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tomcostanza
Joined: 19 Oct 2008
Posts: 49
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 Posted: Mon Jun 29, 2009 10:39 am Post subject: Battery failure |
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I have read a lot on this forum about alternator failures and e-busses and backup alternators. Is it possible that the battery can fail in a way that short circuits the bus, causing a total power failure? If so, is there sufficient probably that I should design around that?
My RV will be equipped for light IFR, and I'll have an e-bus with sufficient reserve to fly for an hour after an alternator failure. I may add a 2nd alternator at some point, but for now, just a single battery/alternator.
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_________________
Clear Skies,
Tom Costanza
-- in year 17 of a 3 year project |
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longg(at)pjm.com
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| Posted: Mon Jun 29, 2009 12:46 pm Post subject: Battery failure |
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Tom,
What I have read on this list is that yes, batteries fail, but
historically dead shorts are rare and worries are few. Thus, large ANL
protection (200 amp) between battery and contactor are deemed
unnecessary. I have discussed this same topic recently with my
mechanics. They have indicated heat is a real danger of Mr. Battery.
Extreme amounts of it can and will cause dead shorts internally (I've
yet to experience one). Thus, if I am to keep Mr. Battery on the
firewall I am ill advised to protect it with shielding, venting and ANL
fuse to keep it from going poof.
Urban legend? I don't know. Bob will tell us. I do know that while I
have a second alternator, I will add a second battery (in the rear) just
to ensure my plasma III pair keeps on ticking. My design goal? No IFR
landings on rainy nights in the PA woods. Legend also has it that if you
keep the tail really low while landing in the trees on an IFR night, it
won't hurt as bad.
Glenn
Do not archive
--
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nuckolls.bob(at)aeroelect
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| Posted: Mon Jun 29, 2009 7:47 pm Post subject: Battery failure |
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At 01:39 PM 6/29/2009, you wrote:
| Quote: |
<Tom(at)CostanzaAndAssociates.com>
I have read a lot on this forum about alternator failures and
e-busses and backup alternators. Is it possible that the battery
can fail in a way that short circuits the bus, causing a total power failure?
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No. We used to see the occasional shorted
cell in flooded batteries. This could cause
a 12 volt battery to become a 10 volt battery.
Remaining good cells then suffered overcharging.
RG batteries don't do this.
| Quote: | If so, is there sufficient probably that I should design around that?
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No
| Quote: | My RV will be equipped for light IFR, and I'll have an e-bus with
sufficient reserve to fly for an hour after an alternator
failure. I may add a 2nd alternator at some point, but for now,
just a single battery/alternator.
|
Start out with Z-11 which is easily morphed
to Z-13/8 at a later time.
Bob . . .
---------------------------------------
( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
---------------------------------------
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nuckolls.bob(at)aeroelect
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| Posted: Mon Jun 29, 2009 8:07 pm Post subject: Battery failure |
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AeroElectric-List message posted by: <longg(at)pjm.com>
What I have read on this list is that yes, batteries fail, but
historically dead shorts are rare and worries are few. Thus, large ANL
protection (200 amp) between battery and contactor are deemed
unnecessary.
Correct. If current limiters are found in battery
circuits, they are to protect WIRES from the fault
currents that can flow (1000+ amps) when a battery
becomes the SOURCE driving a HARD FAULT.
I have discussed this same topic recently with my
mechanics. They have indicated heat is a real danger of Mr. Battery.
Extreme amounts of it can and will cause dead shorts internally (I've
yet to experience one). Thus, if I am to keep Mr. Battery on the
firewall I am ill advised to protect it with shielding, venting and ANL
fuse to keep it from going poof.
RG batteries contain a relatively small amount
of moisture compared to the flooded
cells of yesteryear. When an RG battery is
deliberately abused by either hard fault or
overcharge, the manufacturer is well advised
to design cases to withstand over-pressures
(typically 2 PSI) whereupon the vent valves
open and the battery goes "sssssss" not
"boom".
I used to have running conversations with
the Navy's battery test facilities in Crane
Indiana where EVERY battery in Navy inventory
and many other batteries were test for worst
case outcomes in horrible failure modes.
Our friends at Concorde were routinely subject
to Crane's House of Battery Horrors to verify
their suitability to task in military programs.
It's an easy and not terribly expensive
thing to try for yourself. Hook 3 golf cart
6v batteries in series and hook 17 a.h.
RG in parallel with it. Thermocouple the
battery, watch the current as the 18v
source "cooks" the 12v battery. Test #2 calls
for throwing a dead short across the same
fully charged battery and tracking temperatures
and currents.
Early on in the history of all batteries,
there were some spectacular outcomes for
these tests. However, given the ubiquitous
presence of the SVLA battery in everything
from video cameras to toys to airplanes to
submarines, the family of suppliers have
pretty much all become righteous citizens.
When you see fuses in series with batteries,
it's to protect external things from the energy
the batteries contain . . . not vice versa.
Bob . . .
---------------------------------------
( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
---------------------------------------
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