Just another comment from a disinterested 4th party. A digital
voltmeter is even better. I have found the odd analogue
miscalibrated, but more importantly the ability to read volts down
to a tenth of a volt, used intelligently, let's you monitor
battery condition - ie deterioration over time. You can even have
fun picking off the voltage drop during starting.
Mike L.
60A,67E,59Bug
----- Original Message -----
From: Barney Gaylord <barneymg@ntsource.com>
To: <Duinhoven_Hans@emc.com>; <djw69@idt.net>;
<mgs@autox.team.net>
Sent: October 20, 2000 4:11 PM
Subject: RE: adding an ammeter to a RBB (& water)
> Okay guys, break it up. It's obviously time for a
disinterrested third
> party to referee this event.
>
> When your electrical system is working properly, either an amp
meter or a
> volt meter will fluctuate in basically the same direction at the
same time
> and tell you pretty much the same thing, that all is well, and
therefore
> the primary effect of these instruments will be to take your
eyes off the
> road occasionally. In this case the voltmeter wins, because
it's easier to
> wire and carries less current through the wiring harness, but in
reality
> they're both pretty useless, which is why most cars have neither
of these
> instruments installed from the factory. You know the factory
would never
> want to imply that anything could ever go wrong with the
electrical system.
>
> The real value of these instruments comes when something does go
wrong with
> your electrical system. Here you may have overcharging,
undercharging, or
> no charging at all.
>
> When overcharging, the amp meter will be reading a few extra
amps. But you
> may get the same reading if the battery is simply discharged and
the dynamo
> is working properly, so that indication is not particularly
definitive.
> However, a volt meter pegged at 20 volts is a pretty good
indication that
> your generator is about to suffer internal meltdown.
>
> For mild undercharging, the amp meter could show zero, or
possibly a few
> amps in the red, and the ignition warning light may still not be
glowing.
> But you could see this same condition if you just turn on the
headlights
> and all of the accessories at the same time, particularly if you
have a
> generator with more limited current capacity than an alternator,
so that
> indication is not particularly definitive either. An amp meter
sitting
> constantly at 0 tells you practically nothing, but a volt meter
sitting
> constantly at 12 volts tells you that the battery will surely be
discharged
> eventually.
>
> For the no charge condition the ignition warning light comes on,
so you
> already know it's discharging, so what more can these
instruments offer?
> An amp meter reading -10 tells you it's discharging, but you
already knew
> that, and that reading won't change until you switch off some
accessories
> or kill the ignition switch. The volt meter on the other hand
offers up
> some useful information at this time. With the charging system
disabled
> and the car running on battery only, your most important concern
may be the
> state of charge of the battery, especially if you are at the
time on an
> expressway to nowhere and have no service port handy. 11.5-12.5
volts
> means a fully charged battery and some time allowed to get where
you're
> going without having to panic. 10.5-11.0 volts means about half
> discharged, and you had better have plans on where you will be
in the next
> hour or so. Anything under 9.0 volts means about the same thing
as a fuel
> gauge pointing at empty, you had better be home, or at a service
shop, or
> be prepared to walk.
>
> The point of all this is that the amp meter is indicating the
rate at which
> current is flowing in or out of the battery, but those same
readings can be
> seen under very different circumstances, some good and some bad.
As a
> result you may become unnecessarily concerned, or otherwise
ignore a real
> warning signal, because the thing is not telling you WHY. The
volt meter
> tells you much the same thing that the amp meter does while the
system is
> working properly, but when it's in trouble the volt meter is
telling you
> the state of charge of the battery, which could be a lifesaver.
But then
> that is why I carry a small DVM in the tool box.
>
> My money says $20 for a DVM, a dime for the volt meter, a nickle
for the
> amp meter, and two cents for my own advice.
>
> $.02,
>
> Barney Gaylord
> 1958 MGA with an attitude
> http://www.ntsource.com/~barneymg
>
>
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