Dual LSI - Sanity Check

N130WN · #1 01-03-2012, 07:24 PM

My RV-7 project (night VFR, dual AFS 4500 with internal battery, VP-X) has dual Lightspeed plasma II ignitions. I'm weighing my options as far as the electrical system goes, and I find that I like this approach, from the Lightspeed web site:

Quote:

-Is a back-up battery necessary?
If you are using a battery to start your engine, you have sufficient energy to run an ignition system for several hours after an alternator failure. Therefore, a back-up battery is not required when running a dual electronic ignition system. Once the system is running, less than 5V are required to maintain operation, giving a further safety cushion. At this low power level, most other electrical equipment will have long stopped functioning giving the pilot adequate warning.

Sounds good to me -- no unnecessary complexity, cost or weight. But, I have never heard of anyone here using this approach. Many projects, even those with only one electrically dependent EI, have backup alternators and/or batteries. Whats wrong with the simple approach above?

cubdriver · #2 01-03-2012, 08:41 PM

My $.02 - If you have a problem with the battery - your in trouble. While not likely I would not be without a backup. According to Lightspeed, a very small backup battery will power the system in the event of a total failure - even a 9 volt batter. My guess is that it would weigh no more than 1/2 pound.......so why not.

Kahuna · #3 01-03-2012, 09:10 PM

Quote:

Originally Posted by N130WN

My RV-7 project (night VFR, dual AFS 4500 with internal battery, VP-X) has dual Lightspeed plasma II ignitions. I'm weighing my options as far as the electrical system goes, and I find that I like this approach, from the Lightspeed web site:

Sounds good to me -- no unnecessary complexity, cost or weight. But, I have never heard of anyone here using this approach. Many projects, even those with only one electrically dependent EI, have backup alternators and/or batteries. Whats wrong with the simple approach above?

Are you saying that you have never heard of anyone using a dual LSE setup w/o the use of a backup battery systems to run them? Most are in fact set up this way. I assume that by 'simple' approach you mean no backup battery to run the LSE.

If thats what your asking.... Yes its simple, and yes lots of installations are this way.

As a general rule, adding the complexity of redundancy should in fact address a problem or failure mode that is likely or common enough to justify the added risk of the complexity. You will find that Klaus does in fact recommend no battery back up since as a rule, batteries dont fail. And if wired per his instructions, ie. directly to the battery or very close to it, and having the rest of your systems wired with good practices, well then you have a very reliable rig.

rocketbob · #4 01-03-2012, 11:53 PM

Paul Lipps had a backup battery in his Lancair...he designed the Lightspeed systems so if that tells you something....and I understand Klaus something fail and found out the hard way his backup battery wasn't charging.

David-aviator · #5 01-04-2012, 06:53 AM

Seems like the most practical, simple answer is one EI and one mag. The advantage of 2 vrs 1 EI's is hardly noticeable. Unless the pilot has an instant alternator failure indication, the battery won't last as long as needed with other stuff sucking it down.

But then we fly with just one prop, crankshaft and cam; why not one source of power for the ignition system?

Actually, that's not right. There are 2 sources of power in every airplane - #1 is the alternator and #2 is the battery. The battery sits in reserve, the alternator does all the grunt work. Hook a load meter up to it and another to the battery and it is the alternator doing all the work at its higher voltage.

Claus's thinking is not all that far out.

Walt · #6 01-04-2012, 07:18 AM

I don't see anything wrong with a no "back-up" system design, as long as your electical system has been designed and assembled to the highest standards and has proven to be reliable. After 2 years of running with 1 mag and 1 EI, I then switched to dual EI. I added a BUG (back up gen) at that time but that was to provide for redundancy on long cross countries where I may not want to stop or I could continue to the airport of my choice.

As long as you have a good electrical system monitor to let you know when your alternator has quit, and you are prepared to load shed and land soon if that occurs, then I don't see any issue with it.

rocketbob · #7 01-04-2012, 08:04 AM

One of my good friends had the battery contactor fail in flight in his RV7. The coil shorted internally. Total shutdown of the electrical system. As he was diagnosing the problem, he shut the alternator off and it wouldn't come back on due to the field wire requiring battery power to energize. The alternator I use does not require battery power to excite. I tested this in my -6.

A few years ago I had a battery in my Jeep fail shorted. Instantly shut everything down. I doubt this is very likely in an AGM-type battery but probably is still possible.

All it takes is a small 4-5aH battery isolated with a diode and a resistor or mosfet to limit charging current to have some insurance.

flion · #8 01-04-2012, 08:25 AM

For a dissenting voice, let me say that while batteries rarely fail outright, it happens. And it's not just the battery, it's the whole battery circuit (one reason Klaus recommends attaching the EI directly to the battery is to avoid a bus failure ... but that assumes that the direct circuit to the EI won't fail. I note that the dual PIII system I just received included the backup circuit diagram and a zener diode. A small backup battery that lives behind the panel somewhere is cheap insurance. But I agree in principle with Walt; it's not a big deal if you don't have it.

rocketbob · #9 01-04-2012, 08:35 AM

Using the backup charging diagram as shown by Lightspeed will eventually cause the backup battery to become damaged. The reason is that the regulator is a device designed to charge a battery and limits charging current based on sensed voltage and temperature. Since the main battery is of much lower impedance when it is in a discharged state the backup battery will overcharge. I talked to Paul about this very thing and he agreed and even said that he found this out to be true via the school of hard knocks and designed a circuit using a mosfet to limit current of the backup battery. As I recall his backup battery bulged severely and came close to exploding.

With a dual battery setup current limiting is not required individually because the batteries have identical impedance, or close to it, and because they're larger, are more tolerant of improper charging.

Walt · #10 01-04-2012, 08:40 AM

Quote:

Originally Posted by rocketbob

One of my good friends had the battery contactor fail in flight in his RV7. The coil shorted internally. Total shutdown of the electrical system. As he was diagnosing the problem, he shut the alternator off and it wouldn't come back on due to the field wire requiring battery power to energize. The alternator I use does not require battery power to excite. I tested this in my -6..

If he would have had a "standard" Z-11 with an E-bus design this would have been a non issue.. good design and implementation is very important. As noted in the EI diagrams everything is bypassed and the EI is directly hooked to the battey via swtch and CB, so again the above situation would be irelevent to the EI

Quote:

Originally Posted by rocketbob

A few years ago I had a battery in my Jeep fail shorted. Instantly shut everything down. I doubt this is very likely in an AGM-type battery but probably is still possible.

Sure anything can happer but a highly unlikely event with modern AGM batteries, is it possible, sure anything is "possible", but not likely. No more likely to fail than your Napa oil filter