The voltage produced by an alternator in a running car will have about
10-15mV of DC ripple on it. Battery voltage is quite flat.

Cam Bremner  -  Canadian Coast Guard Technical Services
"There are 10 kinds of people in the world; those who understand binary and
those who don't."


-----Original Message-----
From: Herbert Graf [mailto:mailinglist@FARCITE.NET]
Sent: December 3, 2003 1:54 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [EE:] Voltage measurement


> >         Unfortunately the problem with just this is that the lowest
> >"engine on" voltage may be below the maximum "engine off" voltage
> >when you
> consider all
> >conditions. For example, on a really cold day the "engine
> running" voltage
> >might be below the "engine off with fully charged battery" voltage.
> >TTYL
> >
>
> I would find that surprising, but I'll grant that it's possible.
>
> Measure the current.  It will flow one way as an indicator of engine
> start-up, and the other way until the battery is re-charged from the
> effort...

        Hmm, now that's a good idea, only problem I can see if you'd have to
build your shunt such that it could pass the hundred or more amps needed
during starting, without suffering a large drop. TTYL

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