Thanks for the idea! This would work too :)
But...  I found the prize of theese I2C and SPI controllable pot's to be
insane.
I think the resistor and FET idea will be easier.  I won't require that many
steps.

Kyrre Aalerud


----- Original Message -----
From: "John N. Power" <jpower@BCPL.NET>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Wednesday, November 19, 2003 10:04 PM
Subject: Re: ] Sampling battery voltage with PIC ADC.


> > From:         Kyrre Aalerud[SMTP:kreature@C2I.NET]
> > Sent:         Wednesday, November 19, 2003 3:38 PM
> > To:   PICLIST@MITVMA.MIT.EDU
> > Subject:      [EE:] Sampling battery voltage with PIC ADC.
>
> > Hi ppl!  I've been quiet for some time now on the list as I've had far
too
> > much to do.
> > Now I'm back on the horse with a little question.
>
> > I am making a battery-charger/cycler and want to make the actual circuit
as
> > simple as possible so that fellow hobbyists can make it without too much
> > hassle.
>
> > For battery-voltage I was thinking of using a simple voltage divider to
get
> > signal within 0-5v for ADC.
> > From my understanding I would need to "see" a voltage drop over time of
> > something like 10 mV for NiCD in order to correctly terminate the
> > fast-charge.  For NiMH I should stop the charge when the voltage no
longer
> > climbs within some set timeframe.  So, I would need a resolution of
about 10
> > mV or better pr cell.  If I simply use the 10 bit ADC directly after
scaling
> > voltage down to 0-5v I have would give 10.23 volts as the max
cell-voltage.
> > This would not be enough.  I need charger to work with 8 cells, but it
would
> > be cool to get it working with 1-12 cells.  A max cell-voltage of 2v
should
> > be used I think as peak-voltage increases with charge-current.
>
> > Any ideas as to how I can adjust the scaling of battery-voltage based on
> > number of cells charging ?
>
> > I though of using a I2C capable digital potmeter, but they all seem to
max
> > out at 0-5v.  I would need one where I could place the pL and pH points
of
> > potmeter between the battery + and - poles on a 12 cell battery,
preferably
> > even higher.  (18 volts or more!)
>
> Put resistor Rbig from the positive terminal of the battery to the ADC
input.
> Put a digital pot from the ADC input to ground. Setting the pot to zero
will
> give zero ADC input; setting it to max will give Vbat * Rpot/(Rpot +
Rbig).
> The scale factor will not be linear in Rpot, but it can be calculated from
> Rpot and Rbig. Choose Rbig to limit the pot current to allowed maximum
> under all possible conditions.
> For protection of the ADC and the pot, put a diode from ADC input to +5
> (cathode to +5).
> Since the very low end of the pot range will be useless, you can put a
> fixed resistor in series with the pot to compress the useful range. Just
> change the formula for calculating the scale.
>
> John Power
>
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