Lightspeed ignition / Dual batteries

[mburch]

Hi folks,

I'm setting up my airplane with one magneto and one Lightspeed Plasma III ignition. The airplane has one Odyssey PC680 for its main battery, plus another ~4.5 Ah battery under the baggage floor that's there to keep the most important avionics alive during engine cranking (similar to the setup described in this recent thread). Since I have this second battery already, I wanted to find a way to let the LSE ignition also get power from the aux battery during engine cranking. I don't want to use a separate "power select" switch as described in the Lightspeed manual, since I already have my panel cut for a single switch. I've read the Lightspeed documentation and all the various threads about wiring the LSE ignition, and I haven't come up with a solution that I find totally satisfactory. Here's what I have sketched so far:

• A pair of diodes (or half a bridge rectifier) is used to let the ignition draw power from either battery.
• As per the LSE instructions, I've drawn this using the ignition power wire's shield to provide the ground connection. Obviously you wouldn't want to ground the shield at both batteries (ground loop) so I show it grounded next to the main battery only. This part of the LSE documentation seems like voodoo to me, but I'm trying to follow it as best I can. That's going to be a lot of shield connections, though!
• I don't like unfused conductors attached to batteries, so power from the main battery comes from the always-hot battery bus fuse block on the firewall, right next to the main battery. Power from the aux battery comes from a similar fuse block next to the small aux battery.
• I drew provisions for a filter capacitor next to the ignition box, per this thread.

I know the ignition will certainly get power from both batteries if I hook it up this way. However, I find all those extra shield connections to carry around the ground side of the circuit to be worrisome. So I guess my main question is: For those who are flying with Lightspeed ignitions, did you follow the instructions to use the power wire's shield for ground? If so, or if not, have you experienced any problems with comm, nav, or audio interference from the ignition?

thanks,
mcb

[AlexPeterson]

You will not get radio noise from the low voltage side of the system. It is the tens of thousands of volts actually firing the spark plugs that can cause noise, and those don't start until you are at the coils on the engine.

I followed LS's instructions on the connections/wiring.

One wire, from main battery, through a fuse/breaker and on through your ignition switch will work perfectly. The recent LS systems can tolerate low cranking voltages. Every connection is a potential failure point. KISS applies.

BTW, unrelated to your question, why would avionics be on at all during cranking?

[mburch]

Originally Posted by AlexPeterson

You will not get radio noise from the low voltage side of the system. It is the tens of thousands of volts actually firing the spark plugs that can cause noise, and those don't start until you are at the coils on the engine.

Well, certainly there will be noise from the high-voltage side. But my understanding is that the switching power supply the ignition box itself can feed noise back to the supply side. There must be something to this, or else the install manual wouldn't be so keen about the strange use of the power wire shield as a ground return.

One wire, from main battery, through a fuse/breaker and on through your ignition switch will work perfectly.

So did you do the shielded wire thing, and jumper the shield around the ignition switch, as the manual specifies? Or did you just run a piece of regular unshielded wire from the LSE box to ground?

BTW, unrelated to your question, why would avionics be on at all during cranking?

To monitor the engine. I don't believe in avionics master switches, but that's a topic for a different thread.

mcb

[shuttle]

Hi Matt,
I'm not flying yet and am certainly no expert on EI but, for what it's worth, all my research has led me to the same setup as your solution (except for the capacitor, which I had not found out about but will now look to include thanks to your post today).

I concur with you on the low volt side ground shielding. "Ripples" was the word I recall seeing used to describe what the shielding was protecting against. My plan is, as you say, to 'jumper' the shield around fuses/diode/switch.

I find it reassuring that independently we've each arrived at the same place.

[gasman]

My LSI is wired with standard A/C wire direct to the battery post, ground and pos. The pos goes to a fuse, then to the mag switch. switches were used and a start button. I have no noise from this installation.

As far as a alt. power source, I would just install a switch under (behind the panel) to switch between batteries when, or if ever needed...........