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Electronic Ignition: Two alternators or two batteries

Davea320

Davea320

Well Known Member
Looking for electrical power and backup for electronic ignition and I am asking for the pros and cons of a two battery system with one alternator or a two alternator system with one battery. Thoughts?

Dave
 
Based on my personal choice and safety assurance;
Two alternator, one main alternator that runs/supports the whole ship's power and one that is built-in to the EI and works a self generating EI.
 
I'm planning on an SDS CPI2 ignition. My intent is to have two alternators, as well as the integrated backup battery solution offered with the CPI2. I'm leaning towards the two-alternator system because, to me, it offers fewer concerns in the case of a primary alternator failure. That is, rather than needing to worry about battery capacity, I can effectively fly indefinitely. If I have both alternators fail, then the backup battery will give me enough juice to get on the ground.
 
I wonder how many of us have actually used that second alternator that we haul around 100% of the time. I have. It allowed us to continue a week long trip without a care in the world until we got home. ONCE. We're all different, and I've gone back and forth, but I think that one use, saving our vacation enjoyment, was worth it to haul around the ballast for the last 12yrs. Hmm, or was it. A personal choice that the math can't always support.

I've also failed both at the same time in a distant foreign country across a big body of water... (which we won't talk about).
 
Davea320 said:
Looking for electrical power and backup for electronic ignition and I am asking for the pros and cons of a two battery system with one alternator or a two alternator system with one battery. Thoughts?

Dave
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It’s good to define what each is best suited for. You need at least one of each, but for a third supply of electrons, which form of supply should it be?

A) Batteries are a short term reservoir of electrical power with decreasing capacity over time, capable of high loads for a short period of time, and topped up by the alternator.

B) Alternators are a constant supply of electrical energy good for supplying some/all constant loads and recharging the battery (reservoir).

Choosing one or the other involves considering failure modes of components and of the electrical system in general.

I have 2 Pmags (they self generate electrical power) one lithium battery, one main alternator, one smaller standby alternator. That makes 4 alternators total and one battery, which is easily changed during routine maintenance. To each their own but these are my choices.

Bevan
 
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One battery, one alternator, one electronic ignition, and one engine. A man?s man.
 
Davea320 said:
Thanks for all the quick replies. Leaning towards two alternators and one battery.

Dave
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Not the best option. The second alternator mitigates only one of the risks to loosing power to your panel, loss of the primary alternator. While this might be the most probable casualty, loosing the alternator just means you have X minutes of battery reserve (whatever you designed). The more severe risks all result from the loss of the single battery. That could mean total loss of power, regardless of how many alternators you may have. While a well maintained (not abused) battery is about the most reliable thing you can put in an airplane, the stuff you connect to it to get battery power where it needs to be, is not.

Of course you could, as option, add a covey of backup batteries and such that tend to have limited reserve capacity. I offer that two main batteries (e.g. PC-625 or PC-680) provide a far more robust basis for power in our aircraft.

Carl
 
I?ve done it both ways. First rocket had dual alternators. Second one had two batteries. With some boat parts to control the charging between the two batteries, the wiring and the ?thinking? part during a failure scenario is easier and cleaner with two batteries. Just my opinion
 
DanH said:
Do those alternators work well with the battery disconnected?
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Exactly. To the OP, this is the question you need to answer first, or more importantly have your alternator mfg answer.....

And on a different front, can you prop your motor if your battery is dead on a dark and stormy night? Does that matter for the mission you fly? Its not the same answer for everyone.
 
Two PC680's one alternator. Alternate the battery use for each flight. Simple A B switch.
 
Dave320 - You didn't state which ignition system you are looking at, or if you plan a single or dual EI setup. Different scenarios require different charging solutions. Please elaborate so the responses can be more tuned to your setup.
 
One alternator and two batteries

I’m going with one B&C alternator and a two battery setup in my -7 build. My primary battery that’ll be mounted on the firewall will probably be a 625 and the backup battery will be the vented Earth-X ETX900-VNT. I know it’s probably overkill using the big EXT900 as a backup, but it only weighs 5.4 lbs. I’m planning on mounting it behind the baggage compartment, but due to its lightweight, I think it’ll be okay with regards to W&B. I’ve considered wiring it in so that it’s part of the starting system, but I think I’ll set it up where it’s only there for backup power. If I loose the alternator, I think I’ll have ample juice to get me on the ground safely to an airport of my choosing. I’d probably run out of gas before I’d run out of electrons. I’m installing the EFII bus manager, so mixing and matching a led-acid with a lithium battery won’t be a problem.
 
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One nice thing about 2 batteries is that you can use them in parallel for engine start. That much more battery capacity is nice.
 
330drvr said:
One nice thing about 2 batteries is that you can use them in parallel for engine start. That much more battery capacity is nice.
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If you can't do that with single battery, then you've got either a battery/wiring problem or a starter problem.
 
Or you left something (maybe master?) ON. Nice to have a second try if you are somewhere remote, or just want to get going.
 
tracy said:
One battery, one alternator, one electronic ignition, and one engine. A man’s man.
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Ooh Rah. Just be sure to have that extra hand held radio so you can say "mayday, mayday, mayday" and inform others of your plight come the event one of those fail. ;)

Personally, I do two batteries with the second battery on a separate emergency bus, which has a voltage maintainer circuit and monitoring on the EFIS. If both my alternator and main battery actually fail at the same time (consider the odds of that compared to the average 500 hour MTBF of a magneto set), then a flip of a switch provides me 1.5 hours to find a place to land before voltage drops below 11.8 volts. {tested}.
 
Got it! So when that one engine fails, you have all your radios to scream mayday. Wait....no...you would be in the same boat as me. :)
 
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