Voltage variation

[Jay Honeck]

I still haven't had time to work with my A&P to remove my (newly)
flakey JPI EDM-700 unit. (As you may recall, some of the LED segments
that make up the number display are going in and out.)

Someone here suggested that it might be due to low voltage, possibly
indicating an alternator problem. I've started watching the voltage,
and have noticed something odd:

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. (I can't
remember if it's up or down...)

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

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

Thanks!
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

[Kyle Boatright]

"Jay Honeck" > wrote in message
ups.com...
>I still haven't had time to work with my A&P to remove my (newly)
> flakey JPI EDM-700 unit. (As you may recall, some of the LED segments
> that make up the number display are going in and out.)
>
> Someone here suggested that it might be due to low voltage, possibly
> indicating an alternator problem. I've started watching the voltage,
> and have noticed something odd:
>
> 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. (I can't
> remember if it's up or down...)
>
> a) Is this normal? Why would they read different voltages?

There are numerous reasons, which might be additive:

Gauge calibration.
Different wire paths to the gauges leading to different voltage drop before
the electrons get to the gauge..
Different wire sizes, leading to different voltage drop.
Corrosion in connections.
Loose connections.
Etc.

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

13.4 is a *little* low, but that could easily be a gauge calibration issue
or a voltage regultor that needs a slight tweak.

>
> Thanks!
> --
> Jay Honeck
> Iowa City, IA
> Pathfinder N56993
> www.AlexisParkInn.com
> "Your Aviation Destination"
>

[David Lesher]

Jay Honeck > writes:


>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. (I can't
>remember if it's up or down...)

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

Yes normal. Many reasons, all of the don't bother ilk.

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

That is low. A charging L-A battery will be at/near 14.4v.
You'll have some drop between it & the buss, but...

....Under what load do yue see 14.4? How long after starting?

What is the voltage at moderate RPM with no loads,
& with the JPI on so you can see its reading?

With every frigging thing on, including the Jacuzzi and the landing
lights? [We'll skip cycling the gear....]

--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

[Jay Honeck]

> >b) What voltage is normal? Is 13.4 too low?
>
> That is low. A charging L-A battery will be at/near 14.4v.
> You'll have some drop between it & the buss, but...
>
> ...Under what load do yue see 14.4? How long after starting?

That's 13.4 or .5 in cruise flight, with two nav/coms and strobes on.
Probably turning 2300 RPM, say, ten minutes after launching.

> What is the voltage at moderate RPM with no loads,
> & with the JPI on so you can see its reading?
> With every frigging thing on, including the Jacuzzi and the landing
> lights? [We'll skip cycling the gear....]

Dunno. I'll have to record these, next time I fly.

The sealed battery is maybe a year old. Cranks just fine.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

[randall g]

On Sun, 23 Sep 2007 13:31:10 -0700, Jay Honeck >
wrote:

>I still haven't had time to work with my A&P to remove my (newly)
>flakey JPI EDM-700 unit. (As you may recall, some of the LED segments
>that make up the number display are going in and out.)
>
>Someone here suggested that it might be due to low voltage, possibly
>indicating an alternator problem. I've started watching the voltage,
>and have noticed something odd:
>
>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. (I can't
>remember if it's up or down...)
>
>a) Is this normal? Why would they read different voltages?
>
>b) What voltage is normal? Is 13.4 too low?
>
>Thanks!



I have found that the JPI is very sensitive to voltage. I have a
persistent voltage drop in my system which is exacerbated by putting the
landing gear up or down. This causes a large current draw by the gear
pump motor, which due to resistance somewhere, causes a voltage spike
downwards. My JPI reboots as a result of this, and is the only actual
symptom. Other avionics are unaffected, so this problem is not severe,
but it does bug me.

This effect has been noted by many members of Cardinal Flyers Online
over the years, as I found when I was researching it.

I used to have an inflight voltage of under 13.0 when all electricals
were on, or maybe 13.3 with the "usual" load. I have had some work done
on my electrical system, including replacing old components and wiring,
which cleared the problem up for a time. Now my voltage is a steady
13.7, but I think this is still somewhat low, and the JPI reset problem
is starting to occur again.

