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Rear mounted battery - what did you do to protect the long wire run?

moespeeds

Well Known Member
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For those that mounted the battery back behind the passenger seat, what was done to protect that long 2ga wire run against a dead short?

My concern is in that unlikely situation, the solenoid contactor could get welded closed and you'd be unable to kill the power from the master switch.

Thoughts?
 
Alternator Shunt

There should be a shunt between the alternator and the battery and the main bus.
The starter contactor should be near the battery, so that the big wire from the battery to the starter is only hot while cranking the starter (which removes the possibility of it shorting or anything else except when cranking, which should be an acceptable risk).
The battery contactor is also near the battery, but the 60amp shunt is on the firewall, so if for some reason you had an over current from the alternator, everything downstream is cut off once the shunt burns out.
The battery and downstream of the shunt all starts at a bus that is all fuses, so nothing will get burned up there. Then, I guess the battery contactor could get welded open (connected), but there is also a main bus fuse that protects everything after that.
That just leaves the wire chafing through and dumping the battery to the airframe, which I guess would overheat/melt/fire your battery and maybe arc-weld through the spot where the wire chafed, but surely you would have caught something like that happening over time before it was catastrophic?
 

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I wrapped my 15 ft battery cable in abrasion resistant polyester sleeving that meets ASTM E162 for flame resistance (McMaster Carr). All of the holes it passes through have M22529/2 grommet edging installed (Aircraft Spruce). As it was installed after the aircraft was built to shift the CG aft, snap bushings and Adel clamps were not practical. If you are concerned about contactors welding shut, you could use something like a InPower SSC20-200 Solid State 200A DC Contactor.

John Salak
RV-12 N896HS
 
...

Thoughts?

I had installed a battery in the back and used #0 welding wire to get the power from the tail to the starter. After the availability of super-light lifepo4 batteries, I just pulled all that heavy stuff out, and put the light battery on the firewall. Simpler, lighter, but some additional risk due to the "complexity" of a lifepo4 battery.

W&B is fine - I have a metal constant speed hartzell, parallel valve io-360, and Grove Aluminum gear.
 
For those that mounted the battery back behind the passenger seat, what was done to protect that long 2ga wire run against a dead short?

My concern is in that unlikely situation, the solenoid contactor could get welded closed and you'd be unable to kill the power from the master switch.

Thoughts?

I would think you would have the exact same thing happen for a firewall mounted battery - so location is not an issue unless you think the wire run itself is a risk. But to answer then specific question (double fault - a shorted #2 wire and a failed master solenoid), I would guess the master solenoid would fail (with perhaps some flame), as in the contacts would no longer exist and the casualty would end - other than the fact you now have no battery power to the panel (assuming the only battery feed to the panel is via the master solenoid - which I do not recommend).

Perhaps someone would run an experiment to prove or disprove this theory.

As a side note, my amateur testing of #2 welding wire for such runs provides me some indication that welding wire is more resistant to abrasion than standard #2 Tefzel.

Carl
RV-8 with one PC-625 mounted in the forward battery well, the second PC-625 mounted aft.
 
For Untainted123. I’m pretty sure the GSA 28 pitch servo should be installed on port side of RV8. I think the picture you’re showing is an RV8 but with servo on starboard side of aircraft.
 
Yes, that is an 8 installation. I don't see it called out in the 8 plans, and at any rate, isn't the direction and throw configurable in the G3x (page 9-68 says it is)? Plane isn't flying yet, so really haven't thought much about it.

The 14 plans show it on that side though. I don't think we even bothered to really consider which side was "correct", since it's all configurable.

Is this something I am missing? Where is this called out in the plans or a critical point?
 
There should be a shunt between the alternator and the battery and the main bus.
The starter contactor should be near the battery, so that the big wire from the battery to the starter is only hot while cranking the starter (which removes the possibility of it shorting or anything else except when cranking, which should be an acceptable risk).
The battery contactor is also near the battery, but the 60amp shunt is on the firewall, so if for some reason you had an over current from the alternator, everything downstream is cut off once the shunt burns out.
The battery and downstream of the shunt all starts at a bus that is all fuses, so nothing will get burned up there. Then, I guess the battery contactor could get welded open (connected), but there is also a main bus fuse that protects everything after that.
That just leaves the wire chafing through and dumping the battery to the airframe, which I guess would overheat/melt/fire your battery and maybe arc-weld through the spot where the wire chafed, but surely you would have caught something like that happening over time before it was catastrophic?

