ammeter vs. voltmeter

[Tom Jennings <tomj@xxxxxxx>]

On Fri, 10 Jun 2005, Armand Eshleman wrote:

> The ammeter is a source of car fires in many instances because it must have
> a good bit of juice flowing through it to wok. I'd consider replacing the
> stock ammeter with an aftermarket volt meter. It doesn't need a load running
> through it.

Ammeters (you'd think they'd be called ampmeters) are simple, but
no longer a good idea unless you get a remote-shunt type.

The idea is, all the current to and from the battery -- except the
starter -- flows through the ammeter. You can watch charge go into
and out of the battery. Sounds great.

However, when they were popular, 15 - 20 amp generators were
common, and 35-amp alternators were "large". Charging systems were
unreliable (mechanical regulators and generally lower standards).

For even 20 amps using a decent wire gauge is easy, perfect crimps
less important, in a really simple vehicle (with few wires through
the firewall and room under the dash) it's not that big a deal.

In modern cars with tight dashes and harnesses (heat, pinching,
bends, lack of visibility) and huge alternators with big currents
you'd need to use really large wire and near-perfect connections
to run say 60 amps from the alt, to the dash, back to the
battery/alt area. That's a dozen feet or more of heavy copper and
a lot of heat.  Short that, and you have a fire.


There are shunt-type ammeters. A shunt is simply a very low-value,
high-power resistor. You wire it in series with the battery as a
regular ammeter, but you can put it under the hood close to the
battery, etc. Then you run skinny wires to the guage part. These
wires carry only the "signal" and not the full load.

[The dash-type ammeters simply have the shunt built into them.]


Voltmeters only need a really simple, tiny, safe wire, to any
reasonable point in the system.

It turns out, simply monitoring battery terminal voltage is a
really good indication of battery state -- and it was true in
1950.  I don't know why it wasn't done then. I think it was
basically, people wanted to see "juice" going into the battery.

I have a voltmeter on my Rambler. It's useful. Car off, it reads
12V or so. Running, over 1000rpm, voltage lifts to 14V or so.
Idle, fan, lights, etc on, voltage drops; rev it up, back to 14V.
YOu can see the alternator and loads working.

When the alternator crapped out, and the voltage shot up to 16+
volts, I could see it. On an ammeter it would have showed "+"
current, charging.  Same difference.



> Great Idea !  Where would be the correct spot to connect a voltmeter into
> the wiring on a 70 Javelin with Motorola alternator?

Close to the battery (electrically, that is) is best. On an old
car, close to the "BAT" terminal on the ignition switch.  (My
rambler's voltmeter shows battery voltage with the key off. They
draw nearly zero current.) Just don't stick it out at the end of a
wire that feeds some heavy load -- for example, if you put it on
the fused wire to the fan, say, when the fan is on, drawing
current, there is a voltage drop in that wire; the voltmeter would
show the dropped voltage.