In principle this is not hard to troubleshoot. With the engine running
and a full electrical load, simply measure the voltage starting at the
alternator and moving back to the avionics buss and battery, at all
possible points. At the alternator it should be over 14.0 I think. This
should isolate where there is a voltage drop, and therefore too much
resistance.

So I think 13.4 is too low, at least it is in my plane. I would like to
see 14.0. And you should determine exactly where the JPI and your panel
voltmeter are connected, because a few tenths of a volt difference is
certainly pointing to excess resistance between them.




randall g =%^)> PPASEL+Night 1974 Cardinal RG
http://www.telemark.net/randallg
Lots of aerial photographs of British Columbia at:
http://www.telemark.net/randallg/photos.htm
Vancouver's famous Kat Kam: http://www.katkam.ca

[David Lesher]

Jay Honeck > writes:

>> ...Under what load do yue see 14.4? How long after starting?

>That's 13.4 or .5 in cruise flight, with two nav/coms and strobes on.
>Probably turning 2300 RPM, say, ten minutes after launching.

>> What is the voltage at moderate RPM with no loads,
>> & with the JPI on so you can see its reading?
>> With every frigging thing on, including the Jacuzzi and the landing
>> lights? [We'll skip cycling the gear....]

>Dunno. I'll have to record these, next time I fly.

>The sealed battery is maybe a year old. Cranks just fine.


OK, I asked "how long" because the voltage post-cranking may
be lower.

The real question is: what've the voltage at the battery terminals?
But that's hard to measure in flight...




--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

[David Lesher]

randall g > writes:


>In principle this is not hard to troubleshoot. With the engine running
>and a full electrical load, simply measure the voltage starting at the
>alternator and moving back to the avionics buss and battery, at all
>possible points. At the alternator it should be over 14.0 I think. This
>should isolate where there is a voltage drop, and therefore too much
>resistance.


The problem is; on a car you have to dodge the radiator fan, but if I'd
ever worked on a plane (not me!!!! That would Be Wrong..) I'd wonder if
that big fan out front did not make life lots harder... and THAT's while
it is sitting still.

But a good test is, after the voltage is stable [ie the starting drain
has been recouped..] is to go from no load to fax load while looking at
the voltage.

What we are asking is: what's the voltage drop between the battery
and the avionics buss? If it is drop in the wiring, looking at no load
will show a voltage ~identical to that at the battery.



--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

[[email protected]]

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

[Roy Smith]

In article >,
wrote:
> 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.

The bench test sounds fine, but the same gauge and lengths of wire part is
kind of silly. If these things are pulling more than a 100 mA, I'd be
surprised. Worrying about the voltage drop along a couple of feet of any
kind of wire you're likely to have laying around is just silly.

#18 copper wire is 7 mili-ohm/ft. If this thing were drawing 100 mA, there
would be a 0.0007 V drop along the wire. The problem reported was one
reading being off by several 1/10ths of a volt. Don't worry about matching
wire gauges and lengths on the bench.

As for a reference voltage source, any reasonable DVM you can buy at Radio
Shack fo $20 will be more than accurate enough for this kind of work.

[Darrel Toepfer]

Jay Honeck > wrote:

> I still haven't had time to work with my A&P to remove my (newly)
> flakey JPI EDM-700 unit. (As you may recall, some of the LED segments
> that make up the number display are going in and out.)
>
> Someone here suggested that it might be due to low voltage, possibly
> indicating an alternator problem. I've started watching the voltage,
> and have noticed something odd:
>
> 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. (I can't
> remember if it's up or down...)

Your Lowrance 2000C has a voltage indicator that can be configured on a
screen, it will replace one of your other displayed options...

[Robert M. Gary]

On Sep 23, 1:31 pm, Jay Honeck > wrote:
> I still haven't had time to work with my A&P to remove my (newly)
> flakey JPI EDM-700 unit. (As you may recall, some of the LED segments
> that make up the number display are going in and out.)
>
> Someone here suggested that it might be due to low voltage, possibly
> indicating an alternator problem. I've started watching the voltage,
> and have noticed something odd:
>
> 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. (I can't
> remember if it's up or down...)
>
> a) Is this normal? Why would they read different voltages?
>
> b) What voltage is normal? Is 13.4 too low?