I’m a little confused about all this. A shunt is not a fuse and doesn’t protect anything from an over current. It measures current through a conductor. At least that’s the way I understand it. I agree that a fat unprotected wire on an aft mounted battery to your starter is an acceptable risk, because you could just release the starter current in the event of a short that you became aware of. However, a ANL fuse on your alternator output wire is basically useless if it’s upstream of your battery contactor/main bus feed if there isn’t also a fuse on the wire after the battery contactor. The only thing in your airplane capable of exceeding the current carrying capability of a properly sized main bus feed wire (~6 awg) is the battery. The OUTBOUND feed from your battery should be fused. A 60 amp alternator is basically self limiting. It can’t possibly put out much more then 60 amps. Putting a fuse (not shunt) on the wire from your alternator to your master contactor only protects the wire running from the master contactor to the alternator. And the only protection is from the battery dumping huge amounts of current (momentarily) into the wire to the alternator - FROM THE BATTERY - not the alternator, since it cannot exceed the current carrying capability of the wire. So if you don’t have protection on the wire feeding power from the battery contactor to the main bus, you don’t have protection. In an aft battery configuration I would think you would need protection on the fat wire going from the alternator firewall foreword to the aft battery and another fuse/CB protecting the fat wire coming forward from the battery contactor the main bus. If you didn’t have that second fuse/CB then a short in a feed wire to main bus (alternator is out of the picture at this point because the battery is unfused) would result in a very high amp overload on that fat wire that would melt things.
 
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For Untainted123. You might want to check out G3X installation manual. In edition AN look at pages 18-28 & 18-31. There’s other good information in that section as well. I think the installation would work in reverse - it wouldn’t work - on starboard side. Could also contact [email protected]. Hope that is helpful. Happy Thanksgiving!
 
Best answer I ever heard was "Most high amp shorts are self-fusing", i.e. they melt open pretty quick when the shorted structure is light gauge metal.

With that in mind, consider a different view. The goal isn't to protect the cable. Good aircraft practice and and craftsmanship takes care of the cable. Be concerned with things a shorted cable might damage. For example, don't run the cable in contact with fuel lines or wiring bundles.
 
A shunt is not a fuse and doesn’t protect anything from an over current.
You're right, I conflated the 2 from memory. I meant fuse when I said shunt.

A ANL fuse on your alternator output wire is basically useless if it’s upstream of your battery contactor/main bus feed if there isn’t also a fuse on the wire after the battery contactor. The only thing in your airplane capable of exceeding the current carrying capability of a properly sized main bus feed wire (~6 awg) is the battery. The OUTBOUND feed from your battery should be fused.
The outbound feed from the battery IS fused, at the main bus fuse blocks. See below though regarding the ability of the alternator to dump out more than 60amps.

But OP's concern seems to be more of the wire chafing or contacting somewhere along it's path between the battery and the main bus fuses. Someone mentioned proper wire path considerations, and wire protection; that's pretty much the only thing you can do there, besides as you mention placing a fuse right at the battery + terminal, which I don't think is done regularly, so the possibility and risk of that seems pretty low, assuming proper wire routing and securement.

A 60 amp alternator is basically self limiting. It can’t possibly put out much more then 60 amps.
I don't think this is true, I was under the impression that a bad alternator could dump out way more than 60 amps (even momentarily), and was fuse (ANL) protected for that reason.

I have attached the diagram I went by (simplified somewhat for this conversation), showing the ANL fuse right after the alternator (thank you Aerotronics!). I assumed it was to protect everything downstream from the alternator.

I'll admit, I am no electrician, I just know the it hurts when you touch the wrong wires (your fingers or your wallet :D), so please correct anything written here.
 

