At 04:53 PM 6/23/2003 +0100, Dominic Stratten wrote:

> >I was going to make a similar type of unit for measuring Car battery
>charge/discharge.
>
>It looks pretty easy until you try it !!!!

The story of my life!  Money, career, romance, the whole bit. :-(

> >Problem is when the current flows the other way round, you get a negative
>voltage which the pic cant handle.

Yeah.  Looks like I got several replies, and most of them seem to revolve
around using an op-amp.  So guess I need to now educate myself on those
little suckers.  It would appear to me, intuitively, that the way to handle
the negative voltage is to have a reference voltage which is higher than
the negative ever will be.  (That term "reference" is used loosely).  When
the battery is charging, the "reference" will go even higher.  When the
battery is discharging, it will go lower but never go negative.  Therefore
the A/D on the PIC will always see a positive voltage, and will have the
smarts to do the calculation.  But, as always, the devil is in the details. :-)

I'm going to do some detailed study of the different responses and figure
out what approach to take.  I know it can be done -- just don't know how to
do it.  In the meantime, I have bought a TriMetric monitor, which will
serve for now,  and besides I like the way they talk on the
telephone.  Solar energy people are always nice.

Cheers,
John

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