Charging?

PaulaJay1 wrote:
: I have a voltmeter plugin in the cig lighter socket. It reads about 11.5 when
: I trun on the master. Reads about 12.8 after engine start and after an hour
: flight has worked up to maybe 13.7 or so. I plan on taking a known accurate
: meter out Sat to check these reading.
: My A&P says maybe the alternator needs rebuilt (at about $250). He says that
: the voltage reg is solid state and either works or not - ie no adjustment.
: I've had a couple of times when the bat needed a boost, so something is wrong.
: Any comments before I start throwing money at the problem.

The only way for voltage to drop is if there's current in the measurement
chain. There are also a number of reasons that could cause these symptoms, and having
a bad alternator isn't very high on the list.

Since it's difficult for most people to measure current, it's worth measuring
battery voltage with a number of load conditions to narrow down the search. To reduce
the number of variables, measure the voltage *AT* the battery with master off, on (no
engine), on (above idle some), on (cruise RPM). Do this with and without heavy loads
on (pitot heat, nav, and landing light on/off together make for a good 25-40A
transient). It would also be good to measure the DC and AC output of the alternator
*AT* the alternator post during these tests. Possibly even the voltage at the
regulator input to the alternator.

Not all those measurements would likely be completely necessary, but with all
of them the problem can almost certainly be found. Possible reasons:

- Bad diode(s) in alternator
- Bad connection on field to alternator (it will get a bit hot)
- Bad connection on output of alternator (it will get a lot hot)
- Bad connection with current draw between bus and cig lighter
- Bad regulator (either adjustment or replacement if fixed)
- Bad alternator (unlikely, since if it's turning and the windings aren't faulted,
there's nothing else to be wrong)
- Bad battery (unlikely since if it's dead it won't draw enough current to keep the
voltage down for a long time)

Lots of reasons... not quite enough info to determine the cause.

-Cory
--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

- Bad battery (unlikely since if it's dead it won't draw enough
current to keep the
voltage down for a long time)

Don't excuse the battery too quickly, although more accurate voltage
and voltage drop measurements are certainly in order. Check also that
the engine/alternator is grounded with respect to the battery negative,
and do it under heavy load. Check also that the regulator is
adequately grounded with respect to the battery negative. Remember
that the airframe isn't necessarily a good conductor.

Wires are used to carry current. The airframe only might do so.

From reading these and other posts, Pipers seem to have an inordinate
amount of electrical problems. Is it because of the aluminum wiring on
the main cables?

nrp wrote:
: Don't excuse the battery too quickly, although more accurate voltage

Actually, the battery is quite likely screwed if the charging system is broken
somehow. If a flooded lead-acid battery sits dead for even a short period of time
(like overnight), it can be considerably damaged. If you replace the battery without
fixing the charging system (a *very* common practice, BTW), you'll only succeed in
destroying a brand new battery in a short period of time.

When a battery goes bad, it almost always has a weak cell. In that case, It
won't absorb any current without easily floating up to the max voltage. The
"time-dependent" voltage rise he mentioned isn't consistent with that. If the battery
is somehow taking lots of current but not charging, the energy is going somewhere else
(read: heat)... badness will ensue in this unlikely failure mode.

-Cory

--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

On 27 Jan 2005 23:44:49 GMT, (PaulaJay1) wrote:

I have a voltmeter plugin in the cig lighter socket. It reads about 11.5 when
I trun on the master. Reads about 12.8 after engine start and after an hour
flight has worked up to maybe 13.7 or so. I plan on taking a known accurate
meter out Sat to check these reading.
My A&P says maybe the alternator needs rebuilt (at about $250). He says that
the voltage reg is solid state and either works or not - ie no adjustment.
I've had a couple of times when the bat needed a boost, so something is wrong.
Any comments before I start throwing money at the problem.

Chuck
Archer 2185B


Sounds like what I just went through a few months ago. Turned out
that some of my windings were burnt (and maybe a bad diode). Got a
rebuild, charged the battery, and the starter spun the prop like I
hadn't seen in a long time!

HTH.
z