[tectweaker]

So I have laid out the wiring diagram few times and finally I am beginning to understand this clearly. However, one big question is where to route the alternator output wire.

There are three options as I have drawn in the pdf linked.

https://www.dropbox.com/s/ebaq9rh0od5hv3m/Wiring options.pdf?dl=0

1- Alternator goes to the starter contactor. This is simplest for me since the airplane is already wired with a starter contactor and battery contactor. I will use a current limiter in this case which I already have. I am not quite sure why I can't use this approach.

2- Alternator goes to the battery contactor. This would be same as option 1 except that I will run a wire from firewall to the battery (battery is in the baggage compartment)

3- Run the wire to a 60A CB in the panel and then a wire from there to the battery contactor. Are there any advantages to this approach?

I do have wires and other things available for each option so it is just a matter of time and complexity. Love to hear thoughts.

 

[Auburntsts]

I wired my RV-10 like option 1. It's simple and is the lightest option with the battery being back in the tail. I see no advantage to option 2 over option 1 and option 3 is really just a variation of option 2.

 

[rv7charlie]

#3 forces 2 more runs of heavy wire, plus an expensive CB, into the cockpit (assuming a forward of the firewall battery). And, you'll have the additional small but measurable losses of the extra wire, contact points of the CB, etc before the energy arrives at the battery. And the extra failure points; especially the CB. And, if I'm interpreting your drawing correctly, there's significant risk that the alt can bootstrap itself in continued operation, keeping the buss hot, even if you open the master contactor (emergency shutdown of electrics while engine is running; ex: electrical fire).

 

[Raymo]

Check out the price of a Klixon 60 amp circuit breaker. It will help make the decision.

#3 is closer to how my Cherokee 140 was wired. Alt > amp meter > CB > buss > starter contactor. Of course, this is comparing apples to old rotten oranges.

 

[tectweaker]

SD8 Option

Makes sense. I am going to use option 1.

One more thought - I might be using SD8 since it looks like it would be sufficient and would actually fit (I previously thought it would not).

There is no difference I am assuming if I were to use a SD8 to route the output to the starter contactor. The wiring diagram for SD8 is the only one which shows not going to starter contactor on B&C's website. I will check with them as well.

 

[ccrawford]

https://www.dropbox.com/s/ebaq9rh0od5hv3m/Wiring options.pdf?dl=0

1- Alternator goes to the starter contactor. This is simplest for me since the airplane is already wired with a starter contactor and battery contactor. I will use a current limiter in this case which I already have. I am not quite sure why I can't use this approach.

Where would you mount the 60A fuse? I'm using #2 (fwd battery) and can't see where to easily/securely mount a 60A fuse between the alternator and the starter (in #1) unless you are using an inline one?

 

[rv7charlie]

Corey's question begs another. Current philosophy is to size the alt B lead big enough to carry anything the alt can supply, therefore, there's no danger of melting/burning it from the alt's output. Then no protection from the alt is needed. However, the battery can overwhelm any practically sized wire, so the alt B lead should be protected at the battery end. Which raises the question: How far apart are the master contactor and the starter contactor? Is it any easier to wire to the starter contactor than the master?

Note that if you're planning on using the internal contactor in the starter instead of a separate contactor, you must supply a very heavy duty start switch to handle the surge loads of most built-in contactors. Additionally (assuming a built-in contactor), the current drawing leaves the fat cable to the starter 'always hot' any time the master is on. Unlikely to be a big deal, unless something gets out of place in flight & trys to short it to ground. Then you've got high energy electricity making sparks around fuel & oil while in flight.

Most of this stuff is covered in the AeroElectric book, & on that list.

Charlie

edit: If the starter contactor is on the firewall with the master, then it's fairly simple to mount a fuse holder. In either case, using fuseable link wire solves the mounting problem. You can buy wire made specifically for this. You just size the link wire (~6") four sizes under the actual B lead size. (12 for 8, 10 for 6, etc)

 

[RV6_flyer]

Makes sense. I am going to use option 1.

One more thought - I might be using SD8 since it looks like it would be sufficient and would actually fit (I previously thought it would not).

There is no difference I am assuming if I were to use a SD8 to route the output to the starter contactor. The wiring diagram for SD8 is the only one which shows not going to starter contactor on B&C's website. I will check with them as well.

Just saw the thread and was going to post Option 1 for the reasons others gave.

One of my good very respected friends suggested that I look into not using a belt driven alternator and look at using one off the vacuum pad. Just passing on his idea to you. You may be able to use a lighter alternator that way and still have the output capacity needed.

I was thinking like you to have a backup alternator on my RV-8 project. I keep asking myself is it worth the extra cost and weight to have one? How often does a good B&C Alternator system fail? Part of me wants the backup but the cost, weight, and need for one keep saying that I do not need one.

Yes I had an intermittent alternator issue 5-minutes after an IFR departure. I asked for a return to my departure airport (home base) and was on the ground about 20-minutes after the request. Was already on an IFR Flight Plan. The IFR departure path was a 180 to put me on the IFR arrival approach.

 

[Bavafa]

Mine is wired based on option 3. The added cost is about 4-5 feet of wire (in terms of weight) and one CB but the added protection was worth it for me.

 

[Auburntsts]

Makes sense. I am going to use option 1.

One more thought - I might be using SD8 since it looks like it would be sufficient and would actually fit (I previously thought it would not).

There is no difference I am assuming if I were to use a SD8 to route the output to the starter contactor. The wiring diagram for SD8 is the only one which shows not going to starter contactor on B&C's website. I will check with them as well.

