2nd alternator? Why not alternator and generator?

[tetonpass]

Notice Virgil has 2 alternators and busses on his #5 I am new to airplane building but a skilled mechanic Do not see the point in two components which can both fail unexpectedly. Recent youtube video of a lady who had to return to base with that exact problem.Alternators cannot charge a dead battery but a generator can. Hard to kill a generator but a short will fry the diodes on an alternator. in a NY minute.Ask me how I know. Why not a gen and an alternator? I also noticed Colin(?) having to load 150 lb iron aft for CG. Why not something useful like a big battery which could help keep your IO540 and the electronics running after you loose the alternator?

[jaredyates]

There's kind of a lot to the electrical system architecture design. A good resource to learn more is the Aeroelectric Connection book.

One way to go about designing a reliable system is to make a list of all of the failures and threats that you might encounter, and decide which of those you want to mitigate.

Having the battery so dead that it can't energize an alternator field is a possibility. What might cause that? Leaving the master on? Battery failure? If the batteries are tied together, leaving the master on will kill them both. Things can always break but we tend to think we have a good strategy for understanding when a battery will quit working. There is eye-watering level of more detail on the two alternators vs two batteries in the AEC book.

I think the alternator's efficiency and ability to produce high current at a broad range of RPMs outweighs any benefits that a generator may offer. On our last plane we used a B&C SD-8 on the vacuum pad, and there's a fine line in the terminology between whether it is an alternator or a generator, being that it has permanent magnets. 14 years ago when I was building that plane, the SD-8 was an attractive choice but these days we can get 40A alternators for the vacuum pad. They don't have as much output at idle RPM as a belt driven unit but for just a little more space and weight they offer a whole lot more capability than the SD8.

N619MS was originally outfitted with 150 pounds of ballast but in flight testing it was determined that at most, half of that was sufficient. And this was with a giant (too big) engine. I wouldn't say there is a need to intentionally add weight in the form of batteries but am happy to hear from other BH5 folks who might disagree.

A key goal is to reduce the empty weight.

[alaskabearhawk]

Battery cables, when sized correctly for the distance, are heavy and expensive too!

[TimTall]

If you put batteries in the back, they'll always be in the back. Use ballast to get the CofG right when you're empty so you'll have more useful load when you need it. I have all of my tools and a survival kit aft of the rear bulkhead. That puts me just inside the envelope when I'm solo at min fuel. If flies fine at the forward limit.

[tetonpass]

There's kind of a lot to the electrical system architecture design. A good resource to learn more is the Aeroelectric Connection book.

One way to go about designing a reliable system is to make a list of all of the failures and threats that you might encounter, and decide which of those you want to mitigate.

Having the battery so dead that it can't energize an alternator field is a possibility. What might cause that? Leaving the master on? Battery failure? If the batteries are tied together, leaving the master on will kill them both. Things can always break but we tend to think we have a good strategy for understanding when a battery will quit working. There is eye-watering level of more detail on the two alternators vs two batteries in the AEC book.

I think the alternator's efficiency and ability to produce high current at a broad range of RPMs outweighs any benefits that a generator may offer. On our last plane we used a B&C SD-8 on the vacuum pad, and there's a fine line in the terminology between whether it is an alternator or a generator, being that it has permanent magnets. 14 years ago when I was building that plane, the SD-8 was an attractive choice but these days we can get 40A alternators for the vacuum pad. They don't have as much output at idle RPM as a belt driven unit but for just a little more space and weight they offer a whole lot more capability than the SD8.

N619MS was originally outfitted with 150 pounds of ballast but in flight testing it was determined that at most, half of that was sufficient. And this was with a giant (too big) engine. I wouldn't say there is a need to intentionally add weight in the form of batteries but am happy to hear from other BH5 folks who might disagree.

A key goal is to reduce the empty weight.

Thank you Jared and Tim. I have an off grid camper ND COMMERCIAL N Pacific fishboat with just such an electrical system, High capacity alternator and backup generator. I do not see the point of TWO alternators. One alternator with spare parts(regulator, diodes etc) and robust overcurrent protection. I am torn getting a 4 or the #5 kit. Not much difference in price. I have rebuilt alternators, mostly Densos which are durable and child's play to take care of, and fully field repairable. New brushes every 100,000 miles or 1000 hrs . Bearings never fail.

[svyolo]

That was my choice, at least so far. I was going to carry a spare VR, but ended up mounting it next to the primary, and put them on switches.