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I don't think this is true, I was under the impression that a bad alternator could dump out way more than 60 amps (even momentarily), and was fuse (ANL) protected for that reason.

I have attached the diagram I went by (simplified somewhat for this conversation), showing the ANL fuse right after the alternator (thank you Aerotronics!). I assumed it was to protect everything downstream from the alternator.

As previously described, a circuit interruption device on the alternator B-lead is there to open the path from the battery, in the event the B-lead or alternator is shorted to ground.

Consider I=E/R. Amperage is a result. The regulator (often backed up with an overvoltage device) controls the system voltage. System resistance varies when we add or subtract loads. To increase amps we must increase volts or lower resistance. It doesn't just happen.

Suppled wiring diagrams often do a poor job of identifying the physical location of components. The ANL goes on the firewall, not next to the alternator. We fuse the current source with the voltage we can't control (the battery) to protect against a short (low resistance) in the length of wire which runs alongside the hot, vibrating thing.
 
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You're right, I conflated the 2 from memory. I meant fuse when I said shunt.

The outbound feed from the battery IS fused, at the main bus fuse blocks. See below though regarding the ability of the alternator to dump out more than 60amps.

But OP's concern seems to be more of the wire chafing or contacting somewhere along it's path between the battery and the main bus fuses. Someone mentioned proper wire path considerations, and wire protection; that's pretty much the only thing you can do there, besides as you mention placing a fuse right at the battery + terminal, which I don't think is done regularly, so the possibility and risk of that seems pretty low, assuming proper wire routing and securement.

I don't think this is true, I was under the impression that a bad alternator could dump out way more than 60 amps (even momentarily), and was fuse (ANL) protected for that reason.

I have attached the diagram I went by (simplified somewhat for this conversation), showing the ANL fuse right after the alternator (thank you Aerotronics!). I assumed it was to protect everything downstream from the alternator.

I'll admit, I am no electrician, I just know the it hurts when you touch the wrong wires (your fingers or your wallet :D), so please correct anything written here.

As Dan said, and I tried to - not very clearly obviously - is that you need to prevent your battery from dumping damaging current into your B lead wire going forward toward the alternator from either a short in the B lead wire or alternator itself. The alternator probably isn’t capable of dumping anywhere near that much current into that B lead. The battery is a huge capacitor with lots of stored energy, capable of dumping it out in one big belch. The alternator stores nothing and can only create energy mechanically as designed, 60 amps for example, within a tolerable range.
The RV6 I am building has the battery and both contractors mounted on the forward side of the firewall, but is technically similar to the OP’s aft battery question, regarding protected heavy conductors inside the airplane. On mine I have an ANL fuse protecting the alternator B lead wire mounted right next to the master relay/contactor. I also have a 50 amp Maxi fuse mounted near there (I have a 40 amp B & C alternator) protecting the 6 awg main bus feed wire before it goes through the firewall to my main bus fuse panel. I could have used 8 awg wire with the 50 amp fuse, but I had #6 so that’s what I used. The OP’s system could have the same basic setup with the fuse near the aft battery location protecting the B lead going forward, and also a fuse, circuit breaker, or any current limiting device protecting his main bus feed going forward from the business side of his master relay to his main bus fuse/CB panel.
My goal is to not have any unprotected wires inside the airplane. The battery is the bomb capable of shorting damaging amounts of energy through shorted fat wires into anything that fat wire is touching. Be careful what you let that fat wire touch.
 
For me I would go with a fuse or breaker at the battery along with whatever other overcurrent protection is normally used. Also I would consider putting the AWG 2 into Electrical PVC conduit, if one is really concerned about short-circuit protection.

I don't see any problem with using multiple current-protection devices on the same conductor.
 
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Put a EarhX on the firewall, if you need ballast in the back, have your wife oack a ivernighter and stowe.
 
A shunt is not a fuse.

Thecnically a shunt is not a fuse.
But a shunt rated for 100 A will act as a fuse when subjected to 200 A or more.
Thats probably why you don't see a lot of ANL fuses in older aircraft.
 
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