Two sets of comments from reasonably authoritative sources - both of
them prefer voltmeters.
>From Stewart-Warner web site:
Voltmeter vs. Ammeter
A voltmeter measures electrical potential (pressure) in units called
volts that is necessary for current measured in amps (a quantity) to
flow through your electrical system. If there is NO pressure (volts),
there can be NO current flow (amps). This is analogous to having no oil
pressure and, therefore, no oil flow. You must have one to get the
other!
Ammeters were popular when generators and alternators typically averaged
35-40 amps of output, and vehicle electrical systems were much less
complex. Today, alternators commonly have output capacities of 100-110
amps with more complex wiring and higher accessory loads. This makes it
more difficult to install an ammeter.
Just as we measure oil pressure rather than oil flow, we prefer to
measure volts rather than amps, so, the overwhelming choice for
performance applications today is the voltmeter. In fact, ammeters are
not available as a Stewart Warner Performance product, but they are
available in our regular Stewart Warner line should you need one.
Contact our tech support team if you have additional questions about
ammeters or voltmeters.
The bottom line...an ammeter is NOT an effective solution for racing
applications!
Following is from Summit Racing Equipment
Voltmeter vs. Ammeter, Which is better for monitoring a vehicle's
electrical system - a Voltmeter or an Ammeter?
Short answer: A voltmeter, by far. Electrical guru Mark Hamilton of
M.A.D. Enterprises points out that amperage is a measure of current
flow, so an ammeter is actually a "flow meter" that's intended to
measure current flow to the battery (under normal conditions) or
discharge from the battery (in the case of alternator system failure).
On a typical flow meter, all output must be directed through the device
to obtain an accurate reading. In the ammeter's case, that means all the
alternator output used to recharge the battery must first be routed
through the ammeter under the dash. Which requires a heavy-gauge cable
and presents a possible fire hazard. The ammeter itself must be able to
handle all this current flow, so it must have a higher current rating
than the alternator's maximum rated output.
All this might be worth the hassle if the ammeter produced reliable
information, but the ammeter can only measure the amount of current
output to the battery for recharging purposes. When the alternator
recharges a "low" battery, the ammeter indicates a high charge rate.
With a fully charged battery, the voltage regulator reduces alternator
output, and the ammeter is supposed to indicate a very low charge rate.
But how can you really tell the regulator has reduced alternator output.
Because the battery is fully charged? Maybe a diode in the alternator
rectifier failed, or the alternator belt slipped after it warmed up,
just as if the battery were fully charged. Or maybe the meter indicates
a medium charge rate most of the time-does the battery want this much or
could the voltage regulator be overcharging the battery?
On the other hand, a voltmeter works like a fuel pressure gauge-but
instead of measuring fluid in psi, the voltmeter measures electrical
system pressure in volts. Just like a fuel pressure gauge, a voltmeter
only needs to tap into a circuit; all the fuel (or electricity) does not
have to detour through the gauge itself. Voltmeter installation is easy,
quick, and safe. It hooks up to a fused, ignition-switched "off/on"
source and does not require any modification of the circuit used to
recharge the battery or any part of the alternator/regulator system.
In short, the voltmeter installed at the dash will be a stand-alone
circuit. The voltmeter directly measures the result of charging-system
performance. With normal alternator/voltage-regulator function, battery
voltage is maintained at 14.0 to 14.5 volts-and this is reported
directly by the voltmeter. In the event of alternator-system failure,
voltage will be low and continue to drop as the battery discharges. In
the event of an "overcharge" condition, the voltmeter will climb above
its normal zone.
In summary, there is no chance for misinterpreting a voltmeter's
readings as can happen with an ammeter.
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