Normal voltage drop

In looking for some 43.13 in html, I found this thread. Just had to
comment...
1A was quoted, 1B is current and the relevant passage is in Section 5,
11-66.b

According to AC 43.13-1B -- "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. The tabulation
shown in table 11-6 defines the maximum acceptable voltage drop in the
load circuits between the bus and the utilization equipment ground."

Two different current paths are being addressed; one from the generation
source to the bus, and a second from the bus to the load. Dissecting that
sentence a bit to get two, one for the generation source and one for the
battery.

1) The voltage drop in the main power wires from the generation source to
the bus should not exceed 2 percent of the regulated voltage when the
generator is carrying rated current.

2) The voltage drop in the main power wires from the battery to the bus
should not exceed 2 percent of the regulated voltage when the battery is
being discharged at the 5-minute rate.

Two totally different statements. The current in the gen/alt circuit
should never exceed it's charging capacity; hence that limitation, however
the discharge current from the battery to the bus (in a fault condition)
could approach that 5-minute rate they speak of. That's the rate where
you have a dead battery in ~5 minutes and approximates the battery
ampacity*12. For most of the small batteries, say a size 35, the 1C rate
is in the low 20's. The Concorde 25 size is rated 21Ah; the 35 size at
25Ah. 21*12=252. That's a lot of current! It's not exactly linear
either, the actual current is less, but I haven't seen too many published
12C rates.

So the first sentence says you cannot exceed 2% drop from the generation
source to the BUSS, at rated current. For most of us with 14V (pick your
reference) systems that is ~ 0.28V. That statement is for the circuit to
the generator/alternator, only and is not too difficult to comply with
with a 50 or 60A source. Even with the 100-150mV or so of drop across the
gen/alt breaker, you still have a bit over 100mV of drop in a wire to work
with. Fifty amps through ten feet of AWG-2 is about 85mV. This is the
easy part to comply with.

That second one is the killer, especially for those with an aft mounted
battery. Assume the 5-minute discharge rate isn't really 252A, but (for
ease) only 200A. Ohms law R=E/I 0.28V/200A=0.0014 ohms. That's only a
bit over eight feet of the #2 cable, and if you have an ammeter shunt in
the battery cable, subtract another 50mV from your budget and shorten up
that cable even more.

The last statement, "The tabulation shown in table 11-6 defines the
maximum acceptable voltage drop in the load circuits between the bus and
the utilization equipment ground." clearly states voltage drops between
the bus and equipment ground; effectively from the breaker unique to the
equipment to the ground connection for the piece(s) connected to that
breaker (powered from the bus). For most loads these are easy; 0.5V
continuous and 1.0V intermittent, for 14V systems. Only high current
loads are much of a problem; heated pitot tubes, gear/flap motors,
landing/taxi lights and such. Most all under 20A.

So back to the question, acceptable bus voltage. The comments on
regulator stability are prime, especially for charging, and (I think), the
intent of the question, as the AC reads "2 percent of the regulated
voltage". What is the regulated voltage? As was posted earlier,
depending on temperature Concorde suggests 13.7514.75V so that 0.28V
should be decent estimate for a 14V system. Measure your alt/gen terminal
voltage (under load directly across the terminals) and to be legal per
43.13 your bus should not be more than 0.28V less, and your battery should
charge...

My apologies for my long (and first) post. I hope to create no ire from
the group and hopefully contribute something of value for those
interested.

Ron
'73 BL17-31ATC

"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

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

Dan

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.

writes:

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. [...]

If the voltage drops in flight, I would suspect the alternator
system(s) rather than the battery.

- FChE

Frank Ch. Eigler wrote:
writes:

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. [...]

If the voltage drops in flight, I would suspect the alternator
system(s) rather than the battery.


I agree. I've had several charging system problems over the years
and 12.1V is what I see whenever my alternator goes offline. That's
equal to a charged battery (~12.6V) minus the load from a few radios
and accessories.

John Galban=====N4BQ (PA28-180)

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

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.

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