Alternator B Lead

[B737NG]

I am starting to design the electrical system for an aft battery, RV-8.

I have been reading the AeroElectric Connection material and I have read to connect the alternator B lead directly to the starter contactor on the firewall (via in-line fuse) and not bring the noiseiest wire into the aircraft.

My question is: With the aft mounted battery would the long wire running from the battery to the starter contactor not also turn into a noisey wire?

Thanks for your input.

Paul

[gasman]

What Aero Elec. wants to see is the power from the alt. go direct to the battery. And then, from the batt to whatever.

In the case of a "7" with all on the firewall, this would work like he says in his book. The batt and master and starter are all there before the power goes into the main buss

The wire from the alt to the batt IS the noiesy wire. The batt is the noise filter.

[airguy]

But a couple hundred thousand microfarad capacitor placed on the firewall with the B lead on the positive side and ground on the other will kill the noise, then continue the B lead all the way back to the battery. Go to a car stereo shop, they sell capacitors in this size range to the high school kids with big car stereo setups so the amplifiers keep seeing good voltage on loud music "hits". They are not too bad expensive, either.

I'd put about a 100-ohm resistor in line with the ground connection on the capacitor, to prevent a short circuit in the capacitor from taking out the whole electrical system. A fuse small enough to protect you in this case would also blow when you first hooked it up, but the resistor will be fine. If the capacitor shorted internally, 100 ohms resistance would give you 150 milliamps of current draw - about the same as a couple LED lights.

[gmcjetpilot]

Originally Posted by B737NG

I am starting to design the electrical system for an aft battery, RV-8.

I have been reading the AeroElectric Connection material and I have read to connect the alternator B lead directly to the starter contactor on the firewall (via in-line fuse) and not bring the noiseiest wire into the aircraft.

My question is: With the aft mounted battery would the long wire running from the battery to the starter contactor not also turn into a noisey wire?

Thanks for your input.

Paul

YES Paul! No critizisim to Bob but when he is making an argument he brings in all kind of "facts" that may have been true in the past or some applications or may be over stated, but alternator wires should NOT be noisy, even if its in the cockpit.

You will have the aft battery and master contactor near the battery, than large gauge wire fwd, thru the cockpit and firewall to the cowl area, where you will have the starter contactor. You can run the B-lead thru the big inline "bussman" brand fuse right to the bat side of the starter contactor. This is all per Bob's "Z" dwgs.

You are right it's ALL connected. Electrically the evil 'b-lead' is running all through the plane (electrically). Don't over think but you are right, it matters not as long as you have good grounds and no flacky devices spewing out RFI.

Now I would not run the B-lead or battery cable all over and wrap other wires around it, but NOISE comes mostly for BAD GROUNDS and RFI (radio freq interferance). Some devices do MAKE noise, but this is not the old generator days with old mechanical voltage regulators.

I would disagree that the b-lead is noisy. If it is there is a problem with the alternator or voltage regulator. If you plane has good grounds the modern alternator with modern electonrics (digital functions in IC chips) you'll be fine.

What alternator do you have?

[B737NG]

Thanks for your help.

I purchased a Plane Power 60 amp alternator.

Paul