I have a PlanePower 30 amp FS1-14 backup alternator on the vacuum pad but it's wired to battery side of my battery contactor. So that means I have a run of 10 and 12 ga wire distributing power from the FS1 back to the battery and then to my VP-200 control unit. The rationale is this gives me an emergency redundant path that will power 85% of my normal total electrical requirements in the event of a battery contactor failure. I have a Vertical power VP-200 so my electrical architecture is a bit non-standard.

So to recap, my primary 70 amp alternator is wired up like option 1, and my back-up 30 amp alternator is wired up similar to option 2. Also I have an appropriate sized ANL for each alternator vs. CB's.

 

[rv7charlie]

Mine is wired based on option 3. The added cost is about 4-5 feet of wire (in terms of weight) and one CB but the added protection was worth it for me.

What does it protect?

 

[N941WR]

...
One of my good very respected friends suggested that I look into not using a belt driven alternator and look at using one off the vacuum pad. Just passing on his idea to you. You may be able to use a lighter alternator that way and still have the output capacity needed.

I was thinking like you to have a backup alternator on my RV-8 project. I keep asking myself is it worth the extra cost and weight to have one? How often does a good B&C Alternator system fail? Part of me wants the backup but the cost, weight, and need for one keep saying that I do not need one.

....

Ask yourself this, "What happens if my alternator fails?" If you don't like the answer, then add the redundancy.

In my case, a VFR ship is just fine without all the crazy backup systems people add. The SkyView, handheld GPS, and Dynon PocketPanel all have backup batteries. All that I am really missing is a T into my external antenna for a battery powered handheld radio.

 

[Low Pass]

Ask yourself this, "What happens if my alternator fails?" If you don't like the answer, then add the redundancy.

In my case, a VFR ship is just fine without all the crazy backup systems people add. The SkyView, handheld GPS, and Dynon PocketPanel all have backup batteries. All that I am really missing is a T into my external antenna for a battery powered handheld radio.

Another question one needs to ask themselves is, "Am I going to return home, cancel or alter my trip when one of my two alternators fail?" Having two gives you (roughly) twice the chance of experiencing a failure. So, do you then take a spare with you??

 

[tectweaker]

I now have both SD8 and plane power 60 A sitting in my hangar. Both of them were sourced from the local field.

I realized that I don't have a gear assembly on the back of the engine. It was capped off. Looks like PP might be the only option now. Otherwise; SD8 might be totally sufficient for my needs.

If anybody has a vacum gear assembly with housing by chance for o320A1A (Narrow Deck) then please let me know...

 

[Carl Froehlich]

Ask yourself this, "What happens if my alternator fails?" If you don't like the answer, then add the redundancy.

In my case, a VFR ship is just fine without all the crazy backup systems people add. The SkyView, handheld GPS, and Dynon PocketPanel all have backup batteries. All that I am really missing is a T into my external antenna for a battery powered handheld radio.

Three things:
- Two batteries are more reliable than two alternators - if and only if you incorporate the one of the various simple power distribution schemes to have them independent from each other. Most two alternator schemes have at least one common failure point that would take out both units. The easiest way to do this is to have two identical batteries that combined provide the cranking amps you want. That way nothing is just along for the ride "in case".
- Dynon and others have backup batteries and such, but this does not support things like XPDRs, radios or electric boost pumps.
- Do not use a BNC 'T' to connect a handheld to an existing antenna connected to any other radio. You will be feeding RF energy into the panel radio - and a little bit can do a lot of damage. Use a BNC double female connector and first disconnect the antenna lead from the panel radio and then attach to your handheld.

On the original question, a 70 amp alternator breaker mounted on the firewall in between the alternator and the battery side of the starter solenoid protects from the unlikely but "real bad outcome" of discharging a battery to a failed alternator - or a broken off alternator lead that is shorted to engine ground. If you have a 60 amp alternator use a 70 amp breaker. If the breaker trips you will not be able to reset in flight - but if it does something bad has happened and you don't want to reset it. Two batteries like the PC-625 provide at least 3 hours of IFR flight.
http://www.aircraftspruce.com/catalog/elpages/70amp.php?clickkey=37667

Carl

 

[Bavafa]

What does it protect?

protection and isolating the power from the alternator via a CB. I know many of the alternators have OV protection built-in, including my alternator but I have had experience in the past that the built-in OV protection did not kick-in quick enough. The extra cable was actually about 3 feet for me from the master solenoid to the panel where I have a CB and that is little cost in terms of $ or weight for the added protection.

 

[rv7charlie]

Hate to tell you, but a CB will not protect your equipment from an overvoltage event. All it will do is open if you have a catastrophic short to ground (with current supplied by the *battery*), or, if you rate it too close to the alternator capacity (which is almost always higher than its rating), it will 'nuisance trip' if the alternator tries to meet heavy loads while recharging a 'soggy' battery.

The only way you get overvoltage protection is with a dedicated overvoltage circuit that is designed to either open the alternator field, or open a contactor between the alternator and its load(s). If the alt goes overvoltage without dedicated protection, the voltage will go up until {voltage divided by the load resistance = alternator amps capacity} stops the voltage increase. This will take a looong time to happen in electronics time units, if at all.

edit: What I mean is that if the CB is rated higher than the alt's max capacity, as it should be to prevent nuisance trips, then it will *never trip* due to an overvoltage event, because the alternator 'self-limits' its output, by nature. It cannot produce more current than it can produce. If its rating is, for example, 60 amps, then it will likely be able to produce 65-70 amps in the real world. And it will do that, if asked, at 14 volts, or if you have a regulator failure, at whatever voltage it can get to & still produce that maximum current. So, if you supply a CB that's safely above the alt's capacity to avoid nuisance trips, then current never rises high enough to trip it. The CB has basically no interest at all in voltage, until it exceeds the insulation capacity of components inside. Then you'll get internal arcing; not a trip.