I was going to make a similar type of unit for measuring Car battery
charge/discharge.

It looks pretty easy until you try it !!!!

Measuring current flow one way is easy, use one side of the shunt as ground
and the other to a Pic A2D for the current sensing, by adding the current /
time you can sense the charge.

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

One idea I had was to amplify the voltage across the shunt using a single
rail opamp.

Lets say there is a 10mv drop across the shunt per Amp drawn. Using an opamp
to amplify this by 100 would give 1v/Amp output. Feed this into an A2D input
on the pic and you've got say a charge current. For sensing the discharge
current, use an opamp grounded on the other side of the shunt with the same
amplification, you'll get a slight offset of the voltages but in theory, I
cant see why it wouldnt work.

As I said, its easy to sense one way but to try to sense both ways appears
to be a bit of a pain. If you come across any easier solution (i.e. running
the pic from a split rail supply) then let me know :-)

Regards

Dom
----- Original Message -----
From: "John Nall" <jnall01@ALLTEL.NET>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Monday, June 23, 2003 4:39 PM
Subject: [PIC]: Counter for how many amps a battery is down


> My sailboat has (or at least had) a battery monitor on it.  You start with
> a full battery, and this monitor displayed a number which represented how
> many amp hours had been used.  (Usually a negative number, but you could
> overcharge the battery and have a positive number).  Alas, my monitor
seems
> to have been murdered. :-(
>
> The boat took a lightning hit while in the slip, and apparently some of
the
> electronics in the monitor got zapped.  Although I could buy another one,
a
> large part of the cost is the shunt, which was not harmed.  They won't
sell
> the unit without the shunt, so I figure I should be able to use a PIC to
> build another monitor.  The shunt is just a resistor, and is described as
> follows:
> "When current flows through the shunt, a small voltage is developed across
> the shunt which is proportional to the current flow. The battery monitor
> accurately measures this very low voltage and converts it to the "amps"
> reading on the meter. The resistance, which is the ratio between the
> voltage across the shunt and the current flowing through it, is a constant
> for any particular shunt"
> So the PIC would measure the voltage coming from the shunt and use that to
> calculate the corresponding current, and use a timer to convert to amp
> hours.  The shunt itself is a dual MKB-500/50 (the dead monitor was a
Link,
> made by Heart Interface).
>
> NOW, please don't flame me for not having a specific, detailed question to
> ask!!  I merely want to know if someone on the Piclist might have done
> something like this, or have information that would be helpful to me in
> designing and building such a critter.  If so, would appreciate such info
> -- offline or online, as you choose. :-)
>
> Thanks,
> John
>
> --
> http://www.piclist.com hint: The PICList is archived three different
> ways.  See http://www.piclist.com/#archives for details.

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.