Normal voltage drop

"Robert M. Gary" wrote in message
oups.com...
When I turn on a lot of accessories on my Mooney F '76 I can drop the
bus voltage down to around 13.6. My A&P says 13.6 is normal with
accessories on. Does this sound right?

-Robert

At cruise RPM? Sounds like you may have 1 or 2 of the three diodes burnt
out in your alternator. Another possibility is that your voltmeter is off.
Try checking it with a digital voltmeter, and comparing the two.
--
Jim in NC

Morgans wrote:

At cruise RPM? Sounds like you may have 1 or 2 of the three diodes
burnt
out in your alternator. Another possibility is that your voltmeter
is off.
Try checking it with a digital voltmeter, and comparing the two.

Any RPM between 800 and 2700 gives the same voltage, no change. I
normally read the voltage in 4 ways
1) My EDM shows it on the screen
2) My regulator flashes a voltage light on my panel when low
3) My voice annunicator says, "Check bus voltage" and
4) My A&P has been putting his probe all over the place

Since I can reproduce this at idle on the ground its easy for the A&P
to hook up under the panel. I should also note that...
1) Have a new battery, new master switch, and new regulator
2) I took my regulator and alternator to an aviation electrical
overhaul shop. They bench tested the combo at 14v up to 100amp draw.
They didn't find any diode issues, but I'm willing to believe they
could be wrong.

-Robert, M20F '76

I guess my question is; do all planes generate about 13.6v with
accessories on? I also teach in a large variety of planes, but none
have a volt reader in the panel.

On 27 Dec 2004 19:15:04 -0800, "Robert M. Gary"
wrote:

I guess my question is; do all planes generate about 13.6v with
accessories on? I also teach in a large variety of planes, but none
have a volt reader in the panel.


I'm not a mechanic, but 13.8 volts comes to mind as what I've read on
my battery while the engine was running. I'd think .2 volts less
under load would be acceptable. You still have 1.6 volts of excess
power to charge the battery with so I really wouldn't worry about it.
I think sometimes, with the new digital equipment, readings can be
more accurate than the tolerances of the old stuff used to build it in
the first place.

What's the tolerance of your voltage regulator? If it's +/- .5 volts,
you've got your answer.

My .02.
z

"zatatime" wrote

You still have 1.6 volts of excess
power to charge the battery with so I really wouldn't worry about it.

Actually, the voltage for a lead acid battery should be 12.8 volts, at full
charge, not 12 volts.
--
Jim in NC

"Robert M. Gary" wrote:

I guess my question is; do all planes generate about 13.6v with
accessories on? I also teach in a large variety of planes, but none
have a volt reader in the panel.

According to AC 43.13-1A -- "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 charged at the 5-minute rate. The following tabulation shows the
maximum acceptable voltage drop in the load circuits between the bus and the
utilization equipment."

The table indicates that 1/2 volt is acceptable for a steady load and 1 volt for
an intermittent load. This passage is intended to be used to determine whether
wiring is properly sized.

According to that, you should be seeing at least 13.72 volts at the bus, but as
little as 12.72 is acceptable at an appliance that produces a temporary load.

George Patterson
The desire for safety stands against every great and noble enterprise.

G.R. Patterson III wrote:
"Robert M. Gary" wrote:
a volt reader in the panel.

According to AC 43.13-1A -- "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 charged at the 5-minute rate. The following tabulation shows
the
maximum acceptable voltage drop in the load circuits between the bus
and the
utilization equipment."

It's interesting that if I take the alternator and regulator to the
bench I can hold 100amp without dropping below 14v. I guess the
in-airplane application is a lot different than the bench environement.
-Robert

"Robert M. Gary" wrote:

It's interesting that if I take the alternator and regulator to the
bench I can hold 100amp without dropping below 14v. I guess the
in-airplane application is a lot different than the bench environement.

What I posted are the minimal requirements for minimizing voltage drop in the
wiring from the regulator to the bus and from the bus to appliances (such as the
NAV/COM). The voltage at the regulator on the bench will certainly be different
than that measured at the bus.

George Patterson
The desire for safety stands against every great and noble enterprise.

"Robert M. Gary" wrote:

It's interesting that if I take the alternator and regulator to the
bench I can hold 100amp without dropping below 14v. I guess the
in-airplane application is a lot different than the bench environement.


Oh? Well that points to a poor connection between the alternator
and the battery. It could be the hot side, or my favorite topic:
Bad Grounds. I suffered from the hot connection being loose on a
borrowed 91 CRX, on the way to a funeral. I hung a voltmeter on the
dash and noticed I was hardly gaining despite going at 60 mph. (I'd
run the battery down in traffic with headlights and blower on...)

You should be getting 14.4vdc to charge the battery; 13.8 is the
oft-quoted static voltage of the "12 volt" battery.

Since working on a running engine with a big people-eater spinning
on the front never appeals to me; try this. Run it for a while; shut
it down. Start feeling connections back to the battery. When you
hit the bad one, you'll burn your fingers. (Trust me...)

If all joints are cold; start checking grounds. Or rather, inspect
and clean them. Alternator bond, if any. Engine to frame. Regulator
grounds. You name it.
--
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

On Wed, 29 Dec 2004 00:08:40 +0000 (UTC), David Lesher
wrote:

snipped for length, not for content.

You should be getting 14.4vdc to charge the battery; 13.8 is the
oft-quoted static voltage of the "12 volt" battery.

Technically, the voltage to "charge the battery" should be determined
by the aircraft manufacturer's maintenance manual. Ideally, the
voltage should be determined by the data presented by the manufacturer
of the specific battery installed.

For instance, Concorde recommends 13.75 v 90 degrees F, 14.0-14.2 v
@ 50-90 degrees F, 14.75 v 50 degrees F based on battery operating
temperature.

http://www.concordebattery.com/produ...r%20manual.pdf

The numbers for a Gill flooded-cell battery can be found within:

http://www.gillbatteries.com/battery...ice_Manual.pdf

Another interesting sidenote, is that while Concorde recommends 14.4 v
for constant potential charging on the bench for their flooded-cell
batteries, 14.1 v is recommended for the valve-regulated RG batteries.

Depending on how and where an aircraft is being operated, a bus
voltage of 14.5 v (recommended by several airframe manufacturer's) or
even 14.4 v can drastically shorten the life of both flooded-cell and
"sealed" batteries. Trip/leg length/time is also a factor. RG
batteries seem to be more sensitive to high bus/charging voltages, in
my experience.

The open-circuit or "static" voltage of the typical fully charged "12
volt" lead acid aircraft battery is usually close to 13 v, 26 v for a
"24 volt" battery, however, open-circuit voltage does not necessarily
reflect the state of charge or the amp/hour capacity present.

Regards;

TC

snip