Low battery voltage level

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.
|
|
|