electronic ignition question

My MZ202 engine uses dual CD ignition, with each CD unit firing a plug in each of the 2 cylinders. But these are DC CD's, not the Rotax AC type. This means they require a 12v battery to run, & the engine will die if the battery fails, shorts, or rattles a terminal loose. Because of this, I have fitted a large bright LED voltage meter on my panel.

But to try to get away from a single point of failure, I'd like to fit a 'standby' or auxy battery as backup. As an aside, my W&B measurements tell me I need some 2 or 3kg of ballast in the nose, so I'd rather use a battery than carry deadweight.

At the panel, the master switch supplies separate switches for CD1 & CD2. All 3 switches must be closed for operation. I would like to do one of the following:

1/. Feed power from auxy batt to one CD unit via one of the CD switches. Thus in event of one battery or wiring going down, I still have single ignition on both cylinders.

2/. Have auxy batt 'standing by', wired in via a voltage sensing electronic relay which would change over to aux batt in event of primary batt failure. (This would be for ignition only - too much hassle to wire the aux batt into starter system. So would need to be a few milliseconds in operation to keep engine running).

3/. To charge aux batt, either an off the shelf auto dual battery charger (as used in 4WDs & campervans to charge service + domestic batt), or just charge the aux batt in the hangar occasionally. Even a small battery will outlast fuel reserves.

I would value the observations of those who understand such things. As a mechanical engineer, I struggle with the 'white man's magic' that is electronics. In particular, I am having trouble finding a voltage sensing changeover relay. I'm hoping these exist - I'm sure there must be other applications they'd be used in.

Or am I just being paranoid? I never used to worry about such things, back in the day, cheerfully taking off with single ignition & no instruments whatsoever. Ah, ignorance is bliss. . .

Bruce

Rotax, AC feeds the battery on most setups, so you would only need to isolate the battery supply to your CD ignition units,

Like a couple of diodes and chokes in the supply wire, smoothing the AC spikes out.

spacesailor

Thanks for your reply, Spacey. I don't understand it though!

My CD's use DC power, so alternator is only charging the battery. I'm trying to wire 2 batteries to the ignition circuit such that if one goes down it doesn't take the other out. Charging of the batteries is a lesser issue.

why not wire the ignition into the alternator circuit as well as the battery?

Because the alternator produces AC current and the ignition units need DC.

how does your alternator charge the battery?

Because the alternator produces AC current and the ignition units need DC.

So all the electrical system sees is DC, pulsed DC, but still DC.

http://www.bcae1.com/charging.htm

As I said, white man's magic.

But any opinions on my original question? The alternator is not wired into the ignition system - only the battery.

Just visit Burnsco ( I think they have Aussie branches ) and get a RV/Motorhome dual battery isolator circuit.

Pure Off-The-Shelf self contained electronics. hook two leads to the main battery/alternator circuit at the main battery terminals (to charge the secondary from the main), anything to be isolated and run separately off the second battery goes on the Other set of +/- leads where they attach to the second battery terminals. They allow a few dozen amps across from the main battery and nothing back the other way, enough to charge the secondary and run a few low draw devices.

They're used so if you flatten the house lights in the campervan you don't flatten the starter battery, If the main battery voltage drops to a certain point they stop allowing current through leaving enough to run the starter motor, so you can still start your motorhome after falling asleep with the lights on rather than flattening both batteries. They also isolate the secondary batteries from starter draw since deep cycle house batteries aren't made to deliver lots of grunt for a short while like a car battery,but rather a low draw for a long time, so having them hooked into the starter circuit would kill them in short order.

Exactly what you want, just hook the second set of wires to one DC Mag instead of your cockpit reading lights.

The ones I have seen are about the size of a couple of packs of fags, and a couple of hundred grams at most.

why not wire the ignition into the alternator circuit as well as the battery?

All the info you need is at Aircraft Electrical Systems

The manual is worth way more than Bobs asking price. Many wiring diagrams & explanations on how to setup a safe system, including for dual electronic ignition with dual battery.

In system's that must be reliable and isolated from any fault that may occur we use two diodes in parallel, their outputs connect together and each diode connects to the battery's. Hence if a system goes down for any reason the other diode instantly takes over. Use a similar reverse system for charging two batteries, but you must set the charging voltage 0.6 volts higher to account for the voltage loss through the diodes. But with the ignition power you must put a monitoring system to monitor each stage to each diode otherwise one might fail and you will never know until the second system fails. These techniques are used in control systems that must work, eg driverless trains etc. Their probability of failure is very low,. Their mean time between failures is in the millions of hours.

Thanks to all for your help.

I am leaning toward the simplicity of simply wiring the 'ballast battery' into one of the CD units via a panel switch, & charging it via a diode from the main battery.

This way, each CD unit has an independent power supply. I can check voltages on each battery via a panel mounted toggle switch.