Z electrical system selection

[Camillo]

Hi. I searched on this site and found only few threads on Aeroelectric connection systems (Z-11, Z-12, Z-13 and Z-14).

I read Bob book and would like to go with a single battery, dual alternator system, to fly very light IFR and eventually upgrade later.

Apart from SD-8 and SD-20 selection (I think SD-8 would not be enough...still have to do some calculation), I saw in Bob FAQ that Z-12 is not raccomended for a new design (I guess he suggests Z-13 or Z-14). My question is: why?

I understood that Z-12 is a single layer system, but you can use at your choice main alt. (main alt. switch ON and aux. alt. switch OFF) OR aux. alt. (main alternator switch OFF and aux. alt. ON). Aux. alt. is connected to the same layer of main alt. (this is the limit...), but one can lower the current consumption simply switching off some circuits (i.e.: EFIS, which has its own battery, ecc...). And this seems a simple system. One can also design it only for one alt. and upgrade it later.

I understood that Z-13 is designed to make the aux. alt. work on endurance bus only. So, if main alt. goes out, you can simply switch the other on (I guess that if you have SB-1 instead of LR3 this is automatic). But you can also let current go from endurance bus (aux. alt. only working) to main bus switching on battery contactor. So, one may have the same functionality of system Z-12 with a difference: in Z-13 you shall not switch secondary systems off to stay within aux. alt. supply, because this is the natural design feature of this system.

Well, if this is true (and I fear I missed something), why go to the more complex Z-13 if one knows that will use an SD-20? With SD-8 I can deduce that you save 1 amp. (on total 8...not bad) without closing battery contactor. But with SD-20, why complicate your life for one poor amp. on a total of 20?

Thanks.
Camillo

[f1rocket]

I can't comment on the differences between the Z-12 and Z-13 approaches. I have a dual alternator system on my Rocket and I use an endurance bus. All the wiring diagrams are on my web site.

I can (and have) switched off the primary alternator in flight to simulate a failure. The primary alternator light comes on to warm me that the alternator is offline, and the auxiliary alternator carries the load. If it cannot, the auxiliary light blinks to warn me to shed load. I can shed load by turning the endurance bus on and the master off and if that's not enough, I can switch off remaining items individually. When the SD20 can keep up with the load, the blinking light stops blinking.

I keep both alternators on all the time. It is a "set it and forget it" system that works just like it is supposed to, in spite of the fact that some on this list have claimed otherwise. I find it very simple to operate. I use all B&C alternators and control units.

Now, I will admit that it's not the most cost effective approach. The B&C stuff is pricey but it does work and it lasts. However, you might want to consider a single alternator, dual battery system. When configured correctly, I believe it provides the same redundancy for fewer dollars.

[N941WR]

Originally Posted by f1rocket

... However, you might want to consider a single alternator, dual battery system. When configured correctly, I believe it provides the same redundancy for fewer dollars.

IMHO, this might be the better solution. Should the single alternator fail, the two batteries should get out of trouble. With the double alterntor and one battery, if that single battery fails, I think (please correct me if I'm wrong) you are SOL as the alternators no longer have a place to store their output.

In most of the US, a half hour is long enough to get you out of trouble but a full hour would be best.

[Camillo]

Randy, compliments for your projects and web site.
As far as I can understand, your schematics are similar to Bob Z-12.
I can't understand the following: (if main alt. shuts down,) if you disconnect battery contactor, aux. alternator will not be linked to battery; how can it work?

Bill, a second battery will be more economic, but I will add more weight. In such instance, maybe I should go with Z-14 (eliminating the second alternator).

Thank-you.
Camillo

[johnp]

Originally Posted by N941WR

IMHO, this might be the better solution. Should the single alternator fail, the two batteries should get out of trouble. With the double alterntor and one battery, if that single battery fails, I think (please correct me if I'm wrong) you are SOL as the alternators no longer have a place to store their output.

In most of the US, a half hour is long enough to get you out of trouble but a full hour would be best.

exactly my reasoning -- i'll have dual pc680's.

