Normal voltage drop

"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.

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.

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

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