Jay Honeck wrote:
The JPI has its own built-in voltage indicator. It will usually read
13.4 or 13.5 volts. We also have a stand-alone, panel-mounted digital
voltage/amperage meter, mounted a few inches below the JPI. It
usually reads a few tenths of a volt different than the JPI.

a) Is this normal? Why would they read different voltages?

Disclaimer: This is based on experience with ground vehicles and
equipment. I don't have an A&P; I don't even have a TG&Y. Your
mileage may vary.

Probably. The reasons have been well covered by other posters. If the
JPI is coming out soon, it might be interesting to pull the other meter
as well. Connect both of them to a 12 V battery sitting on a table -
with the same gauge and lengths of wire - and see what they read. If
they disagree under "ideal" conditions, they'll never agree in the
plane. (Alternatively, leave the other meter in the panel, but
disconnect the wires to the plane temporarily, and hook up wires from
the meter to a stand-alone battery.)

If the meters don't agree, the next question is "How do you know which
one is right?" The absolutely correct answer is to measure a known
standard voltage, and the pretty good answer is to compare to a
known-accurate meter. The trick is to find either of these things.
I guarantee that one (probably both) of them exist up in Cedar Rapids
at that little radio company up there. Getting to _use_ them is
another story. A new or nearly-nuke Fluke DMM with a fresh battery is
probably the easiest thing to get your hands on that has a decent chance
of being accurate.

"A man with a watch knows what time it is. A man with two watches is
never sure." -- Segal's Law, as quoted at
http://tycho.usno.navy.mil/clocks.html

b) What voltage is normal? Is 13.4 too low?

It depends. If you're measuring at the battery terminals, anything
below about 12.6 or 12.7 V with the engine running at cruise speed
is too low - this means the alternator/generator is most likely not
charging the battery. Anything above about 15.0 V measured under the
same conditions is too high. Anything in between could be considered
"normal". Some numbers that get thrown around for a nominal system
voltage for a 12 V battery include 13.8 V and 14.0 V. I have a mid-60s
King nav/com (hollow state!) that lists 13.75 V on the name plate.

If you want to get specific, AC 43.13-1A Change 3, paragraph 442, says
"The voltage drop in the main power wires from the generation source or
the battery to the bus should not exceed 2 percent of the regulated
voltage, when the generator is carrying rated current or the battery is
being discharged at the 5-minute rate." There is also a table that
shows the "maximum acceptable voltage drop in the load circuits between
the bus and the utilization equipment." For a 14 V nominal system, this
is 0.5 V for continuous operation and 1 V for intermittent operation.
In other words, if your generator or alternator is designed for a
regulated voltage of 14.0 V, the minimum bus voltage would be 13.72 V,
and the minimum voltage you would expect at a continuously-powered piece
of equipment would be 13.22 V. If the regulated voltage is 13.8 V,
these change to 13.52 V at the bus and 13.02 V at the equipment.

In practical terms, if you want to power something from a 12 V nominal
vehicle electrical system, it should probably _work_ on anything from
about 11.0 V to 15.0 V, _survive_ anything from 10.99 V down to zero,
and not explode immediately if fed more than 15.0 V. This is somewhat
dependent on how much current the thing draws - it's a little easier
to make a low-current device work over a large range of input voltages
than a high-current one. It also depends on intended application. It's
OK if your in-flight DVD player cuts out at 12.0 V; your landing light
should still work down to 9.0 or 10.0 V in case you have to land at
night with a dead alternator and discharged battery.

Disclaimer: This is based on experience with ground vehicles and
equipment. I don't have an A&P; I don't even have a TG&Y. Your
mileage may vary.

Matt Roberds