Depends, is the landing light on? Few GA planes can maintain 14v with
the landing light on.

-Robert

[[email protected]]

Roy Smith > wrote:
> In article >,
> wrote:
>> 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.
>
> The bench test sounds fine, but the same gauge and lengths of wire part
> is kind of silly.

Quite possibly. However, if I _don't_ say that, somebody will try this
test with jumper cables for one meter and two 500 foot spools of 30 gauge
wire for the other meter, and then write me an indignant email when they
buy a new meter and its reading still doesn't match the other one.

> As for a reference voltage source, any reasonable DVM you can buy at
> Radio Shack fo $20 will be more than accurate enough for this kind of
> work.

Precision is cheap. Accuracy costs a little more.

Matt Roberds

[[email protected]]

Anytime you are looking for electrical problems, you need to
keep in mind Ohm's Law. Voltage is equal to resistance times Current.
Reading voltages at various places in the airplane gives you little
information other than that the conductor is not entirely
disconnected, unless you remember that current flow affects voltage
drop.
I once forgot this principle and spent money that didn't need
spending. The strobe on the tail wasn't working, so I checked to see
that there was power available to it by pulling the feed wire off,
measuring the voltage there, and finding battery voltage. OK. Must be
the strobe's power supply shot. Bought another ($500) and installed
it. It didn't work, either. Then I remembered: E=IxR. With the strobe
turned on (but not working, of course) I took voltage measurements at
the bus (OK), at the circuit breaker's output input terminal (OK), at
the breaker's output terminal (not OK!) and realized that the breaker
was internally defective. There was full battery voltage there with
the power supply disconnected, because the meter requires only a few
microamps to drive it and the hundred ohms or so in the breaker's
corroded contacts wasn't enough to drop the voltage to it. Connecting
the power supply introduced a much lower resistance, increased the
current demand to two or three amps, and caused a huge voltage drop so
that the strobe was dead. The airplane needed a $15 breaker, not a
$500 power supply.

Dan

[[email protected]]

On Sep 24, 9:03 am, wrote:
> Anytime you are looking for electrical problems, you need to
> keep in mind Ohm's Law. Voltage is equal to resistance times Current.
> Reading voltages at various places in the airplane gives you little
> information other than that the conductor is not entirely
> disconnected, unless you remember that current flow affects voltage
> drop.
> I once forgot this principle and spent money that didn't need
> spending. The strobe on the tail wasn't working, so I checked to see
> that there was power available to it by pulling the feed wire off,
> measuring the voltage there, and finding battery voltage. OK. Must be
> the strobe's power supply shot. Bought another ($500) and installed
> it. It didn't work, either. Then I remembered: E=IxR. With the strobe
> turned on (but not working, of course) I took voltage measurements at
> the bus (OK), at the circuit breaker's output input terminal (OK), at
> the breaker's output terminal (not OK!) and realized that the breaker
> was internally defective. There was full battery voltage there with
> the power supply disconnected, because the meter requires only a few
> microamps to drive it and the hundred ohms or so in the breaker's
> corroded contacts wasn't enough to drop the voltage to it. Connecting
> the power supply introduced a much lower resistance, increased the
> current demand to two or three amps, and caused a huge voltage drop so
> that the strobe was dead. The airplane needed a $15 breaker, not a
> $500 power supply.
>
> Dan

I should add: Many batteries get replaced because the engine
doesn't crank very well. You need to take voltage drop measurements
across the master and starter solenoids while cranking; you'll often
find that their contacts are burned and introducing resistance with
the large current flow. Resistance checks mean nothing on these
things; even a quarter ohm will cost a bunch of voltage, and with
large current flows heat is generated, which increases the resistance
further. Same goes for cable and terminal crimps and connections.

Dan

[David Lesher]

writes:


> I should add: Many batteries get replaced because the engine
>doesn't crank very well. You need to take voltage drop measurements
>across the master and starter solenoids while cranking; you'll often
>find that their contacts are burned and introducing resistance with
>the large current flow. Resistance checks mean nothing on these
>things; even a quarter ohm will cost a bunch of voltage, and with
>large current flows heat is generated, which increases the resistance
>further. Same goes for cable and terminal crimps and connections.

