Battery Contactor Questions

[Glen Robinson]

I purchased and installed a B&C S701-1 battery contactor on my firewall. I'm leery of wiring it the way B&G suggests. The contactor has a jumper wire that runs from the battery post to the positive coil terminal. The negative side terminal is to be connected to a wire that runs to a switch, then to ground. The only protection I see, is that the very thin jumper wire will probably burn out, if there is a short. Because of the location, I wouldn't be able to diagnose the problem in flight (or do anything about it). Out of all the electrical equipment I have installed, this is the only component that doesn't have a fuse, or breaker protection. I have two ways I could possibly wire the battery contactor, to give me the overvoltage protection I would like. I did contact B&C, but I wasn't happy with their advice View attachment 114462 View attachment 114463 View attachment 114465 . I am new member, so hopefully I can upload some diagrams. Does anyone see any problems with the way I want to wire the battery contactor?


Glen

Thanks,
Glen

 

[mondtster]

Either of your proposals would work. What type of aircraft is this on and how was the circuit originally set up?

The way B&C suggested to wire it is common. The likelihood of having a 2" long wire get pinched and cause a short is pretty small.

 

[Doug Reid]

Over voltage protection is normally a job for the voltage regulator. I see nothing wrong with the two diagrams with breakers. Most failures I have found are either open coils, or bad contacts after many hours of use. Most installs are like your first diagram...just saw mondster's post...I agree.

 

[DaleB]

Either of your proposed methods would work, but each adds several feet of unprotected wire going through your firewall and behind the panel. I’m no EE but my non-expert opinion is that running it to your panel would be considerably less safe.

The wire to the switch in B&C’s diagram will only carry as much current as the contractor coil allows. A wire from the hot side will carry the maximum current that wire is capable of before it finally melts, and it will keep on doing so as long as it can.

 

[Daleandee]

I have one of these in my experimental airplane. Wired as recommended (switched ground) and it has worked as advertised for over 10 years.

Either way you wire it would get the same result but using a ground to trigger it means one less wire to short out if that wire should somehow come into contact with bare metal. B&C also sells a really good OV protection set-up (experimental only) if overvoltage is a concern ...

 

[mondtster]

Either of your proposed methods would work, but each adds several feet of unprotected wire going through your firewall and behind the panel.

This is a point worth emphasizing. The fuse is intended to suppress wire fires, not protect the equipment in the circuit so it should be as close to the power source as practical. There are a lot of type certified aircraft that have long runs of unprotected heavy gauge cable in them, which always bothers me.

 

[Bell206]

Because of the location, I wouldn't be able to diagnose the problem in flight (or do anything about it).

Perhaps explain what your specific concerns are? As noted above the B&C method is very common. And as also mentioned there's no circuit protection on the 4AWG from the battery. So....

 

[Glen Robinson]

I'm building a Sports Performance Aviation Panther. I haven't finished the wiring yet. I'm leaning towards the battery to circuit breaker, then switch, then back to the positive side of the coil. The negative post would go right to ground. Any short in the wires (both positive or ground), or coil should trip the breaker. The nice part about tripping the breaker, is it gives an immediate indication of where the trouble might lie. I have 2 firewall tubes that will safely carry all my cockpit to engine compartment wires through the firewall. Although wire protection is the main job of a fuse or circuit breaker, if sized right, they can offer some protection to their electrical components.

Glen

 

[geezer]

You are proposing to remove a two inch insulated wire, un fused, and not near any sharp grounded parts, with:

A several feet long, unfused, insulated wire, shaking with vibration, and going through a rubber grommet in the sharp, grounded, firewall, and then through a densely wired space to a protective circuit breaker.

You are proposing to multiply the exposed risk by at least 10, and the likelihood of failure by hundreds. Wires through firewalls are among the most likely failures.

The manufacturers have many years of data from which they have selected the recommended wiring that the manufacturer has shown in his diagram. His engineers and those elsewhere in the industry are designing from many years of experience.

 

[Daleandee]

I'm building a Sports Performance Aviation Panther.

That's a really great choice of a kit to build. Dan is a fascinating man and his Panther design is well thought out on so many levels. Congrats. What is your engine choice?

 

[Dan Thomas]

Wire it exactly as per the B&C diagram. That's how Cessna and Piper and everyone else does it. NO breaker or fuse, as it takes a hot unfused wire through the firewall, as DaleB pointed out.

The manufacturers do it the way they do for good reasons. With the wire that runs to the switch being grounded when the switch is closed, the max current flow in the wire is what the contactor coil will allow. If the wire shorts against some sharp or torn metal in a crash or forced landing, all it does is close the contactor. The wire doesn't get hot and start a fire.

