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finn.lassen(at)verizon.ne
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 Posted: Tue Nov 12, 2024 4:28 pm Post subject: OVP relay in B+ lead bad idea |
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Putting a relay between the alternator B+ lead and the battery for over
voltage protection is a bad idea!
If the battery fails open and the alternator voltage momentarily drops
below voltage to energize the relay, you no longer have system power: no
battery and no voltage from alternator even if alternator works.
This happened to me Saturday resulting in a hay field landing with a
ground loop to avoid a fence. Buckled outer part of right wing and tore
off right landing gear leg with damage to engine mount, firewall and
forward part of fuselage. (In the RV-4 the landing gear leg sits in a
socket in the engine mount.)
In the process of trouble shooting noise and other issues in my
electrical system I changed from a 160Amp schottky diode to a relay. My
original thinking was that the diode would act as a fuseable link in
case of an alternator over voltage run-away event, and leave the battery
unaffected in case of an alternator short.
I had experienced the breaker to the alternator field popping when
reducing RPM. So I restored the alternator to its original integrated
voltage regulator configuration and added the relay to ensure the
alternator could be disconnected from the system in case of an
alternator run-away event (over voltage crowbar shorting the power to
the relay).
Interestingly the alternator voltage regulator start-up input apparently
also draws enough power to pop a 5A breaker when RPM is suddenly
reduced, as I had this happen even after restoring the alternator to
factory configuration.
So this event could be the result of both a bad battery and alternator.
Finn
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user9253
Joined: 28 Mar 2008
Posts: 1944
Location: Riley TWP Michigan
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| Posted: Tue Nov 12, 2024 7:45 pm Post subject: Re: OVP relay in B+ lead bad idea |
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Finn, I gather that your engine ignition system is electrically dependent.
What type and brand is it?
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Joe Gores |
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finn.lassen(at)verizon.ne
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| Posted: Wed Nov 13, 2024 6:13 am Post subject: OVP relay in B+ lead bad idea |
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Yes, I guess I should stated "... for an electrically dependent engine".
Mazda 13-B Renesis, two spark plugs per rotor. Electronic fuel injection
and ignition.
Battery: Power Sport YTX20L-BS.
Alternator: Mitsubishi reman'd 14910. 1986-1988 Mazda RX7.
Battery mounted on firewall on engine side.
Gets hot under the cowling. As much as 190F.
Now I'm beginning to question the wisdom of mounting an AGM battery in
the engine compartment. Van's doing it in their newer models, so I
figured it'd be OK. But a casual search seems to indicate that high
temps drastically shortens the life of AGM batteries. However, battery
easily started the engine, so more likely an intermittent (now
permanent) internal disconnect in the battery. That and a relatively
slow acting voltage regulator in the alternator appear to be the causes
of the total electrical power loss (the OVP relay being the weak design
point).
In retrospect prioritizing protection of avionics from alternator
over-voltage event over reliable power for engine was a bad idea. On the
other hand, a major alternator over voltage event would likely also take
out the engine controller.
Appears the real solution would have been a backup battery to power
essential engine electronics. Problem would be how to keep that isolated
from alternator and main battery and still keep it charged. Seems there
will always be some point of possible failure. Perhaps a double-throw
switch to disconnect it from charging diode and connect it to the engine
bus. But would also need to be able to disconnect the engine bus from
alternator and main battery. So it all comes down to a super-reliable
switch, but still single point of failure.
Finn
On 11/12/2024 10:45 PM, user9253 wrote:
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user9253
Joined: 28 Mar 2008
Posts: 1944
Location: Riley TWP Michigan
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| Posted: Thu Nov 14, 2024 4:15 pm Post subject: Re: OVP relay in B+ lead bad idea |
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I also had a PC680 battery fail open due to a broken internal weld. You can read about it here:
http://forums.matronics.com/viewtopic.php?t=16762906
Luckily my Rotax 912uls does not depend on the aircraft electrical system.
So the engine kept running fine.
.
Below is a circuit with an aux battery for the engine bus. Suggestions welcome.
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Joe Gores |
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finn.lassen(at)verizon.ne
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| Posted: Thu Nov 14, 2024 6:34 pm Post subject: OVP relay in B+ lead bad idea |
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Fascinating! Mine is a Power Sport YTX20L-BS but symptoms very similar.
I managed to charge it up to 13V over night. But when lightly loaded
with an engine monitor it dropped to ~5V. Perhaps the 5V is just where
the EM voltage regulator cuts off. I'll try with a landing light.
I looked at the thread you referenced. Did Bob ever do an autopsy on
your PC680?
I don't really like relays -- prefer switches if reliable.
I'm thinking a backup battery on the cockpit side of the firewall (the
PC680 in a metal case has a temp limit of 140F and I measured as much a
190F in the alternator -- granted, the alternator mounted on the engine
-- significant thermal transfer from engine to alternator housing).
Currently my engine bus is always live (connected to battery via
fuseable link).
I'm thinking of inserting a DPDT switch between the main battery and
engine bus. In normal operation engine bus would be connected to main
battery and the second pole to a diode from alternator/main battery to
charge the backup battery. When throwing the switch that diode would be
disconnected and the engine bus connected to only the backup battery. Of
course that makes that switch a single point of failure.
The way I see it I want to protect against:
1) Overvoltage from alternator reaching battery, main and engine bus.
2) Alternator short to ground shorting battery, main and engine bus.
3) Dead main battery and/or alternator not affecting back-up battery and
engine bus.
Obviously also need to be able to shred non-essential loads to extend
flight time. There a backup battery would extend running time in case of
failed alternator and good main battery.
Finn
On 11/14/2024 7:15 PM, user9253 wrote:
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user9253
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Location: Riley TWP Michigan
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finn.lassen(at)verizon.ne
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| Posted: Fri Nov 15, 2024 6:18 am Post subject: OVP relay in B+ lead bad idea |
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Thank you Joe.
Wish I'd seen or paid attention to that back in 2016!
So an intermittently electrically open battery is definitely something
to allow for in an electrically dependent engine installation.
Certainly would explain the "noise" issues I was trying to solve.
I unfortunately bought into the "battery is backup for alternator and
alternator is backup for battery" idea.
When you have intermittent failure or weaknesses in both, that just
doesn't work.
I still have to investigate the alternator. Is it really possible for
its output voltage to drop low enough and long enough for a relay to
open when RPM drops? Or was it a heat and/or vibration issue?
RX-7 alternator re-man'd in 2002. I think I may even have replaced the
internal voltage regulator in the process of troubleshooting noise, etc.
Finn
On 11/14/2024 10:26 PM, user9253 wrote:
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user9253
Joined: 28 Mar 2008
Posts: 1944
Location: Riley TWP Michigan
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| Posted: Fri Nov 15, 2024 9:46 am Post subject: Re: OVP relay in B+ lead bad idea |
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It takes a lot less voltage and current to hold a relay closed than it does to first energize it.
Just because a circuit breaker is marked "5A" doesn't necessarily mean that it will trip at exactly 5 amps.
It should be bench tested with an adjustable current power supply. It might be tripping at much less than
5 amps. A loose connection will generate heat which can result in a breaker tripping at
less than its rated current. The alternator field circuit breaker should not be tripping.
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Good job making a forced landing and walking away. Many pilots panic when the engine
quits and pull back on the stick to avoid going down. The wings stall with fatal results.
The ground doesn't give. Bob Hoover quote:
"If you're faced with a forced landing, fly the thing as far into the crash as possible."
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