Yes. I measure, for example:

a) On the battery term.
b) On the lug
c) On the wire in the lug.(That's why you have a sharp pointy probe.)
....

The hassle is, you have to do this under load, which usually means
"cranking" which means various dangerous parts are moving...


--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

[David Lesher]

Thinking more about Jay's undervoltage issue...

I can't see why he can't do all the testing with the engine off. That's
LOTS safer. Yes, he'll drain the battery slightly but we're talking only
a few minutes here.

He'll need a DVM. In my experience, even really cheap [as in $5 at Harbor
Fright] DVMs seem to be surprizingly close. (But you can always test it.
Ask your avionics shop to compare to their lab meter at 13v and see.)


Then make a simple schematic:

JPI

1-----------2breaker3---bus4----[master contactor]5-------6Battery+

and measure two ways; from each point to a known clean ground, and
from one to another, such as 2-3, 3-5 etc.

Each technique has its own advantages and disadvantages. "To ground"
will be hard to see small drops; 'across' is twice as many good
connections to make in tight places.

But what you are looking for is: How much does the voltage drop from the
battery along the way? With no/little load; the answer should be 0, no
drop, even if there are bad connections. Under load, the voltage drop
will be more visible.

So the first thing needed is this: What is the voltage at 6, the battery
post? And 1, where the other device connects? If the JPI (and the master
relay) are the only thing on, and it's more than one volt of drop; I'd look
deeper.

If the battery is much less than 13.8v, with the master off; I'd wonder how
it cranks the plane.

--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

[[email protected]]

David Lesher > wrote:
> I can't see why he can't do all the testing with the engine off. That's
> LOTS safer. Yes, he'll drain the battery slightly but we're talking only
> a few minutes here.

The test results with engine on vs. engine off probably depend somewhat
on where the battery is. If it's under the cowl, then engine on or off
probably won't make that much difference - all of the battery-recharge
current will be mostly under the cowl and won't affect the readings
much. If the battery is back in the tail someplace, then I would expect
more difference between engine on and engine off - there will be several
amps flowing from the generator/alternator back to the battery,
especially if the engine has just been started.

> Then make a simple schematic:
>
> JPI
>
> 1-----------2breaker3---bus4----[master contactor]5-------6Battery+
>
> and measure two ways; from each point to a known clean ground, and
> from one to another, such as 2-3, 3-5 etc.

The only thing I would add to this is to test both sides of everything,
like:

> JPI1---2breaker3---4bus5---6[master contactor]7---8Battery+

This helps show if you've got bad contacts in the master contactor or on
the bus.

> "To ground" will be hard to see small drops; 'across' is twice as many
> good connections to make in tight places.

Tip: Get some heat-shrink tubing and cover up all but the last 0.1-0.2"
(2-4 mm) of the meter probe tips. This helps prevent shorts when poking
around in tight places. You can also do this with electrical tape, but
the heat-shrink stays put better.

> If the battery is much less than 13.8v, with the master off; I'd wonder
> how it cranks the plane.

I'd expect the voltage on a fully-charged battery to be something like
12.6 to 12.8 V. 12.3 or 12.4 might still crank the engine, but slowly.
12.0 or 12.1 is totally discharged.

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

[Paul kgyy]

My JPI routinely runs 13.4 - 13.5 in flight after initial charging at
13.6 - 13.8, no problems with display.

[Deane Judd]

Paul kgyy wrote:
> My JPI routinely runs 13.4 - 13.5 in flight after initial charging at
> 13.6 - 13.8, no problems with display.
>
Mine is similar. Initial charge voltage is ~14.0 volts, drops to ~13.6
as charge rate decreases after start. It has never made sense to me, but
I assume it is a characteristic of the voltage regulator. I believe it
is a solid-state version, replaced several years ago.

So far I've had no problems with my JPI 700, display or otherwise. It
has been installed since 1998 in my Cessna R172K. Flown about 400 tach
hours since that time.