nuckolls.bob(at)aeroelect
Guest
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 Posted: Fri Apr 03, 2009 5:50 am Post subject: Effects of Battery Disconnect with Good Alternator? |
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At 12:57 PM 4/2/2009, you wrote:
| Quote: |
Bob,
There was a question that a few of us were debating and would be
interested in your input on.
Assuming you have a single battery installation and a good
(regulated) alternator doing its thing as expected (putting out
13.8ish volts),
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Actually, nominal setpoint for for the alternator
should be in the 14.2 to 14.6 range . . .
| Quote: | what would be the effect on the system if the battery was
disconnected (like if primary battery cable came undone or was
disconnected)? I know the battery acts like a capacitor to dampen
the bus voltage (and fluctuations from the alternator), but would
the alternator continue to output the right voltage in the quantity
(current) as dictated by the active loads on the bus? In other
words, could you fly indefinitely as long as the active load did not
exceed the alternator capacity (and assuming the regulator was
working appropriately)?
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The battery does offer a modicum of noise mitigation
but it's not really great . . . and alternator noises on
a battery-less system are not a whole lot greater
when the battery is off line. In fact, the Bonanza and
Barons of many years had separate and independent alternator
and battery switches. Further, although not by original
design, the alternators on these airplanes were capable
of coming on line without a battery. There are procedures
in the flight manual that speak to operations without
a battery.
It's not a real simple answer. It's easy to see how
system dynamics can be materially altered if the battery
were not present. The alternator's response to sudden
application or off-loading of demand is not real
fast. This means that high-inrush loads like pitot heaters,
landing gear pumps, etc. MAY cause the bus to sag below
the minimum operating points for some electro-whizzies.
The duration is but tens of milliseconds and the potential
for deleterious effects are similar to the starter brown-out
phenomenon that has been discussed extensively on these pages.
Assuming that your particular alternator is NOT capable
of coming on line without a battery assist, then sudden
application of load might cause it to "stall" . . . and
stay off line. Assuming your alternator is being loaded
to some high rate of delivery and the load is suddenly
removed, the upward transient in bus voltage during the
alternator/regulator's recovery time MAY be a bit much
for other electro-whizzies . . . including the alternator's
own regulator. The automotive world defines a
"load dump" event as loss of battery connection while
the battery is a significant total of the alternator's
present load.
Bottom line is that one should not plan on routine
operations with battery off line. Now, assume you
have any way to become AWARE that your battery contactor
has opened due to some failure (exceedingly rare).
The best modus operandi is to continue flight to
comfortable arrival without making big changes
to system loads. But without specific monitoring
facilities to detect loss of battery, your first
notice of malfunction just might be a stall of
the alternator when you turn on landing lights
or extend gear.
This is not even a moderate probability event . . .
or at least it's never been mentioned as something
"we needed to go fix" in my career of working on
airplanes. The turbine aircraft are another matter
in that the majority use starter/generators. Generator
only ops are not a big deal for a variety of
little details concerning voltage regulation
dynamics and the self-exciting nature of generators.
Bob . . .
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