From a practical psychological standpoint, a warning should
be only for a serious failure, and not a routine variation.
When you get a flashing red light with every power
reduction, pretty soon the red light will have lost its
warning effect. From a pilot's perspective, I would like an
alert that the voltage is below a level that will maintain
the battery, about 13.5 volts with a steady yellow light,
but I'd want to have a flashing red if the voltage was below
the 12 volt level. This would happen only with a dead
cell in the battery or with a complete charging failure and
be soon enough to give time to reduce loads and make a safe
landing, but not cry wolf too often.
With some computer logic, you could monitor rpm of the
engine and the alternator and perhaps even the performance
of the voltage regulator (dwell time?).
You might contact Bob Nuckolls [Wichita, KS](see EAA Sport
Pilot magazine back issues for his address/contact info).
He is an engineer and pilot who might have some useful ideas
and FAA certification input.
Obviously you can make an engineering research device to
sample the electrical system for data every second and to
store data on a flash memory chip (USB drive) to include
temperatures, rpm, volts, amps, data which can be used in a
laboratory. But a pilot doesn't need to know more than it
is working or it isn't...time to land ASAP.

I like the airplane to have a volt meter and a load meter.
If the voltage is "in the green" and the load is less than
85% continuous, with all the lights and loads ON, I'm happy.


--
James H. Macklin
ATP,CFI,A&P

--
Merry Christmas
Have a Safe and Happy New Year
Live Long and Prosper
Jim Macklin

"markp" wrote in message
...
| Thanks Jim.
|
| I'm an electronics designer, I have been asked by my
customer to provide a
| warning when battery volts drop below 12.5V. I already
measure voltage with
| a data logger. The logger has an accuracy of +/- 2% on bus
rail voltage
| monitoring (this is around +/-0.25V at 12.5V). My customer
wants a warning
| when battery voltage goes below 12.5V irrespective of
whether the engine is
| running or not. To achieve this the threshold must be set
at 12.75V to
| guarantee catching 12.5V, but it could of course be the
other way round and
| we'll actually warn at 13V. All a bit hairy really.
|
| Given the inherent inaccuracy, I'm concerned that this
voltage warning will
| be triggered un-necessarily and is essentially impractical
when set to
| 12.75V. Personally I'd put the threshold to 12.5V nominal
and have a large
| time constant filter on it to catch glitches and momentary
dips - this would
| trigger at a minimum of 12.25V, max 12.75V. I can also
measure engine speed
| with the logger and inhibit warnings under 50% rpm when
the alternator is
| not charging, this is currently what the prototype sytem
does and is
| reliable, but my customer wants warnings below 12.5V
regardless.
|
| Any comments welcomed!
|
| Regards,
|
| Mark
|
|
| "Jim Macklin" wrote
in message
| news:22mqf.30260$QW2.2267@dukeread08...
| The 12 volt generator/alternator will be at 14-15 volts
| nominal when within the working range of RPM. The
voltage
| regulator should keep the voltage within that range
without
| any significant variation.
|
| Just what are you intending to be warned against? The
load
| meter gauge shows the load being carried by the
generator
| system, the ammeter shows whether the generator or
battery
| is supplying the power being used, An ammeter should be
| showing only a slight charge all the time as it keeps
the
| battery "topped off." If the battery is any good, the
| ammeter will return to low indicated charge within a few
| minutes after starting the engine. At a low idle, the
| battery will show a discharge because the generator is
not
| carrying the load and the battery is discharging to
carry
| what the demand is at the time. This is not a problem
in
| the short term, such as a power-off landing or glide,
but
| anytime that the engine is in the power range, the
generator
| [alternator] should be carrying all the load.
|
| Power relays need about 10 volts to stay engaged. If
you
| are building a home-built airplane, you could have a
yellow
| warning light at less than 13.9 volts and a red warning
at
| 12.8. Open cell no-load voltage is 2.2 volts or 13.2
volts
| on a lead/acid battery.
|
| Certificated aircraft require certificated parts. You
can
| get a cigarette lighter plug-in monitor.
|
|
| --
| James H. Macklin
| ATP,CFI,A&P
|
| --
| Merry Christmas
| Have a Safe and Happy New Year
| Live Long and Prosper
| Jim Macklin
| "markp" wrote in message
| ...
| | Hi All,
| |
| | I'm looking for recommendations for a low battery
warning
| voltage level. I
| | was going to use 12.8V - is this liable to be
triggered by
| noise/dips in the
| | bus voltage? My guess is when heaters or motors kick
in
| there could be a
| | momentary drop due to high current that could trigger
my
| low voltage
| | indicator. Also it it worth putting a long time
constant
| filter on this?
| |
| | Thanks!
| |
| | Mark.
| |
| |
| |
|
|
|
|