[DGlaeser]

Originally Posted by Camillo

Randy, compliments for your projects and web site.
As far as I can understand, your schematics are similar to Bob Z-12.
I can't understand the following: (if main alt. shuts down,) if you disconnect battery contactor, aux. alternator will not be linked to battery; how can it work?

Bill, a second battery will be more economic, but I will add more weight. In such instance, maybe I should go with Z-14 (eliminating the second alternator).

Thank-you.
Camillo

Z-19 is Bob's architecture for 1 Alternator and 2 Batteries.

[Camillo]

Yes, I saw it this afternoon. Too lazy to modify again my message.

Anyway, against the double battery, I can say (all studied from Bob book; three months ago I didn't know the difference between amp and watt):
- second battery is heavier than second alternator system;
- second alternator is maintenance free;
- battery failure (as Bob says) is more difficult than (belt) alternator;
- gear alternator (SD-8/SD-20) have 0,5/100,000 fault ratio;
- second alternator (SD-20) provides infinite power, when second battery is limited to its a.h. and efficiency (Bob says half batteries installed are fault).

To finalize my idea:
- if I will go with Z-12 (which is relatively simple), I can build my plane with a single battery/single alternator and upgrade it during construction (when have money for SD-20 and SB1) or later;
- system will be pilot friendly, since if main alt. warning light goes ON, aux. alt. will activate and its light will go ON, too, and will also start blinking in case of consumption bigger than production;
- in such case, pilot may switch some unnecessary stuff OFF (and/or switch OFF switches for items with internal battery, as Trutrak ADI, Dynon EFIS and Garmin GPS296, in my case...they will continue operating with internal batteries);
- if both alt. break (nearly impossible), pilot can switch master OFF and aux. ebus feed ON, thus running e-bus with battery only.

The bad aspect is that I think such architecture will not allow an upgrade to a full dual layer system (dual batteries/dual alternator). That's why, I guess, Bob says Z-12 is not made to be elected in a new design. But...Z-13/20 appears to me too complicated - to build and to fly - to build.

I have a theory (must still read the book for the second time): Z-13/8 has a sense because SD-8 (which provides almost 10 amp.) bypasses battery contactor (so saving 1 amp.) and was born to make builder save money (who couldn't buy an SD-20). But, if you have to go with SD-20...why adopting such complicated (for me) architecture?!

Anyway, Bob has another "Z" for an aux. battery upgrade. This should be enough. I don't have a B777 and...even such airplanes seem to have electrical losses!

Any comment appreciated.
Camillo

[pvalovich]

Single battery -8A with 40 amp main alternator and an SD-8. AFS-3500 with backup batter, and Garmin 496 with backup battery. SL30 and ICOM 1200 for comm. PMA7000B and GTX-327, and an ADI Pilot II.

I initially thought of using an SD-20 vice SD-8, but an email response from Bob N. made me realize that the continuous loading was the most important number, not intermittent loads.

Also, the mindset that a main alt failure is a fairly serious problem, best addressed on the ground. Shedding continuous loads to less than 8 amps by turning off the strobes, nav lights and pitot heat is reasonable. 30+ minutes of batt only capbility is fine - if the decision is made to land at the nearest suitable field. The definition of suitable will vary with the conditions.

Just my biased opinions and the thought process used to design my electrical system. Bottom line - the SD-20 was overkill.

[DGlaeser]

Are you planning to use an electrically dependendent ignition system?
How often do you expect to fly IFR? (hard or light, doesn't matter )
A single alternator, single battery system is actually very robust, as long as you have an E-Bus. A decent RG battery will run a whole panel of today's electronics for quite a while - and if you are careful, quite likely longer than your fuel will last, even if you are IFR.
You mention that 'Bob says half batteries installed are fault'. I think he said that in the context of people who don't replace their batteries until they can't crank the engine. If you replace your battery annually, the lottery is a better bet than battery failure. And even then it's only a problem if the alternator also fails (which means it just isn't your day )
Any architecture can be upgraded. It's just adding parts and adding/moving wires! So if you decide you really want the second alternator, just do it!