny
>
>--
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