Normal voltage drop

In rec.aviation.owning Robert M. Gary wrote:
: 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?

Don't forget that the charging voltage for a lead-acid battery is *supposed*
to be modified by ambient temperature, with a higher voltage at lower temps.
This is commonly done with a thermistor in the voltage regulator. Even
the mechanical (vibrating relay) regulator in my 1977 Datsun has temperature
compensation.

That said, 13.6V is probably OK. If you check the volts on the battery
terminals you'll probably get a little more than the 13.6V on the avionics
bus, because the battery is wired to the alternator with heavier wires.
--
Aaron C.

On 28 Dec 2004 06:51:30 -0800, wrote:

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.


This may be an alternator problem. The output reading is what is
coming off the alternator, not what is being held by the battery.
This happened to me, and I ended up re-building the alternator. The
good news is a re-build is about the same as the cost of a new
battery!

Good Luck.
z

Robert M. Gary wrote:
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?

How dare you doubt the word of your A&P? Don't you know that every A&P
candidate is individually interviewed by an FAA inspector before he's
even allowed to take the written test, and those who are unworthy are
weeded out? ( for the sarcasm impaired )

Seriously, 13.6 volts under load is not unusual.

In theory, the system should be 14 V, and you are allowed a drop of 2%
of that to the main bus, for a minimum voltage of 13.72 V. Now, how
exactly do you think your 20+ year old (by design) regulator holds the
voltage? Think it might be 0.1 V off? Because that's all it would
take. Think it's absolutely insensitive to load (a true voltage
regulator with zero temperature and current dependence)? Think again.
That level of stability is not necessary and generally not present.
Typically the regulator will give you 14 +/- 0.2 V.

At 13.6V, your battery will charge just fine and your accessories will
run. So watch it, and as long as it stays where it is, don't worry
about it.

Michael

"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

There are six diodes in the common three-phase alternator.

Dan

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

On Tue, 28 Dec 2004 17:43:57 +0000 (UTC), Aaron Coolidge
wrote:

snip

Don't forget that the charging voltage for a lead-acid battery is *supposed*
to be modified by ambient temperature, with a higher voltage at lower temps.
This is commonly done with a thermistor in the voltage regulator. Even
the mechanical (vibrating relay) regulator in my 1977 Datsun has temperature
compensation.

Good point, the battery dudes suggest a difference in regulated
charging voltage with varying battery operating temperatures, which
can vary from ambient temps.


That said, 13.6V is probably OK. If you check the volts on the battery
terminals you'll probably get a little more than the 13.6V on the avionics
bus, because the battery is wired to the alternator with heavier wires.

Another good point, voltage drop between a GA aircraft main bus and
the avionics bus is not uncommon.

Regards;

TC

The regulator's sensing is usually taken directly from the
alternator's output terminal, not the bus, and voltage variations at
the bus may not be present at the alternator's sense terminal so it
might not respond to a seemingly low system voltage. This could happen
if there's a bad connection between the alternator output and bus, such
as the crimped connections in the output cable, connections at the
alternator output breaker, or the breaker itself. None of these are
unusual, and the voltage will usually drop further as load is applied
and the resistance in the connections makes itself known.
That said, the regulator is often overlooked in the search for
alternator problems. The regulator controls the field current in the
alternator, and if it's not doing it well the output voltage will be
off or erratic. Lots of alternators have been replaced for no reason
other than a bad or misadjusted regulator.

Dan

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