V G wrote:
> I want to construct a simple device for plotting discharge curves for
> batteries. I want to be able to measure voltage to the nearest
> millivolt.

So go ahead.  This is not hard.  Connect a resistor accross the battery and
measure voltage over time.  The current will be proportional to the voltage=
,
but you can still compute it.

Why do you need mV accuracy?  That seems excessive unless this is some
special lab experiment.  I've done a battery charger and maintainer with th=
e
normal 10 bit A/D of a PIC and it had sufficient resolution for the purpose=
..

> How would I go about doing this? Would an ADC on a PIC suffice?

The A/D in a PIC should suffice for normal purposes.  If you really need 1m=
V
resolution (I suspect you probably don't), then you can:

1 - Use external circuitry to map just the full to empty battery voltage to
the full range of a PIC A/D.  NiMH cells go from about 1.3V full to 900mV
empty for a range of 400mV, so that would work in this case.

2 - Use a external high resolution A/D.  There are pleny out there that
communicate over IIC or SPI.  Since your signal is very slow, a sigma-delta
type sounds appropriate here.  You can get 20 bits or more, which is way
more than you need.  Microchip makes several sigma-delta A/Ds that would
work fine.

3 - Use a PIC with 12 bit instead of the more common 10 bit A/D.  Probably
used directly is good enough, but with proper choice of Vref you actually
get 1mV resolution.  There are PIC 18 and some dsPICs with 12 bit A/Ds.


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