Normal voltage drop

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

"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

"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.

"Robert M. Gary" wrote in message
oups.com...
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.


When I was reading your original post, I thought I read 12.6. I was
reading what I thought was abnormal, but now looking at it, I see I was not.
?.?

13.8 volts is considered normal, so 13.6 is not so bad after all.

Are you getting any warnings, or anunciatiors at the 13.6?

The next question is, are you able to complete a flight with everything on,
and come back and still be reading the same battery voltage (engine off)
that you started with? If so, no worries!
--
Jim in NC

"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

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

My EDM700 (Arrow 200) has shown 13.5-13.7 in flight for a couple of
years. It recently dropped to 12.1, which I've interpreted to mean the
4-year old battery is shot. Just replaced the battery but haven't done
test flight yet.