A fuse or breaker is another failure point, too. Just asking for trouble. In any airplane (or car), performance troubles are 90% electrical.

This is how Cessna does it:

 

[Bell206]

Although wire protection is the main job of a fuse or circuit breaker, if sized right, they can offer some protection to their electrical components.

You are over-thinking this. And don't ever expect a CB or fuse to "save" your equipment 100% of the time. Good luck.

 

[Glen Robinson]

I'm installing a Jabiru 3300A engine.

 

[Daleandee]

I'm installing a Jabiru 3300A engine.

Tony Spicer had a nice one of those ...

 

[Rgbeard]

I'm installing a Jabiru 3300A engine.

people do this intentionally?

 

[Capt. Geoffrey Thorpe]

I wouldn't be able to diagnose the problem in flight (or do anything about it).

Diagnose what? If the jumper wire from the battery terminal to the coil fails in some way, it's game over as far as electrons are concerned no matter what. Running it into the cockpit won't help. On my ride, I put an in line fuse in the short wire, but I'm not convinced that I should have done it. I would probably just use the jumper if I did it again.

I purchased and installed a B&C S701-1 battery contactor on my firewall.

The battery is on the firewall also?

 

[donjohnston]

As others have mentioned, you're over-thinking this. Wire it as recommended.

Are you going with fuses, circuit breakers or a VPX for you primary circuit protection?

 

[Checkout_my_Six]

Circuit breakers and fuses can fail….those safety devices will reduce the overall reliability value.

so, pick your failure modes and go with it. Less reliable but safer is not a bad option….but more reliable but less safe may not be desirable.

 

[NavAir]

My DA40 is wired exactly as B&C shows it, as apparently many (most) manufacturers do. I wouldn't substitute my electrical engineering knowledge and experience for theirs...

 

[Glen Robinson]

"Diagnose what?" That's exactly my point. If the battery coil pops the circuit breaker, I know I don't have to do anymore diagnostics. Electrically I'm done, and I don't have to waste any time figuring out what to do. Time to think about what to do with no electric. The crash scenario mentioned above with a compromised ground wire causing the battery terminal in a crash to stay on, means everything switched on will remain energized after the crash. If the hot wire is compromised, it might trip the breaker, or the breaker could be pulled to kill the battery contactor. My thinking this way comes from looking at wiring diagrams posted on the internet. Most of the wiring diagrams I have scene show no protection for the battery contactor, but a breaker or fuse on the starter contactor.

 

[Bell206]

My thinking this way comes from looking at wiring diagrams posted on the internet.

Time to think about what to do with no electric.

Perhaps look to the aviation side for guidance in why most aircraft do not have a CB/fuse in that circuit? Circuit protection in aircraft is to prevent a safety hazard in flight and mitigate that hazard until on the ground. No hazard, no requirement to have circuit protection. Same if a circuit does have protection but is not essential for safe flight--there's no requirement for a pilot to have direct access to that fuse or CB. Below are a couple aviation references. If your concern is to expedite your decisions in the event of an in-flight electrical failure, perhaps install an emergency bus, connected hot to the aircraft battery, that will provide you the necessary functions to get the aircraft on the ground? Then you can determine what happened.

 

[Dan Thomas]

On my ride, I put an in line fuse in the short wire, but I'm not convinced that I should have done it. I would probably just use the jumper if I did it again.

That thin little jumper will fuse if it or the coil terminal it's connected to is shorted to ground. No fuse is necessary. That wire is sized based on the current it carries, and that is very little. A fuse, in its fuse holder, will get oxide on its contacts and gain resistance that either hampers contactor operation, or the resistance heats the fuse and it fails. Those terminals are covered by silicone nipples when the airplane is built, but they get torn or lost during maintenance and often aren't replaced. They were there for a reason.


They come in various sizes, and all four terminals get them.

The battery is on the firewall also?

That is a very good question. The contactor should be right at the battery, often right on the battery box, to keep that fat unfused positive cable as short as possible. Again, a fuse there is just a failure waiting to happen. The installation is designed and built to preclude any shorting of that cable, but sometimes mechanics will screw it up by replacing a corroded cable with a longer one that starts getting rubbed by something. A classic case is the Cessna 185 that had a too-long cable being sawed at by the upper elevator cable, and it shorted to the cable and welded the various pivoting mechanisms, freezing the elevator position.

And starter circuits need no fuse. If something shorts, you just stop cranking. Shut the master off. Get out.